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The Coronavirus Crisis

Who reviews 'current' evidence on coronavirus transmission through air.

Nell Greenfieldboyce

A photograph from 1940, taken for infectious research purposes at the Massachusetts Institute of Technology, shows respiratory droplets released through sneezing. Bettmann/Bettmann Archive hide caption

A photograph from 1940, taken for infectious research purposes at the Massachusetts Institute of Technology, shows respiratory droplets released through sneezing.

The World Health Organization says the virus that causes COVID-19 doesn't seem to linger in the air or be capable of spreading through the air over distances of more than about 3 feet.

But at least one expert in virus transmission said it's way too soon to know that.

"I think the WHO is being irresponsible in giving out that information. This misinformation is dangerous," says Dr. Donald Milton , an infectious disease aerobiologist at the University of Maryland's School of Public Health.

The WHO says that "according to current evidence," the virus is transmitted through "respiratory droplets and contact routes." By that, the agency means the virus is found in the kind of big droplets of mucus or saliva created through coughing and sneezing.

These droplets can travel only short distances through the air and either land on people or land on surfaces that people later touch. Stopping this kind of transmission is why public health officials urge people to wash hands frequently and not touch the face, because that could bring the virus into contact with the nose or mouth.

Other viruses, however, get shed by infected people in a way that lets the germs actually hang suspended in the air for minutes or even hours. Later, these airborne viruses can get breathed in when other people pass by. Measles is a good example of that kind of transmission — the Centers for Disease Control and Prevention says , "Measles virus can remain infectious in the air for up to two hours after an infected person leaves an area."

The WHO said that this kind of airborne transmission of the new coronavirus might be possible "in specific circumstances and settings in which procedures that generate aerosols are performed," such as when a patient is intubated in a hospital or being disconnected from a ventilator.

Based on that, the agency recommends "airborne precautions" when medical workers do those procedures. Otherwise, the WHO says, health care workers caring for COVID-19 patients could use less-protective "droplet and contact precautions."

That troubles Milton, who says so little is known about this new virus, SARS-CoV-2, that it's inappropriate to draw conclusions about how it is transmitted.

"I don't think they know, and I think they are talking out of their hats," Milton says.

He says people like to think that there's some sharp, black-and-white distinction between "airborne" viruses that can linger and float in the air and ones that spread only when embedded in larger moist droplets picked up through close contact, but the reality of transmission is far more nuanced.

"The epidemiologists say if it's 'close contact,' then it's not airborne. That's baloney," he says.

When epidemiologists are working in the field, trying to understand an outbreak of an unknown pathogen, it's not possible for them to know exactly what's going on as a pathogen is spread from person to person, Milton says. "Epidemiologists cannot tell the difference between droplet transmission and short-range aerosol transmission."

He says these are hard questions to answer, and scientists still argue over how much of the transmission of influenza might be airborne. Some research shows that exhaled gas clouds from people contain a continuum of many droplet sizes and that a "high-momentum cloud" created by a cough or sneeze might carry droplets long distances.

What's more, one study of hospital rooms of COVID-19 patients found that "swabs taken from the air exhaust outlets tested positive, suggesting that small virus-laden droplets may be displaced by airflows and deposited on equipment such as vents."

Another study, in hospitals in Wuhan, China, found that most areas had undetectable or low levels of airborne virus.

In the face of this uncertainty, Milton thinks the WHO should follow the example of the CDC and "employ the precautionary principle to recommend airborne precautions."

"The U.S. CDC has it exactly right," he says, noting that it recommends airborne precautions for any situation involving the care of COVID-19 patients.

Of course, the world is struggling with a shortage of the most protective medical masks and gear. For the average person not working in a hospital, Milton says the recommendation to stay 6 feet away from others sounds reasonable.

He says if someone in a house is sick, it makes sense to have that person wear a mask and to increase the ventilation in the room, if possible, by cracking open a window. People shouldn't cram into cars with the windows rolled up, he says, and officials need to keep crowding down in mass transit vehicles such as trains and buses.

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• 02 April 2020

Is the coronavirus airborne? Experts can’t agree

• Dyani Lewis

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Since early reports revealed that a new coronavirus was spreading rapidly between people, researchers have been trying to pin down whether it can travel through the air. Health officials say the virus is transported only through droplets that are coughed or sneezed out — either directly, or on objects. But some scientists say there is preliminary evidence that airborne transmission — in which the disease spreads in the much smaller particles from exhaled air, known as aerosols — is occurring, and that precautions, such as increasing ventilation indoors, should be recommended to reduce the risk of infection.

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Nature 580 , 175 (2020)

doi: https://doi.org/10.1038/d41586-020-00974-w

Liu, Y. et al. Preprint at bioRxiv http://doi.org/dqts (2020).

Ong, S. W. X. et al. J. Am. Med. Assoc . http://doi.org/ggngth (2020).

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Santarpia, J. L. et al. Preprint at medRxiv http://doi.org/dqtw (2020).

van Doremalen, N. et al. N. Engl. J. Med . http://doi.org/ggn88w (2020).

Yan, J. et al. Proc. Natl Acad. Sci. USA 115 , 1081–1086 (2018).

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Cheng, V. C. C. et al. Infect. Control Hosp. Epidemiol . https://doi.org/10.1017/ice.2020.58 (2020).

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Which form of transport has the smallest carbon footprint?

How can individuals reduce their emissions from transport.

This article was first published in 2020. It was updated in 2023 with more recent data.

Transport accounts for around one-quarter of global carbon dioxide (CO 2 ) emissions from energy. 1 In some countries — often richer countries with populations that travel often — transport can be one of the largest segments of an individual’s carbon footprint.

If you need to travel locally or abroad, what is the lowest-carbon way to do so?

In this chart, we see the comparison of travel modes by their carbon footprint. These are measured by the amount of greenhouse gases emitted per person to travel one kilometer .

This data comes from the UK Government’s Department for Energy Security and Net Zero. It’s the emission factors companies use to quantify and report their emissions. While the overall rankings of transport modes will probably be the same, there may be differences across countries based on their electricity mix, vehicle stock, and public transport network.

Greenhouse gases are measured in carbon dioxide equivalents (CO 2 eq), accounting for non-CO 2 greenhouse gases and the increased warming effects of aviation emissions at high altitudes. 2

Walk, bike, or take the train for the lowest footprint

Over short to medium distances, walking or cycling is nearly always the lowest carbon way to travel. While they’re not in the chart, the carbon footprint of cycling one kilometer is usually in the range of 16 to 50 grams CO 2 eq per km depending on how efficiently you cycle and what you eat. 3

Using a bike instead of a car for short trips would reduce travel emissions by around 75%.

Public transport is usually your best option if you can’t walk or cycle. Trains are particularly low-carbon ways to travel. Taking a train instead of a car for medium-length distances would reduce emissions by around 80%. 4 Using a train instead of a domestic flight would reduce your emissions by around 86%. 5

In fact, if you took the Eurostar in France instead of a short-haul flight, you’d cut your journey’s footprint by around 97%. 6

What if you can’t walk or cycle and don’t have access to public transport?

If none of the above are options, what can you do?

Driving an electric vehicle (EV) is your best mode of private transport. It emits less than a petrol or diesel car, even in countries with a fairly high-carbon electricity mix. Of course, powering it from a low-carbon grid offers the greatest benefits.

The chart above only considers emissions of EVs during their use phase — when you’re driving. It doesn’t include emissions from car manufacturing. There have been concerns that when we account for the energy needed to produce the battery, an EV is actually worse for the climate than a petrol car. This is not true — while an EV does have higher emissions during its production, it quickly “pays back” once you start driving it. 7

The next best is a plug-in hybrid car.

Then, where you take a petrol car or fly depends on the distance. Flying has a higher carbon footprint for journeys less than 1000 kilometers than a medium-sized car. For longer journeys, flying would actually have a slightly lower carbon footprint per kilometer than driving alone over the same distance.

Let’s say you were to drive from Edinburgh to London, a distance of around 500 kilometers. You’d emit nearly 85 kilograms CO 2 eq. 8 If you were to fly, this would be 123 kilograms — an increase of almost one-third. 9

Some general takeaways on how you can reduce the carbon footprint of travel:

• Walk, cycle, or run when possible — this comes with many other benefits, such as lower local air pollution and better health;
• Trains are nearly always the winning option over moderate-to-long distances;
• If travelling internationally, going by train or boat is lower-carbon than flying;
• Electric vehicles are nearly always lower-carbon than petrol or diesel cars. The reductions are greatest for countries with a cleaner electricity mix;
• If traveling domestically, driving — even if it’s alone — is usually better than flying;
• Car-sharing will massively reduce your footprint — it also helps to reduce local air pollution and congestion.

Appendix: Why is the carbon footprint per kilometer higher for domestic flights than long-haul flights?

You will notice that domestic flights have higher CO2 emissions per passenger-kilometer than short-haul international flights, and long-haul flights have even slightly lower emissions. Why is this the case?

In its report on the CO 2 Emissions from Commercial Aviation , the International Council on Clean Transportation provides a nice breakdown of how the carbon intensity (grams CO 2 emitted per passenger kilometer) varies depending on flight distance. 10

This chart, with carbon intensity given as the red line, shows that at very short flight distances (less than 1,000 km), the carbon intensity is very high. It falls with distance until around 1,500 to 2,000 km, then levels out and changes very little with increasing distance.

This is because take-off requires much more energy input than a flight's “cruise” phase. So, for very short flights, this extra fuel needed for take-off is large compared to the more efficient cruise phase of the journey. The ICCT also notes that less fuel-efficient planes are often used for the shortest flights.

The IEA  looks at CO 2  emissions  from energy production alone — in 2018, it reported 33.5 billion tonnes of energy-related CO 2  [hence, transport accounted for 8 billion / 33.5 billion = 24% of energy-related emissions.

Aviation creates several complex atmospheric reactions at altitude, such as vapor contrails, creating an enhanced warming effect. In the UK’s Greenhouse gas methodology paper , a “multiplier" of 1.9 is applied to aviation emissions to account for this. This is reflected in the CO 2 eq factors provided in this analysis.

Researchers — David Lee et al. (2020) — estimate that aviation accounts for around 2.5% of global CO 2 emissions but 3.5% of radiative forcing/warming due to these altitude effects.

Lee, D. S., Fahey, D. W., Skowron, A., Allen, M. R., Burkhardt, U., Chen, Q., ... & Gettelman, A. (2020). The contribution of global aviation to anthropogenic climate forcing for 2000 to 2018 .  Atmospheric Environment , 117834.

Finding a figure for the carbon footprint of cycling seems like it should be straightforward, but it can vary quite a lot. It depends on several factors: what size you are (bigger people tend to burn more energy cycling), how fit you are (fitter people are more efficient), the type of bike you’re pedaling, and what you eat (if you eat a primarily plant-based diet, the emissions are likely to be lower than if you get most of your calories from cheeseburgers and milk). People often also raise the question of whether you actually eat more if you cycle to work rather than drive, i.e., whether those calories are actually ‘additional’ to your normal diet.

Estimates on the footprint of cycling, therefore, vary. Based on the average European diet, some estimates put this figure at around 16 grams CO2e per kilometer. In his book “ How bad are bananas: the carbon footprint of everything ”, Mike Berners-Lee estimates the footprint based on specific food types. He estimates 25 grams CO 2 e when powered by bananas, 43 grams CO 2 e from cereal and cow’s milk, 190 grams CO 2 e from bacon, or as high as 310 grams CO 2 e if powered exclusively by cheeseburgers.

National rail emits around 35 grams per kilometer. The average petrol car emits 170 grams. So the footprint of taking the train is around 20% of taking a car: [ 35 / 170 * 100 = 20%].

National rail emits around 35 grams per kilometer. A domestic flight emits 246 grams. So the footprint of taking the train is around 14% of a flight: [ 35 / 246 * 100 = 14%].

Taking the Eurostar emits around 4 grams of CO 2 per passenger kilometer, compared to 154 grams from a short-haul flight. So the footprint of  Eurostar is around 4% of a flight: [ 4 / 154 * 100 = 3%].

The “carbon payback time” for an average driver is around 2 years.

An average petrol car emits 170 grams per kilometer. Multiply this by 500, and we get 85,000 grams (85 kilograms).

A domestic flight emits 246 grams per kilometer. Multiply this by 500, and we get 123,000 grams (123 kilograms).

Graver, B., Zhang, K. & Rutherford, D. (2018). CO2 emissions from commercial aviation, 2018 . International Council on Clean Transportation.

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by Chris Woodford . Last updated: July 23, 2023.

Photo: Sound is energy we hear made by things that vibrate. Photo by William R. Goodwin courtesy of US Navy and Wikimedia Commons .

What is sound?

Photo: Sensing with sound: Light doesn't travel well through ocean water: over half the light falling on the sea surface is absorbed within the first meter of water; 100m down and only 1 percent of the surface light remains. That's largely why mighty creatures of the deep rely on sound for communication and navigation. Whales, famously, "talk" to one another across entire ocean basins, while dolphins use sound, like bats, for echolocation. Photo by Bill Thompson courtesy of US Fish and Wildlife Service .

Robert Boyle's classic experiment

Artwork: Robert Boyle's famous experiment with an alarm clock.

How sound travels

Artwork: Sound waves and ocean waves compared. Top: Sound waves are longitudinal waves: the air moves back and forth along the same line as the wave travels, making alternate patterns of compressions and rarefactions. Bottom: Ocean waves are transverse waves: the water moves back and forth at right angles to the line in which the wave travels.

The science of sound waves

Picture: Reflected sound is extremely useful for "seeing" underwater where light doesn't really travel—that's the basic idea behind sonar. Here's a side-scan sonar (reflected sound) image of a World War II boat wrecked on the seabed. Photo courtesy of U.S. National Oceanographic and Atmospheric Administration, US Navy, and Wikimedia Commons .

Whispering galleries and amphitheaters

Photos by Carol M. Highsmith: 1) The Capitol in Washington, DC has a whispering gallery inside its dome. Photo credit: The George F. Landegger Collection of District of Columbia Photographs in Carol M. Highsmith's America, Library of Congress , Prints and Photographs Division. 2) It's easy to hear people talking in the curved memorial amphitheater building at Arlington National Cemetery, Arlington, Virginia. Photo credit: Photographs in the Carol M. Highsmith Archive, Library of Congress , Prints and Photographs Division.

Measuring waves

Understanding amplitude and frequency, why instruments sound different, the speed of sound.

Photo: Breaking through the sound barrier creates a sonic boom. The mist you can see, which is called a condensation cloud, isn't necessarily caused by an aircraft flying supersonic: it can occur at lower speeds too. It happens because moist air condenses due to the shock waves created by the plane. You might expect the plane to compress the air as it slices through. But the shock waves it generates alternately expand and contract the air, producing both compressions and rarefactions. The rarefactions cause very low pressure and it's these that make moisture in the air condense, producing the cloud you see here. Photo by John Gay courtesy of US Navy and Wikimedia Commons .

Why does sound go faster in some things than in others?

Chart: Generally, sound travels faster in solids (right) than in liquids (middle) or gases (left)... but there are exceptions!

How to measure the speed of sound

Sound in practice, if you liked this article..., don't want to read our articles try listening instead, find out more, on this website.

• Electric guitars
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On other sites

• Explore Sound : A comprehensive educational site from the Acoustical Society of America, with activities for students of all ages.
• Sound Waves : A great collection of interactive science lessons from the University of Salford, which explains what sound waves are and the different ways in which they behave.

Educational books for younger readers

• Sound (Science in a Flash) by Georgia Amson-Bradshaw. Franklin Watts/Hachette, 2020. Simple facts, experiments, and quizzes fill this book; the visually exciting design will appeal to reluctant readers. Also for ages 7–9.
• Sound by Angela Royston. Raintree, 2017. A basic introduction to sound and musical sounds, including simple activities. Ages 7–9.
• Experimenting with Sound Science Projects by Robert Gardner. Enslow Publishers, 2013. A comprehensive 120-page introduction, running through the science of sound in some detail, with plenty of hands-on projects and activities (including welcome coverage of how to run controlled experiments using the scientific method). Ages 9–12.
• Cool Science: Experiments with Sound and Hearing by Chris Woodford. Gareth Stevens Inc, 2010. One of my own books, this is a short introduction to sound through practical activities, for ages 9–12.
• Adventures in Sound with Max Axiom, Super Scientist by Emily Sohn. Capstone, 2007. The original, graphic novel (comic book) format should appeal to reluctant readers. Ages 8–10.

Popular science

• The Sound Book: The Science of the Sonic Wonders of the World by Trevor Cox. W. W. Norton, 2014. An entertaining tour through everyday sound science.

Academic books

• Master Handbook of Acoustics by F. Alton Everest and Ken Pohlmann. McGraw-Hill Education, 2015. A comprehensive reference for undergraduates and sound-design professionals.
• The Science of Sound by Thomas D. Rossing, Paul A. Wheeler, and F. Richard Moore. Pearson, 2013. One of the most popular general undergraduate texts.

Text copyright © Chris Woodford 2009, 2021. All rights reserved. Full copyright notice and terms of use .

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Why Are Great Circles the Shortest Flight Path?

Why do you fly over Greenland in an airplane flight?

Or why is it that when you see flight paths on a map they always take a curved route between 2 cities?

It’s because planes travel along the shortest route in a 3-dimensional space.

This route is called a geodesic or great circle route . They are common in navigation, sailing, and aviation.

But geodesics can be confusing when you’re looking at a 2-dimensional map as they follow quite the odd flight path. Let’s dig into this concept a bit deeper.

Great Circle Routes Explained

In a flight path from New York to Madrid, if I asked you which line is shorter, you’d say the straight one, right?

However, a straight line in a 2-dimensional map is not the same as a straight line on a 3-dimensional globe .

This is why flight paths travel along an arc between an origin and a destination.

Now here’s what the same flight paths look like on a sphere. Remember that the straight line in the Mercator map above followed the 40° latitude line.

This paints quite a different story, doesn’t it? It’s deceiving to the human eye.

The takeaway is this:

A route that looks longer on the map is because of the distortion created by map projections like the Mercator Projection . In navigation, pilots often use great circles (geodesic) as the shortest distance flight.

Great circles vs small circles

Now that you have a visual understanding of great circles. Here’s a definition of what a great circle is:

• A great circle is a circle on the globe such that the plane passing through the sphere’s center is equal to the circumference of the Earth.
• Alternatively, a great circle is where the radius is equal to that of the globe representing the shortest distance between two points on the surface of the earth.

In basic terms, imagine you’re cutting into an orange. You can cut them at any angle – north-south, east-west, diagonally. As long as you cut two identical portions, then the circle where the cut was made is a great circle.

For example, the equator is a great circle because it’s the maximum possible circle:

You could also cut it at the north and south poles. This longitudinal line also cuts two equal portions. Any meridian line is a great circle as well.

From New York to Madrid, here’s how the plane creates two equal segments.

A great circle generates two arcs with the shorter one being the shortest path. Here is the shortest path and how the plane is angled to create the shortest path.

How about when you follow along the 40° latitude line? Anywhere that it doesn’t cut two equal pieces is a small circle .

While a rhumb line track is at a constant azimuth, a geodesic line changes direction all the time.

This fundamental difference in navigation concepts can have a significant impact on long-distance sea voyages.

READ MORE : Rhumb Lines: Setting it Straight with Loxodromes

How Geodesics Work

Planes travel along the shortest route in 3-dimensional space. This route is called a geodesic or great circle .

While map projections distort these routes confusing passengers, the great circle path is the shortest path between two far locations.

This is why pilots fly polar routes saving time and distance . And this is why pilots often fly over Greenland.

Have any questions? Please let us know in the comments section below.

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38 comments.

Please, I want to use your draws in a aerodynamic book for engineering students (in spanish language).

Please, can you report me if I can use it?. In case of yes, wich will be the conditions?

kind regards !!!

Hi Victor. In this case, you can use these images. We are the creators of these graphics. Check out our guide how to cite – https://gisgeography.com/how-to-cite/

Way to conflate 2 different idioms: “This paints quite a different story, doesn’t it?” I think one paints a different picture or tells a different story. How does one paint a story, or tell a picture? Otherwise, a great article.

Buy a globe and figure it out.

I’m trying to figure out what flight path an LTA aircraft (Larger than Goodyear Blimp) would take from England to Idaho. It’s part of a novel I’m writing. Any suggestions?

You’re definitely on the right path and to use a great circle.

I am a layman. Please describe the route from Mumbai to New York and San Fransico to Tokyo through a diagram to show why airlines prefer polar routes?

A great circle connecting two points is the shortest distance but it requires frequent heading changes throughout the journey. A straight line (rhumb line) drawn on a Mercator projection map produces the constant compass bearing to follow for the same journey; which is easy to draw and much easier to follow. Over a short distance the difference between a great circle and a rhumb line route is negligible so the simple rhumb line route is used. However, over a very long journey the difference in distance will be quite significant so a great circle route is preferred.

In navigation latitude is the angle above (north) or below (south) of the equator. Longitude is the angle east or west of the prime meridian (ie the Greenwich Meridian). Both of these angular measurements are derived from the centre of the global earth mass. 0 degrees latitude, 0 degrees longitude is the intersection of the prime meridian and the equator (just south of Ghana West Africa).

When you try to draw a flat map to depict the surface of a globe there is a huge problem; imagine taking the peel off of an orange and trying to lay it out flat.

On the surface of global earth the horizontal distance measured between two meridians along the equator (0 degrees latitude) will be greater than the distance between the same two meridians measured along any other latitude because the circumference of each latitude gets smaller toward the poles so the meridians are closer together near the poles. However, the distance measured between two latitudes on the same longitude is the same distance on other longitudes because they will lie on a great circle.

For marine navigation the map solution generally accepted is the Mercator projection because of its unique property of representing any course of constant bearing as a straight segment that can easily be drawn on a flat map. Such a course, known as a rhumb line, is preferred in marine navigation because it is easier for a ship to sail in a constant compass direction to reach its destination even though it will be a bit further than a great circle route. For short distances the difference between the rhumb line and great circle route is negligible. For aviation the distance is more critical since it directly affects time, fuel and load. A simple solution is, where possible, create a sequence of short rhumb lines along the corresponding great circle route. In the modern era all of these positioning problems have been overcome by GPS satellite navigation.

These days we all take degrees latitude and degrees longitude for granted because they have been successfully used for over 2,000 years. Around that time there were early attempts to measure the circumference of the global earth. Early mariners setting off on long voyages either came ‘back’ or never returned; which for millenniums gave rise to the flat earth notion. The first circumnavigation of the earth was accomplished by the Magellan fleet sailing westward 500 years ago. I’ve personally used latitude and longitude to circumnavigate the earth 5 times.

I’ll finish with a reminder that latitude and longitude are both angular measurements derived from the centre of the global earth mass. If the earth were flat then for positioning on it there would be only one angle from the centre and a linear distance measurement from it… an idea that has never ever been tried and proven.

Taking all of the info from this page into consideration, how could it be used to strengthen flat earth theory? I was directed here because I wanted to learn what constitutes a “great circle” and I now understand. I seriously want to know how anyone could reach that conclusion.

High-altitude flying brings another problem to the fore. It is called “coffin corner” where if you fly slower you will stall, and if you fly faster you will also stall.

It actually makes sense if you go back to the comparison of cutting an orange. Again understanding how to navigate along a 3-dimensional globe while looking at a 2-dimensional picture is why it seems so confusing. And No Cari it doesn’t support the flat earth theory. Not even its own theory supports it. lol. The entire point of this is because of it’s a globe. If it were flat none of this would be necessary. Not to mention the Japanese would still be en route to Pearl Harbor if it were a flat earth. That’s an exaggeration but still, it would have been quite a long flight. One that those little planes would never have been capable of making. In fact, if the earth were flat many lives at Pearl Harbour as well as at both Hiroshima and Nagasaki would have been spared. It’s a shame that the earth is in fact round.

You have supported the theory of a flat earth. Thanks.

My wife just left Dallas for Tokyo. I looked on the flight plan and it goes up through Montana and Canada and over the edge of Alaska. I don’t see how this could be the shortest route to Tokyo from Dallas. It seems more like they are trying to avoid flying directly over the vastness of the Pacific Ocean in case there is a mechanical problem.

Having read all the above I’ve decided to go by boat.

Odd! I was told that because of centrifugal force, the Earth was compressed at the poles, thus making it wider at the equator. The great arc is a navigational compensation!

Meanwhile, the Egyptians, who started all this, still could not figure out the height of the pyramids even though they had Pythagoras. Not to mention that even today there is no certain way to measure the partial volume of a cylinder.

The Earth is flat and south polar centered. That’s it.

What about the direction of the route? Nobody can convince me that Madrid lies to the north of Newyork as they are saying now.

Maybe thats how they conclude it but its wrong, even if it works. the real reason is because of difference of rotational velocity of a shorter vs longer circumference. if you draw two lines from the center of a circle, each come out of the destinations. then draw two lines, one at altitude and the other at ground level. clearly the one at altitude is longer. it doesn’t matter if these lines are curved or not. what matters is the velocity the earth rotates at. the length of a circumference increases by 2 pi r. this means that at greater radius per starting location, the rotational velocity of a shorter radius vs a larger are varied. at a great altitude, though you are traveling at a perceived pace relative to the motor function, but you are multiplying that velocity by the increase in circumference relative to the ground’s. or vice versa, you can say the ground it traveling quicker as well since two point in the circumference have less distance to travel compared to the greater altitude. but what this really does is multiply the distance you travel at altitude relative to the ground. so one minute of flight at ground level vs one minute of flight at altitude results in greater distance. but its only if you fly against earth’s rotation. the earth is rotating at a constant speed, but the circumference between the greater height points travels at length over time just as the shorter one. but the shorter circumference then has less length to travel in that time, so its technically traveling slower. so as you increase height from the center of a circle, the increase in circumference, with earth’s rotation increase your velocity. if earth wasn’t rotating, then height wouldn’t matter and straight line would. 3 dimensions or 2… sure, you could look at it that way, but its over complicated…

Because the time it takes to fly to the higher altitude is not commensurate to the time lost by not climbing.

Let P = pressure, d = density, R = gas constant, T = temperature, h = height, g = gravitational acceleration.

By Ideal Gas Law PV = nRT -> d = n/V = P/RT

By the law of hydrostatic pressure

dP/dh = -d*g = P*g/RT

We know have a first-order separable differential equation. Solving this, we get:

Integral dP/P = Integral -g/RT dh log(P) – log(P0) = -g*h/RT -> P = P0*exp(-g*h/RT) d = P/RT = P0/RT*exp(-g*h/RT)

Therefore, as you go higher, the weight of the air column decreases reducing atmospheric pressure and density. However, the air also gets colder slightly increasing density. In the Stratosphere, temperatures increase with altitude instead of decrease. However, this layer of the atmosphere is above the troposphere where we live, so it has to be less dense. Thus, the weight of the hydrostatic pressure of the air column has a larger effect on atmospheric pressure and air density at least less than 10 km above sea level.

In the real world of long distance air navigation, great circle routes are *not* the shortest *flight* path, but they do make a good start on figuring out what will be the shortest flight path. The article fails to understand that aeroplanes travel in the air, which is moving, rather than fixed in relation to the earth. The origin and destination are fixed to the earth, if the sea and the air were likewise fixed, a great circle would yield the shortest path. A longer path with regard to the earth can yield flight path savings in the order of 30% to 50%, the classic example being North Atlantic flights in the forties, where savings of this order resulted from using Bellamy drift derived constant heading paths instead of great circle routing.

Aircraft need to travel the shortest route in the air, not in three dimensional space. Sometimes that will be the same thing, but not very often. For an air routing, you could use the barometric pressure at origin and destination plus the latitudes. Using these, you can calculate the drift for the entire flight due to pressure differences and Coriolis effect, the net geostophic winds. However, there may be jetstreams at less optimum altitudes that could provide considerable net improvement, even if more fuel is used per air mile (meaning movement through the body of air, not the 3D relationship with the earth), but lets not go there. Having derived a constant heading, it is flown, and neglecting political geography, with no concern for the position with respect to the position on earth below, other than the destination and not bumping into things.

To help with the lay understanding, imagine a rectangular table top, with a dot at each end. Over the top you have a much larger piece of perspex. You have a toy car which can travel at a speed equal to the distance between the dots over one hour. Over the course of an hour, the perspex moves half that distance to the left of the direct track, then half that distance to the right. If we attempt to stay on the “shortest” path from A to B, at the end of an hour, we will still be at A. If we calculate the net drift between A and B, which in this example means following a constant heading equal to the direct heading at the start of the journey, our path will be much longer than the direct path, a loop heading out to the left. However, after an hour, we will be at B. If we change the total distance moved by the perspex to half A-B, purely left and right, it will still take us one hour on the constant heading, but if we follow the direct path on the table top, we will get there, having taken considerably more than an hour to do so.

On shorter journeys in the real world, there are few wind reversals – there is also little difference between a rhumb line course and a great circle, so you may as well fly a rhumb line. Longer journeys are where this counts, and also where the great circle should be of use, but often is not, due to winds aloft.

The correct term is “Density altitude” which is a calculation of pressure altitude corrected for nonstandard temperature. As temperature and altitude increase, air density decreases. All aircraft have a flight “ceiling”, an altitude above which it cannot fly. As an airplane ascends, a point is eventually reached where there just isn’t enough air mass to generate enough lift to overcome the airplane’s weight. This is why (depending on the airplane) you might not be able to take-off from a high altitude airport on a particularly hot day. I have experienced this (pilot) myself several times.

If you depart from the North Pole with a plane along a meridian, directly from the rotation point (rotation axis) where speed is 0, to a destination at the equator where the rotation speed is around 1660 km / h , would the plane still reach its destination? From North Pole direct to Brazil(circular arc over meridian 60 W).

Thinner air does not give better engine performance. The reason flying high is more economical compared to flying at low altitude is because the TAS is higher due to density. Thus the amount of fuel burned per nm travel is lower.

You have such great figures. May I use one of them in a presentation to illustrate great circle routes?

Yes, you can. But please give credit by referencing to gisgeography.com

Chris, you are wrong. For air, D=PM/(RT), where D=mass density, P=pressure, M=molecular weight average, R=ideal gas constant, & T=absolute temperature. This is why you need to fill your tires with air in the winter. The temperature drop compresses the air, which means you have to add air to fill in the same amount of relative space.

There are many other atmospheric factors in play such as humidity, wind speed, and wind direction to consider as well.

…and yes, thinner colder air is better for fuel efficiency, but bad for lift. The higher you go, the harder it is to maintain altitude, so like everything else, there is a trade-off. Airplanes are designed to take advatange of the thickness of the air at a certain altitude. That altitude is not fixed relative to the ground, but depends on the weather. So on a hot day, they can fly higher. On a cold day, they.have to fly lower. But there is always an optimum “temperature altitude” for any given plane on any given day. You can’t just keep going up forever. You need pressure under the wings, or you’re flying a brick.

Airplanes need air pressure to fly, it doesn’t matter how dense the air is if there’s no pressure, and pressure and altitude have a consistent relationship. Does that make sense?

Actually, you’re all right, but you’ve each only got two-thirds of the puzzle. There is no direct relationship between temperature and density. There IS a relationship between PRESSURE and temperature and density.

In aviation, there is a concept called “termperature altitude.” So the actual relationship is between all three of them, but the focus is only on the two. The higher you go, the colder and thinner the air gets because the pressure is dropping. For every 1000 feet you go up, there is an expected drop in temperature and pressure, and this remains constant. So flying through colder air is like flying higher. The colder air makes the pressure go down while the density remains the same. The airplane acts as if the air is less dense. Got it?

I too, like Mr. Bir, noticed that there was something wrong with the explanation regarding the density of air. Yes, as air gets colder it gets denser, that’s why to start a cold gasoline engine you have to activate the ‘choke’ which in turn will supply the fuel-air mixture with an extra amount of fuel in order to compensate the leanness that otherwise would not start a combustion.

J Bir, unlike liquids the viscosity of gases inceases with increasing temperature. This can be explained using the kinetic theory of gases.

Mr Bir has it right on, as soon as I read it, I questioned it. But we appreciate your explanation of the great circle route. Good article.

Let me correct you. As the air gets colder, it gets denser, not thinner. However, as you increase in altitude, the air gets thinner and colder. Pilots will always want to fly higher as the low density of the air reduces drag and thus increases the efficiency of the fuel.

Every airline in the world has a business route. This route determines where the aircraft can and cannot fly. For example certain routes over the Atlantic are more costly to make but much faster, the final determination is down to the pilot, in regards to how much fuel he has on board, how delayed the aircraft is, in respect to flying faster than he normally would. This is a very significant thing when determining which transatlantic route to take, because there are at least a dozen of which only 3 of those routes are the fastest and most economical, the rest vary with flight cost. In respect to flying over Greenland, there was a very valid reason to do so especially during the winter and early spring months. As the air gets colder, it gets thinner, and the air thins it gets more and more difficult for an aircraft to stay in the sky. This meant under aviation rules, those aircraft with only 2 Jet Engines had to make the shortest distance between land masses, incase an engine stopped working. This way the pilot could make an emergency landing. Aircraft such as the 747 had no such problems, and could easily fly the quickest transatlantic routes. However over the decades Jet Engines have become increasingly reliable, so much so, that they too can now fly over the Arctic to whatever destination. There are very few airlines who actually use Arctic routes, because these routes are extremely expensive to use. An example of a flight from Helsinki (EHFK; HEL) to Barcelona (LEBL, BCN) does not use a Great Circle nor a Small circle, it uses a direct path from point A to B.

Small circles are not always used for short distances, infact they are seldom used in Europe. Instead most short distances are literally from A->B.

Why are small circles used in short distance trips and not the latter?

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The Geography of Transport Systems

The spatial organization of transportation and mobility

5.5 – Air Transport

Authors: dr. john bowen and dr. jean-paul rodrigue.

Air transportation is the mobility of passengers and freight by any conveyance that can sustain controlled flight.

1. The Rise of Air Transportation

Air transportation was slow to take off after the  Wright Brothers breakthrough at Kitty Hawk in 1903. More than a decade passed before the first faltering efforts to launch scheduled passenger services. On January 1, 1914, the world’s inaugural scheduled flight with a paying passenger hopped across the bay separating Tampa and St. Petersburg, Florida.

In its earliest years, the airline industry had a symbiotic relationship with military aviation. World War I, which began just months after that first flight from Tampa, provided a powerful spur to the development of commercial aviation as air power began to be used strategically, and better aircraft were quickly introduced. The war left a legacy of thousands of unemployed pilots and surplus aircraft, along with an appreciation for the future significance of aviation.

After the war, civilian airliners improved rapidly. The non-stop crossing of the North Atlantic in 1927 was a key event as the range and navigational capabilities of the emerging air transport system were tested. For instance, the 8-12 passenger Dutch-built Fokker Trimotor, the most popular airliner in the early interwar years, had a top speed of 170 kilometers per hour and a range of 1,100 kilometers, which is less than the distance between Amsterdam and Rome. By the eve of World War II, airlines worldwide were adopting the USA-built Douglas DC-3 with a capacity of 28 passengers, a speed of 310 kilometers per hour, and a range of more than 2,400 kilometers nonstop, able to  fly across the US with just three stops. The DC-3 made its maiden commercial flight in 1936 between New York and Chicago, a vital business route highlighting the commercial significance of fast-changing technology.

Governments supported the emergence of the airline industry through ownership or subsidies. In Europe, governments established new passenger airlines, while on the other side of the Atlantic, the American government heavily subsidized airmail .  Airmail was one of the earliest commercially relevant applications of air transportation because it helped accelerate monetary transactions and tie together far-flung enterprises, facilitating the emergence of continental and intercontinental enterprises. US airmail subsidies also fostered the emergence of the first major US passenger airlines.

By the eve of World War II, air travel was quite literally taking off. In the US, for instance, the number of passengers grew fivefold from 462,000 to 1,900,000 between 1934 and 1939. Still, aviation remained far beyond the means of most travelers, especially for long-haul routes. For instance, in 1936, Pan American World Airways launched services across the Pacific with a roundtrip fare of $1,438 (about $26,900 in 2020 dollars) between San Francisco and Manila. As in this example, many of the long-haul air services were to colonies and dependencies. Only the elite or government officials could afford such early intercontinental routes .

Yet war again catalyzed the growth of air transportation since airpower became an ever more crucial element of military operations. New airports, vast numbers of trained pilots, great strides in jet aviation, and other aviation-related innovations, including radar, were among the legacies of World War II. Boosted by such developments and the broader economic boom that followed the war, air transportation finally became the dominant mode of long-haul passenger travel in developed countries. By the 1950s, air travel had become more widely advertised, and standardized fare structures were emerging. In 1956, more people traveled on intercity routes by air than by Pullman car (sleeper) and coach class trains combined in the US. For the first time in 1958, airlines carried more passengers than ocean liners across the Atlantic.

The speed advantage for aviation grew with the advent of jet travel in the mid-1950s. In October 1958, the  Boeing 707 took its maiden commercial flight with a Pan American World Airways route linking New York and Paris, with a refueling stop in Gander, Newfoundland. The B707 was not the first jetliner, but it was the first successful one. The B707 and other early jets, including the Douglas DC-8, doubled the speed of air transportation and radically increased airline productivity, enabling fares to fall . Just a few years after the B707’s debut, airlines had extended jet service to most major world markets. The technical benefits of jet planes, such as better ranges, changed the structure of air networks as airlines bypassed airports that conventionally had acted as gateways because of refueling stops. This was the case for Gander in Canada and Recife in Brazil for transatlantic flights.

Jet transportation facilitated the extension of the linkages between people and places . For example, through the mid-1950s, all major league baseball teams in the US were located in the Manufacturing Belt, situated no more than an overnight rail journey apart from one another to permit closely packed schedules. The speed and ultimately lower cost of air transportation freed teams to move to the untapped markets of the Sunbelt. By the mid-1960s, half a dozen teams were strung out across the South and West, complementing and competing against those that remained in the Frostbelt.

In the years since the beginning of the Jet Age, commercial aircraft have advanced markedly in capacity and range. Just 12 years after the debut of the 134-seat (in a typical two-class configuration) B707, the 366-seat (in a typical three-class configuration) B747 made its maiden flight. The economies of scale fostered by the 747 and other wide-body jets helped to push real airfares downward , thereby democratizing aviation beyond the so-called “Jet Set”. Like the B707, the B747 premiered on a transatlantic route from New York City. However, the B747, particularly the longer-range B747-400 version introduced in the late 1980s, has been nicknamed the “Pacific Airliner” because of its singular significance in drawing Asia closer to the rest of the world and because Asia-Pacific airlines have been  major B747 customers .

By the 2010s, the majority of the B747s were being retired and replaced by longer-range and more fuel-efficient twin-engine aircraft such as the B777, the A330, the B787, and the A350. On transpacific routes, the 787, for instance, has a fuel economy of about 39 passenger-kilometers per liter of jet fuel versus about 23 passenger-kilometers per liter for the Boeing 747-400ER. The triumph of widebody twinjets is most evident in the transatlantic and transpacific markets, including the introduction of the A380 in 2007 to develop a niche of a high-capacity aircraft servicing long hauls between major airports.

Air transportation is now overwhelmingly dominant in transcontinental and intercontinental travel and has become more competitive for shorter trips in many regional markets. Low-cost carriers (LCCs) have been instrumental in extending aviation’s reach to short-haul markets. The pioneering LCC, Southwest Airlines, sought to make flying cheaper than driving on the first markets it served in the early 1970s: the Texas “Golden Triangle” linking Dallas, Houston, and San Antonio. Since then, LCCs have proliferated across developed markets and, more recently, in emerging markets. In developing countries, the ascent of LCCs has been partly fueled by the poor quality of land transportation, making air travel an attractive option for national inter-city routes.

Interestingly, since their introduction in the late 1950s, commercial jets have  not improved much in terms of speed apart from a small fleet of supersonic but commercially unsuccessful Concorde jets (which flew on a handful of transatlantic routes between 1976 and 2003). Since the end of Concorde services, the fastest airliners in regular use have had cruising speeds about as fast as the B707s of the early 1960s. However, introducing long-haul aircraft has produced new rounds of time-space convergence. For instance, in 2018, twenty US cities had nonstop services to at least one destination in Asia, up from 13 US cities in 1998. Boston had nonstop links to Tokyo, Beijing, Shanghai, and Hong Kong in 2018, whereas two decades earlier, all those markets would have required a time-consuming connection at a larger hub. Meanwhile, there have been repeated attempts to launch new supersonic airliners. In 2021, United Airlines placed orders for 15 aircraft from Boom Supersonic. The new jets, each seating 65 to 80 passengers and cruising at Mach 1.7, will begin flying in 2029 if all goes according to plan.

Perhaps the most significant improvement in aviation is the reduced risks of accidents . If civil aviation had had the same accident rate per million departures as in the early 1960s, there would have been the equivalent of about three fatal accidents somewhere in the world per day in 2018. Instead, there were nine fatal accidents worldwide for the whole year .

The world’s busiest air routes are mainly short-range sections between cities less than 1,000 km apart, with many of these city pairs found in emerging markets. More generally, short-haul flights predominate despite the expansion of long-haul flights and the increased globalization of the economy. Importantly for the world as a whole, about 59% of airline seats were on domestic flights in 2018, and for larger countries, the share was even higher, such as 88% in China.

Air transportation’s share of world trade in goods is less than 1% measured by weight but more than 35% by value . Typically, air transportation is most important for time-sensitive, valuable, or perishable freight carried over long distances. Air cargo has been central in “just-in-time” production and distribution strategies with low inventory levels, such as for Apple iPhones. Air cargo is also vital in emergencies when the fast delivery of supplies prevails over cost issues. In the early weeks of the COVID-19 pandemic, air cargo carriers were crucial in rushing ventilators and other equipment worldwide. Later in the pandemic, the same carriers helped speed the distribution of vaccines and supported the increasing demand for goods due to online purchases.

2. Civil Aviation and Activity Spaces

Air transportation has transformed society at scales ranging from the local to the global. Aviation has made economic and social activities in many parts of the world faster, more interconnected, varied, and more affluent . Still, those gains have come with externalities such as congestion and environmental challenges.

a. The acceleration of the material world

As the fastest mode, air transportation has been associated with the speeding up of daily life . This effect is most apparent in the astonishing delivery times for goods ordered online from sites such as Amazon.com. In 2019, Amazon offered two-day deliveries to all of the United States for millions of goods and next-day delivery for a narrower range of goods. The speed of the company’s deliveries depended largely on the multiplicity of distribution centers Amazon operated across the country, positioning many goods close to consumers. Still, air cargo has also been vital in rushing goods from global suppliers to distribution centers and consumers. In 2016, Amazon began flying leased aircraft as Amazon Air in the United States, with nationwide flights. The surge in e-commerce during the pandemic propelled the expansion of Amazon Air to a fleet size of 96 aircraft by 2022, a small number of which now operate on routes within Europe.

Passengers move at faster speeds as well. The supersonic Concorde once advertised its service with the slogan “Arrive before you leave”, highlighting the fact that for westbound flights such as London – New York, the local time on arrival (in New York) would be earlier than at departure (in London). As noted above, the Concorde was grounded in 2003. Still, the multiplication of nonstop services means that even at conventional jet speeds (which are about 80 percent the speed of sound), the world is smaller for passengers; the number of unique city pairs served by commercial airlines grew to 22,000 in 2019, about twice the number of twenty years earlier.

The speed of human transportation has changed how people interact in ways that are both positive and negative. For instance, until the advent of low-cost air transportation, the principal means of traveling between Ho Chi Minh City and Hanoi was a 33- to 36-hour rail journey on the Reunification Express or a similarly tedious bus journey. Now, for those who can afford to fly (low-cost carriers have broadened that population), the cities are just 2 hours apart. The route has become among the most densely trafficked in the world, with 60 flights per day each way in 2018. The result has been an improvement in the lives of traders, bureaucrats, students, tourists, and others traveling between Vietnam’s two largest cities, and the same has occurred in countless other city pairs.

On the other hand, the acceleration of passenger flows around the world has also sped up the diffusion of infectious diseases . In late 2002, Severe Acute Respiratory Syndrome (SARS), for instance, began spreading slowly within southern China. Still, within days of reaching Hong Kong in February 2003, the disease was transmitted to Canada, Vietnam, the United States, and the Philippines. Direct nonstop services were an important factor behind a diffusion pattern that may, at first glance, appear random. Ultimately, cases were reported in more than two dozen countries over a matter of weeks, with airports becoming the key frontiers in trying to limit the spread of SARS. Before aviation became widespread, the sheer size of the world afforded a degree of protection from the development of pandemics. But the world is, at least measured in terms of time, much smaller than in the past.

That lesson was repeated on a much larger scale during the COVID-19 pandemic . In early 2020, the coronavirus epidemic first forced the shutdown of large segments of the Chinese air transport system, including international air services to Chinese cities. As the disease spread, travel bans cascaded across the planet, precipitating the worst crisis in the history of the airline industry . In the United States, passengers cleared at Transportation Security Administration (TSA) checkpoints reached a nadir of 87,500 on April 13, 2020 , just 4 percent of the level on the same date a year earlier. By June 2021, with vaccination increasingly widespread in the United States, the number of passengers processed daily by the TSA reached 70% of pre-pandemic levels, with domestic flights the main driver. By June 2022, this traffic level was at 95%, with the demand considered to have recovered to pre-pandemic levels after a two-year hiatus. Still, international travel lagged mainly due to entry restrictions involving vaccine certificates, testing before arrival, and quarantine requirements. By mid-2022, these restrictions were eased for major destinations in North America and Europe, allowing for the resumption of segments of long-distance international air travel.

b. An interconnected world

At any moment in 2018, an estimated 1.4 million people were airborne on commercial airline flights worldwide. Most were on short-haul flights linking nearby cities within the same country, as evidenced by the most densely trafficked sector, the 454-kilometer hop from Seoul to the resort island of Jeju, off South Korea’s southern coast. At the regional scale, frequent flights have amplified the political and economic integration of regions such as the European Union (EU) and the Association of Southeast Asian Nations (ASEAN). In Europe, the phrase “easyJet Generation” refers to young people who have grown up in a region where cheap aviation and porous borders have permitted unprecedented mobility.

At the global scale, increasingly long-haul nonstop services (up to 18 hours in duration ) are both a response and a driver for globalization . Most of the nodes for such flights are world cities , the command-and-control centers of the global economy rank among the best-connected cities in the global airline networks. Yet the links between globalization and the airline industry extend far beyond the main hubs. Manufacturers, especially those producing high-value electronics, rely heavily on air transport to tie together spatially disaggregated operations. For example, by 2019, Zhengzhou, the capital of Henan Province in China and the largest production base for Apple iPhones, was linked by numerous freighter aircraft flights daily to global markets, including a nonstop 747-freighter flight by Cargolux to Luxembourg.

In addition to the trade networks established by multinational corporations, there are also extensive social networks created by migrants involving recurring air travel. For instance, in 1998, Ethiopian Airlines launched services to Washington, DC, the carrier’s first destination in the United States and not coincidentally home to the largest community of Ethiopians outside Africa. The flow of people between Ethiopia and Washington, DC, is one strand in the larger tapestry of global connections expedited by air transportation.

c. A kaleidoscope of experience

Cheap air transport has enlarged the geographic scope of everyday life and, in so doing, has enriched the lives of many with unprecedented variety. Take first the diversity of goods. By one common measure, the United States imported more than four times the variety of goods in 2018 as in 1972. Much of the increase was attributable to the sharp reduction in transportation costs through containerized maritime shipping, but lower-cost air cargo has also played a role. Many perishables, for instance, such as Valentine’s Day roses bound from Kenya to Europe or Colombia to the United States and fresh tuna shipped from around the world to the fish markets in Japan, move exclusively by air. These markets largely did not exist a few decades ago.

Efficient and affordable air cargo has contributed to changes in diet by making available products in seasons during which they would not be available, to changes in retailing, and correspondingly to changes in manufacturing. Examples abound, such as fresh produce grown in the southern hemisphere available in the northern hemisphere during winter (a phenomenon sometimes referred to as permanent global summertime ), at least for affluent consumers.

Likewise, air transport has catalyzed the emergence of an ever-greater variety of tourist destinations . The markets with the fastest growth rely overwhelmingly on arrivals by air from major source tourist markets such as the United States, Europe, and China. The COVID-19 pandemic significantly curtailed air tourism, particularly at the international level, but by 2022, activities were returning to normalcy, and pent-up demand accelerated the recovery of air tourism.

d. The ascent of affluence

Air traffic is correlated with per capita income, but the relationship is interdependent. More affluent populations can more easily afford what is usually the most expensive mode, but aviation has also been catalytic to economic growth.

In 2019, airlines flew approximately 4.5 billion passengers. The total volume of air passengers equaled nearly 60 percent of the global population. Of course, a much smaller share are actually air travelers, as individuals who use air transportation usually do so several times per year. Therefore, the propensity to fly is highly uneven, as observed in the passenger and freight markets. Flights originating in North America and Europe accounted for 47 percent of airline seat capacity in 2018. However, that share has been declining with faster growth in other regions of the world. For instance, flights from China accounted for 14 percent of seat capacity in 2018, up from 3 percent in 1998.

Both passenger and cargo traffic  have grown rapidly as higher incomes translate into higher values for time and a stronger preference for what is the fastest mode. In fact, air passenger and air cargo traffic have outpaced the growth of the broader global economy .

At the same time, lower transportation costs, in terms of time and money, have encouraged faster income growth. The economic impact of air transportation is most strongly pronounced near air hubs, but the catalytic effect of air accessibility extends across the economy. Whole sectors are strongly dependent on aviation. Logistics, advanced business services such as consulting and advertising, and tourism are among the industries for which air accessibility is vital. It is no coincidence, for instance, that all six major Disney theme parks are located near one of the world’s busiest airports. In 2017, passenger volumes at Orlando International Airport were more than 500 times larger than they had been the year before Disneyworld opened (1971), and what was once a medium-sized Florida city had nonstop links to cities across the United States and Canada, Latin America, Europe, and the Middle East. Disney’s other parks include Disneyland near Los Angeles International Airport, Disneyland Paris near Paris-Charles de Gaulle, Tokyo Disneyland near Tokyo-Haneda, Hong Kong Disneyland, which shares Lantau island with the most  expensive airport in history , and Shanghai Disney Resort located just a few kilometers south of the city’s main airport.

e. The high costs of aviation

Yet, the huge increase in traffic in Orlando and the more modest increase globally have not been cost-free. In particular, aviation externalities have risen with traffic volumes. The air transport sector accounts for about 3.5 percent of anthropogenic climate change, but its share is expected to climb towards the mid-century. Aviation is heavily dependent on fossil fuels and is likely to remain so after other modes have transitioned to more environmentally friendly fuel sources. Some airlines have experimented with biofuels, but their impact remains marginal so far. Between 2011 and 2019, about 175,000 flights were partly powered by biofuels, but in 2019, more than 100,000 flights per day were powered solely by conventional fuels. A landmark was reached in 2021 when a test flight between Chicago and Washington, DC, ran exclusively on biofuels. In 2023, this was the case for the first transatlantic flight.

Battery-electric aircraft are another avenue to ease the sector’s global climate change impacts. Air taxis using this technology are expected to launch as soon as 2024, but the aircraft being developed are small in their capacity (about five passengers) and range (about 250 kilometers). Airships , which might be suitable for freight transportation in remote areas, still comprise another area of innovation.

Aviation also has significant impacts at the local level, including emissions of nitrogen oxides and particulate matter. As with greenhouse gasses, however, growth in emissions (at least when measured per passenger-kilometer) has been stemmed by rapid advances in aviation technology, especially improvements in engine efficiency. The average fuel burn per passenger-kilometer by air transportation fell by 45 percent between 1968 and 2014, and the introduction of a new generation of jet engines portends further gains.

The most apparent externality at the local scale is aircraft noise , and technology has brought impressive gains. For instance, engine manufacturer Pratt & Whitney claims up to a 75 percent reduction in the noise footprint (i.e. the area near a runway affected by high noise levels) for its newest large jet engine compared to similar-sized jets operated with an earlier generation of engines. Still, the huge increase in traffic volumes (at least before the COVID-19 pandemic) partly offsets this and other technical improvements in aviation.

3. The Geography of Airline Networks

Theoretically, air transport enjoys greater freedom of route choice than most other modes. Airline routes span oceans, the highest mountain chains, the most forbidding deserts, and other physical barriers to surface transport. Yet, while it is true that the mode is less restricted than land transport to specific rights of way, it is nevertheless more constrained than might be supposed.

a. Structuring factors

Weather events such as snowstorms and thunderstorms can temporarily create disruptions that cascade through hub-and-spoke networks . Volcanic eruptions may also impede air travel by releasing ash into the atmosphere, which can damage and even shut down turbofan engines. Fear of such calamities forced the closing down of the airspace in much of Europe as well as the North Atlantic for nearly a week following an April 2010 volcanic eruption in Iceland. Meanwhile, on a more regular basis, aircraft seek to exploit (or avoid) upper atmospheric winds, particularly the jet stream , to enhance speed and reduce fuel consumption.

Yet the limitations that structure air transportation are mainly human creations , especially internationally. The Chicago Convention of 1944 established the basic geopolitical guidelines of international air operations, which became known as the freedoms of the air . First (right to overfly) and second (right for a technical stop), freedom rights are almost automatically exchanged among countries. The United States, which emerged from World War II with by far the strongest airline industry in the world, had wanted third and fourth freedom rights (the right to drop off passengers and cargo and the right to pick up passengers and cargo, respectively, in another country) to be freely exchanged as well. Instead, these and other rights have been the subject of hundreds of carefully negotiated bilateral air services agreements (ASAs). In an ASA, each side can specify which airlines can serve which cities with what size equipment and at what frequencies. ASAs often include provisions regulating fares and revenue sharing among the airlines serving a particular international route.

Other constraints on the geography of air services stem from safety and national security concerns . To limit opportunities for midair collisions, air traffic is channeled along specific corridors so that only a relatively small portion of the sky is in use. Jet Route 80, for example, links Coaldale, Nevada, and Bellaire, Ohio, and accommodates many transcontinental city pairs as well as some shorter haul sectors such as Indianapolis-Denver. Meanwhile, airlines within China face widespread capacity constraints because the People’s Liberation Army controls four-fifths of the country’s airspace and prioritizes military flights over passenger use.

Strategic and political factors also influence route choice over larger scales. The Cold War imposed numerous airspace constraints, preventing the use of polar air routes . The opening of the Siberian airspace to Western airlines in the 1990s permitted more direct routes between cities like London and Tokyo or New York and Hong Kong. However, in 2022, the Russian invasion of Ukraine resulted in the closing of the Russian airspace for most Western airlines, forcing international flights to detour along North America/Asia and Europe/Asia routes. For instance, Lufthansa’s flight between Frankfurt and Beijing detoured to the south of Russia (through Romania, Turkey, Azerbaijan, Kazakhstan, and Mongolia), adding hundreds of kilometers and more than an hour of flying time. In turn, Russian airlines were excluded from the airspace of most Western countries. Meanwhile, there has been some progress towards opening up airspace elsewhere in the world. In 2020, some Arab governments opened their airspace to Israeli airlines as part of a broader peace initiative.

b. Liberalization of air travel

These instances of government intervention in airline networks run contrary to the broader trajectory of airline industry liberalization (a term that refers to deregulation and privatization). Since the 1970s, dozens of airlines around the world have been at least partially privatized, meaning that they are now owned by private investors instead of governments. Many airline markets have been deregulated, meaning there are fewer regulations on fares, routes, and other aspects of operations.

In the United States, the Air Deregulation Act of 1978 opened the industry to competition. The results were significant. Once hallowed names, like TWA, Pan Am, and Braniff, sank into bankruptcy, and many new players emerged . Most lasted only briefly, but some have had a more profound, enduring effect on the industry and air transportation. For instance, Southwest Airlines could only serve intra-Texas markets until deregulation freed the low-cost carrier to spread nationwide and beyond.

In Europe, deregulation advanced in a series of stages, culminating in 1997 with the opening of the European market to all European carriers. For instance, the Irish LCC Ryanair operates dozens of bases outside Ireland, its headquarters country, and most of its routes never touch Ireland.

Liberalization has also spread to emerging markets, with a transformative effect in places as different as Indonesia, India, and Brazil. In all these markets, state-owned flag carriers have lost market share to nimbler, privately owned airlines, often including LCCs. The enormous Chinese market has also been partially deregulated, and its leading airlines, while predominantly state-owned, have varying degrees of private ownership.

Meanwhile, in international markets, an important trend in the past few decades has been the proliferation of Open Skies agreements . These agreements remove most restrictions on the number of carriers and routes they may fly between signatory countries. By 2021, the United States alone had Open Skies agreements with more than 128 countries. Perhaps the most important Open Skies agreement links the European Union and the United States. Signed in 2007, the agreement permits any European carrier to fly to any city in the United States and vice versa. It makes it easier for investors from one side of the Atlantic to invest in airlines on the other side and facilitates collaboration among carriers integrated into airline alliances.

Liberalization has fueled the growth of aviation and made the world’s airline networks far more dynamic. Airlines have greater freedom to fly where and when they see commercial potential. For instance, under regulation by the US Civil Aeronautics Board, United Airlines was allowed to add only one city to its network between 1961 and 1978. By contrast, between 1978 and 2018, the airline’s network grew from 93 cities (almost all in the US) to 342 cities worldwide.

Liberalization has not been a one-way street, however. There have been numerous instances of governments reasserting their power in the industry, and the COVID-19 pandemic was an event likely to incite further interventions.

c. Aircraft technology and airline networks

In time, air transportation networks evolved to become increasingly complex, a trend that goes on par with the improvements in the technical capabilities of aircraft, but also their specialization to service-specific markets. Three major categories of passenger jet planes may be recognized, each servicing a specific air transport market :

• Regional market (Short range/haul aircraft) . This market usually involves short flights lasting anywhere between 30 minutes and 2 hours, which means that they can fly between 6 and 10 legs a day. Embraer’s older ERJs and new E-Jets are examples of planes with relatively small capacities (fewer than 150 passengers) that travel short distances. Regional jets (RJs) like these serve smaller markets and feed hub airports on routes such as Appleton, Wisconsin, to Chicago or Maputo, Mozambique, to Johannesburg. RJs also provide high-frequency point-to-point services between large city pairs.
• Regional and international markets (Medium range/haul aircraft) . This market involves flights between 1 and 4 hours in duration, but longer flights of 5 to 6 hours are also possible, which means 2 to 5 legs per day. The Airbus A320 and Boeing B737 are very flexible aircraft that can be efficiently deployed on short hops but also on transcontinental routes. From New York, all of North America can be serviced by the latest versions of the A320 and B737. This range can also be applied to the European continent, South America, East Asia, and Africa for corresponding market areas. These narrow-body jets are the workhorses of LCCs, including Southwest Airlines, the largest 737 operator.
• International and intercontinental markets (Long-range/haul aircraft) . This market involves flights of 7 or more hours, with 12 hours considered ultra-long-range, which means two legs or fewer per day. The North Atlantic is considered in the lower range of this category since the US East Coast and Western Europe can be connected in 6 to 8 hours. This implies a full rotation of 2 legs per day, with European-bound flights leaving the US East Coast during the night, arriving in Europe in the morning, and heading back in the afternoon to arrive on the East Coast in the evening. There is a variety of aircraft combining high payloads and long-distance ranges. Early variants, such as the B707, have evolved into planes offering high capacity, such as the B747 series, which have evolved into extra long-range abilities. Today, the emphasis in this category is on twin-engine wide-body aircraft with high fuel efficiency and range. As of 2022, the longest-range aircraft were the Boeing B787 series (14,800 km range) and the Airbus A350 series (15,600 km range for the normal version, 18,000 kilometers for the ultra-long-range version). Aircraft such as these can link almost any pair of large cities worldwide if there is enough traffic to make the service profitable.

Across all these categories, a notable trend has been ever-longer ranges. One noticeable effect of improved aircraft technology is the bypassing effect, particularly over long hauls with the possibility of direct connections without intermediary stops . The first 737s in the 1960s had a range of just over 3,000 kilometers. Some of the most recent versions can fly more than 7,000 kilometers nonstop. Longer-range aircraft of all sizes facilitate the fragmentation of intercontinental and transcontinental markets and point-to-point services that depend less on hubs. For instance, in 2019, Norwegian Airlines operated a 737 on a 5,300-kilometer route between Hamilton, Ontario, and Dublin, Ireland. This city pair otherwise would have required a transfer to a hub such as Toronto.

d. Differences by traffic type and seasonality

An important aspect of airline networks is the emergence of separate air cargo services operating on separate networks. Most air cargo is carried in the bellyhold of passenger airplanes and provides supplementary income for airline companies. However, passenger aircraft are operated on routes that make sense for passengers but may not attract much cargo or may not operate at times that make sense for cargo shippers. In response to these factors, a growing number of freighter aircraft operations have spread across the world, using airplanes that carry cargo on the main decks and in their bellyholds and operate routes attuned to the needs of shippers. More than half of all air cargo is carried in freighters, including many operated by combination carriers (e.g., Qatar Airways) that carry passengers and cargo and operate mixed fleets of passenger and freighter aircraft.

More specifically, the air freight market is serviced by five types of operations:

• Passenger airlines (e.g., United Airlines) offer the freight capacity in the bellyhold of their all-passenger aircraft fleet. For these operators, freight services are rather secondary and represent a source of additional income, such as carrying mail . It remains an important market as about 50% of all the air cargo is carried in the bellyhold of regular passenger aircraft. However, low-cost airlines usually do not offer air cargo services since their priority is a fast rotation of their planes and servicing lower-cost airports that do not generate cargo volumes.
• Combination airlines (e.g., Korean Air) have fleets with freighters and passenger aircraft able to carry freight in their bellyhold. Most of the freighter operations involve long-haul services.
• Dedicated cargo operators (e.g., Cargolux) maintain a fleet of cargo-only aircraft and offer regularly scheduled services between the airports they service. They also offer charter operations to cater to specific needs.
• Air freight integrators (e.g., FedEx Express) operate air and ground freight services, providing nearly seamless (at least from the customer’s perspective) door-to-door deliveries.
• Specialized operators (e.g., Volga-Dnepr Airlines) fulfill niche services that cater to specific cargo requirements (e.g., heavy loads) that do not fit the capabilities of standard cargo aircraft.

Generally, the most important air cargo hubs, such as Memphis and Hong Kong, are also the hubs of key carriers. One important exception is Anchorage International Airport. Because freighters have shorter ranges than passenger aircraft and because freight is less sensitive to intermediate refueling stops than passengers, many freighters on transpacific routes refuel in Alaska to maximize their payload and clear US customs.

It is not uncommon for older aircraft, particularly wide-body aircraft, to be converted for cargo operations when they complete their commercial life on the passenger market. For instance, in mid-2022, the fleet of Amazon Air comprised converted Boeing 767 and Boeing 737 freighters. Former passenger jets like these have lower acquisition costs, a vast pool of experienced pilots, and the ready availability of parts for maintenance. On the other hand, new-build jets, such as the popular Boeing 777 freighter used by FedEx on many intercontinental routes, have greater reliability, fuel efficiency, and range.

A final feature of airline networks is their seasonality . Air cargo flows tend to peak near the Christmas season. However, some specific products (e.g., Valentine’s Day flowers in February or the shipment of thousands of tons of Beaujolais Nouveau wine from France to Asia each November) have different temporal patterns. For passenger air transport, July and August are the most traveled months overall, corresponding to the peak tourist season in Europe and North America. Elsewhere in the world, other seasonal patterns may be more important. For instance, in China, the busiest air travel days of the year tend to be close to the Spring Festival (or Lunar New Year) in January or February. The Muslim hajj generates millions of trips to Mecca, Saudi Arabia, over a five-day period each year, with the vast majority of pilgrims flying into either King Abdulaziz International Airport in Jeddah or Prince Mohammed bin Abdulaziz International Airport in Medina.

4. Airlines, Hubs, and Alliances

There are several thousand airlines in the world, most of them very small. Only about 1,400 are members of the International Air Transport Association (IATA), and even among IATA members, a relative handful of airlines account for most of the traffic. In 2018, the top 25 airlines accounted for just over 50 percent of available seat-kilometers (ASKs), a measure of capacity.

Most airlines have strongly centralized networks, and the hubs of the largest airlines are among the busiest airports in the world. Hub-and-spoke systems rely on the usage of an intermediate airport hub. They can either connect a domestic (or regional) air system if the market is large enough (e.g. United States, China, European Union) or international systems through longitudinal (e.g. Reykjavik) or latitudinal (Panama City) or both longitudinal and latitudinal (Dubai) intermediacy. An important aspect of an intermediate hub concerns maintaining schedule integrity. Airports that are prone to delays due to congestion are not effective hubs.

The traffic feed through hubs like Dubai enables the hubbing carrier (Emirates in this instance) to offer higher frequency service with larger aircraft at higher load factors , lowering the per passenger-kilometer cost. Traffic feed further permits a carrier to add services to more thinly traveled markets (e.g., in 2019, Emirates extended new nonstop services between Dubai and Porto, Portugal’s second-largest city).

Beginning in the 1970s, deregulation freed airlines to expand, consolidate, and reconfigure their hub-and-spoke systems to optimize their performance. Computer reservation systems and frequent flyer programs amplified the hubbing advantages of large carriers. These systems and programs leveraged the economies of scale provided by large hub-and-spoke carriers to draw still more traffic onto their networks.

The ability of airlines to spread their networks internationally has been limited both by the persistence of regulations and by the preferences that travelers have for their home country airlines. Carriers have overcome these limitations, at least partially, through the formation of alliances . Alliances are voluntary agreements to enhance the competitive positions of the partners. Members benefit from greater scale economies, lowering transaction costs, and sharing risks while remaining commercially independent. Today, the largest alliance is the Star Alliance, which was launched in 1997 by Air Canada, Lufthansa, SAS, Thai Airways International, and United Airlines. By 2022, 21 others had joined those five carriers, and the alliance’s combined network reached 193 countries with a combined fleet of more than 5,000 aircraft. The two other major alliances are SkyTeam (18 airlines led by Delta and Air France) and Oneworld (15 airlines led by British Airways and American Airlines).

Most large airlines belong to an alliance, a testament to the significant advantages of membership:

• Codesharing . Members of an alliance can sell seats on one another’s flights so that, from the passenger’s perspective, a single airline appears to offer a seamless service even though multiple members’ flights might be involved in getting from A to B. Codesharing effectively enlarges an airline’s network and increases the chance of capturing customers.
• Optimization of connections . Alliance members coordinate schedules at key hubs (e.g., Frankfurt for the Star Alliance) to facilitate connections from one member’s network to another. Adjacent gates in shared terminals accelerate connections. For example, all the Star Alliance airlines serving Beijing are co-located in Beijing Capital International Airport’s Terminal 3.
• Geographical specialization . An airline in an alliance can tap global markets while specializing in its home market. Before the COVID-19 pandemic, for instance, Star Alliance member Air Canada served only seven hubs in East and Southeast Asia. Still, via its alliance partners, it gained access to dozens of other cities in the region. In turn, Asian members of the Star Alliance, such as Singapore Airlines (SIA), accessed Air Canada’s vast network in its home country.
• Joint marketing . Alliance members reciprocate in frequent flyer programs and other marketing efforts. Travelers can earn and redeem miles across the members of an alliance.

The leading airlines in the alliances are full-service network carriers (FSNCs), also known as legacy airlines. FSNC refers to the fact that these airlines offer a wide array of services (especially for passengers in first or business class), and their key selling point is the reach of their networks (networks that have been stretched by the alliances). The phrase “legacy carrier” highlights the deep roots of these airlines, some of which, like KLM, Qantas, and Delta, rank among the oldest continuously operating carriers in the world.

Yet by the late 1990s, FSNCs as a group were losing the market share to LCCs. In 1998, there were approximately 60 budget airlines globally, and almost all of them were located in the US, Canada, and Western Europe. Together, they accounted for about 7 percent of all departure seat capacity per week worldwide. By 2018, there were approximately 140 LCCs; more than half were based in emerging markets, and accounting for about 31 percent of all seat capacity. Interestingly, budget airlines are most significant in middle-income emerging markets. In 2018, the countries where LCCs accounted for the largest share of capacity included Slovakia, Malaysia, Romania, India, and Mexico. In these countries (and their neighbors), the population that can afford air travel is growing, and competition from ground transport modes and from full-service network carriers (e.g., Air India) is weak. Conversely, budget carriers are weakest or altogether absent from poorer, authoritarian states with heavily protected state-owned flag carriers (e.g., Uzbekistan).

LCCs are distinguished by several  common features :

• Fleet simplicity . Legacy carriers operate diverse fleets because they serve a diversity of routes, from long-hauls to feeders. LCCs emphasize short-haul routes. The minimal number of aircraft types (Southwest and Ryanair only fly B737s, though several different models) lowers operating costs.
• High seating density . Budget airlines pack more seats in a typically all-economy class configuration. For instance, the budget airline EasyJet fits 180 seats in its Airbus A320 aircraft versus 144 seats on the same plane used on intra-European routes for British Airways.
• Fast turnaround times . LCCs operate their networks in ways that keep their aircraft in the air earning money for a higher number of hours. Minimal inflight service, for instance, reduces the time needed to clean and cater flights.
• Rapid growth . This is not just a product of the LCCs’ success but an element of it. Fast growth enables the LCCs to continue adding aircraft and staff at a steady pace, which keeps the average fleet age and average years of employee service low, both of which help keep operations costs low.
• Emphasis on secondary airports . Secondary airports, such as Houston-Hobby instead of George Bush Houston Intercontinental or Charleroi instead of Brussels National, typically have lower landing and parking fees for airlines as well as a more entrepreneurial approach to recruiting new airline services. However, LCCs have also directly challenged established carriers in major airports.
• Reduced importance of hubs . Most LCCs do have hubs, but for some carriers, hubs are substantially less important than they are for legacy carriers. Southwest Airlines, for instance, distributes air traffic more evenly among the top “focus cities” in its network than is true of any traditional hub-and-spoke airline. Whereas nearly half of all seat capacity on Delta Air Lines is on flights leaving just five hub cities, to reach the same share of capacity on Southwest Airlines requires combining eleven focus cities. Spreading traffic reduces vulnerability to congestion and frees aircraft to keep moving rather than waiting for arriving traffic at a hub.
• Aggressive digitalization . Internet booking has partially neutralized the one-time advantage that legacy carriers enjoyed through their proprietary computer reservation systems. LCCs have been industry leaders in using automated kiosks and smartphones to accelerate the check-in process. Digitalization has also facilitated segmented services and monetized once-included amenities such as seat selection, priority boarding, meals, and luggage allowance.
• Avoidance of global alliances . LCCs have stayed out of the big alliances discussed above because they come with obligations that can increase a member’s costs.

These and other advantages explain the gap between fares offered by LCCs and full-service network carriers. In advanced markets, decades of competition between these two types of airlines have whittled away the differences. In 2016, US network carriers had costs per available seat-mile about 40 percent higher than American LCCs. In developing countries, conversely, the budget airline phenomenon is newer, and the gap between LCCs and legacy carriers is generally wider. For instance, Singapore Airlines had unit costs twice as high as Malaysia-based LCC AirAsia in 2016. Still, the world’s largest airlines are almost all network carriers. Southwest Airlines, the pioneer LCC, is the only LCC to rank among the world’s  20 largest airlines .

LCCs are important in broadening the air transportation market beyond the relatively small affluent population in countries such as India and Brazil. Budget airlines’ slogans frequently highlight this democratizing effect, as in AirAsia’s motto “Now everyone can fly”, Wizz Air’s (Hungary) “Now we can all fly”, and Jambojet’s (Kenya) “Now you can fly”. These are exaggerations, but there is little doubt that LCCs have expanded the affordability of air travel.

Meanwhile, in advanced markets, the notion of a low-cost carrier is losing some of its meaning as budget airlines and full-service network carriers converge in some of their business practices and cost structures. The degree to which FSNCs have emulated low-cost carriers is a testament to the latter’s success, as is the fact that in numerous markets, the largest airline is now a budget carrier.

5. The Future of Flight

The COVID-19 pandemic has been the most severe crisis in civil aviation since World War II. The International Air Transport Association (IATA) has estimated worldwide airline industry losses at $84 billion for 2020. By April 2020, air traffic in most markets plummeted by more than 90% versus the same time in the previous year. By mid-2022, however, traffic levels were back to near pre-pandemic levels in North America and Western Europe. In fact, traffic recovered faster than expected in these markets, causing significant schedule problems. Airlines had sharply downsized their fleets and staff levels early in the pandemic, leaving them ill-prepared for the resumption of high traffic levels in 2022.

Longer-term challenges may emerge from the pandemic. The shift towards forms of teleworking, tele-education, and teleconferences may engender enduring changes in business travel behavior . Leisure travel behavior may also change. For instance, in 2021, tourists preferred shorter, domestic, or regional nonstop flights due to the increased exposure that comes with long-distance travel via hubs, including the burden of regulations, testing, and quarantine procedures associated with international travel. However, such preferences were less noticeable as testing and quarantine procedures were removed for most international travel in 2022.

The pandemic may also accelerate the shift away from full-service airlines toward LCCs. A large number of network carriers’ A380s and B747s parked in desert “boneyards” will never again carry passengers. For instance, Air France retired its A380 fleet in 2022. Between 2020 and 2022, the COVID-19 crisis shifted the balance of the industry toward cargo. Freight rates jumped during the pandemic, and cargo’s share of industry revenue soared from 12 percent in 2019 to 26 percent in 2020. Some airlines even converted a part of their passenger planes into cargo planes to take advantage of historically high freight rates that resulted.

Beyond the COVID-19 crisis, numerous clouds are on the horizon for civil aviation. First, the airline industry must be financially strong enough to continue to afford new generations of aircraft upon which further gains in efficiency and improved environmental performance depend. The development costs of new jetliners, even after adjusting for inflation, are unprecedented, partly because the latest generation of aircraft incorporates so many complex interfacing systems. The financial health of the industry’s largest airlines is particularly important because great carriers have previously provided the launch orders for new airliners. Pan Am, for instance, launched the B707 and B747; United launched the B767 and B777; Air France and Lufthansa provided the launch orders for most of Airbus’ early airliners; and Asian carriers such as Singapore Airlines and All Nippon Airways have been significant launch customers since 2000. By contrast, the LCCs’ focus on a handful of smaller, relatively short-haul aircraft limits their capacity to serve as catalysts for technological breakthroughs in aviation.

Still, both Boeing and Airbus promise that their newest jetliners will offer unparalleled fuel efficiency . That is important because a second fundamental threat to the future of the airline industry is the price and availability of fuel. In 2018, fuel accounted for about 24 percent of the operating costs of airlines globally. As noted above, aviation is less amenable to substituting conventional fossil fuels than ground transport modes, though numerous innovations show promise. The spike in fuel prices after the Russian invasion of Ukraine added impetus to decarbonizing the air transport sector.

A third threat is terrorism and security . The rise of the airline industry was partly facilitated by the steady advance in the safety and predictability of air travel from the early 20th century “Flying Coffins”. Terrorism directed against civil aviation threatens the confidence of ordinary travelers, and added security constraints sap some of the speed advantages of aviation. The September 11 attacks caused a two-year dip in traffic levels. The 2001 attacks were the most significant to affect the airline industry in the United States. Still, before and after those attacks, civil aviation was a frequent target of terrorist attacks in the Middle East, Europe, and other parts of the world.

With the growth of air traffic, airports were facing capacity pressures and congestion before the COVID-19 pandemic, which in some cases, resulted in changes in the  scheduling of flights . In the United States, a flight that arrives more than 15 minutes past its scheduled time is considered late. Airlines are posting longer flight times to maintain the appearance of schedule integrity. For instance, a flight from New York to Los Angeles scheduled to take 5 hours in the 1960s is now scheduled to take more than 6 hours. A 45 minutes flight from New York to Washington saw its scheduled duration extended to one hour and 15 minutes.

Before the pandemic, emerging economies such as India, Indonesia, and Brazil saw a surge in air travel demand, both for domestic and international markets, a trend that strained their air transport systems. An essential means of dealing with this challenge has been the modernization of air traffic control systems , some of which remain highly fragmented. For instance, using satellite-based navigation, air travel can be improved with better flight paths and more direct descents. The outcomes include shorter flight times, improved safety, and lower fuel consumption and environmental emissions. Such innovations are likely to be very important again as traffic levels recover.

Environmental concerns are perhaps the darkest cloud on the horizon for civil aviation. Aviation has accounted for a growing share of environmental externalities , and strategies to curtail emissions and noise could mean higher aviation taxes, higher airfares, and restrictions on aircraft operations (e.g., nighttime curfews). Those most alarmed by aviation’s environmental impacts will likely resist the return to pre-pandemic practices, and governments may have the leverage to do so. The severe financial distress of the airline industry sparked by the COVID-19 pandemic has drawn governments back into the industry. In 2020, airlines received hundreds of billions of dollars in state aid, often with strings attached, giving governments new leverage over carriers. For instance, the French government has pressured Air France to become “greener”, including reducing competition with rail on short-haul sectors, as part of its bailout of the airline.

Ultimately, the speed with which air links have been reopened even during the pandemic speaks to the degree to which “aeromobility” is intertwined into the fabric of everyday life across much of the world. The COVID-19 pandemic, the Russia-Ukraine war, and responses to longer-term concerns about air transportation’s role in climate change will change the trajectory and geography of aviation. Perhaps these crises will hasten the introduction of new, more environmentally friendly technologies such as electric aircraft. However, traffic volumes will almost certainly regain and surpass the heights attained before the pandemic. Air transportation will remain a vital force shaping the contours and tempo of society at scales ranging from the local to the global.

Related Topics

• 6.5 – Airport Terminals
• B.6 – Mega Airport Projects
• 5.1 – Transportation Modes: An Overview
• B.7 – International Tourism and Transport
• B.19 – Transportation and Pandemics

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Yale Climate Connections

Evolving climate math of flying vs. driving

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The Environmental Protection Agency’s June 2015 first step toward regulating greenhouse gas emissions from airplanes comes amidst a drumbeat of analysis and criticism about air travel harm to the atmosphere. Flying, critics maintain, trashes the planet and in many instances represents a wasteful luxury service for the globally affluent, who may engage in “ binge flying .”

Data generally support the concern that planes pollute at inordinately high levels, making a long, energy-intensive flight for your third annual tropical vacation quite questionable, strictly speaking, from the standpoint of climate change mitigation.

But for the sake of discussion, let’s say you live in Detroit and need to visit an ailing family member in Raleigh. Or you live in Phoenix and the viability of your small business hinges on attending an annual industry conference in Dallas.

Do you fly or drive? There are myriad useful Web-based carbon calculators that can help you make most run-of-the-mill consumer decisions: CoolCalifornia ; Nature Conservancy ; ICAO , to name a few. But flying versus driving posess a very tricky example of what transportation planners call “ mode choice ,” a dynamic researchers are forever modeling to figure out how best to incentivize efficient and environmentally sound travel.

Sure, convenience – How close is the nearest airport? – may play a big role in such a decision. But on the environmental question of trips like these, most people are operating with limited information. What’s a traveler with a long, non-optional trip to do, then, short of taking a few weeks to bike?

Grounding the Numbers

The U.S. aviation industry produces 11 percent of total transportation-related emissions domestically, and about 2-3 percent of global carbon dioxide emissions annually are produced by planes. Most projections, including those of the Federal Aviation Administration (FAA), suggest demand for air travel will continue to increase substantially in decades ahead, with 1 billion passengers on U.S. carriers by 2029. A rising middle class around the world would lead to massive growth for global air travel.

This means that, although cars and coal-fired power plants are a bigger problem, curbing emissions from aviation is a non-trivial piece of the puzzle in reducing the risks of climate change.

For those concerned about managing their carbon footprints, facts and figures in three salient areas are worth knowing, though none on its own tells the whole story.

First, in terms of pounds of carbon dioxide produced per gallon of fuel, there is not a huge difference between a gallon of gas for a car and a gallon of either jet fuel or aviation gas: Jet fuel produces an average of 21.1 pounds of CO2 per gallon and aviation gas 18.4, while fuel for cars is 19.6, according to the U.S. Energy Information Administration . This nominal parity in fuel-related greenhouse gas pollution, however, obscures the tremendous amount of fuel that planes use on the runway. Longer flights are more efficient overall, as cruising requires less fuel.

Second, beyond carbon dioxide, emissions by airplanes have some particularly problematic aspects. The Intergovernmental Panel on Climate Change points out that aviation emissions include water vapor, which creates clouds, and releases of ample black carbon, nitrous oxide and sulphur oxide. These in turn contribute even more to a greenhouse effect and the trapping of heat.

Thus, the total plane-related “ radiative forcing ” – a measure of the varying influences on climate change – goes way beyond just carbon dioxide spewed from engines. Aviation emissions have strong and immediate effects. Indeed, a 2010 study from researchers at the University of Oslo’s International Institute for Applied Systems Analysis found that “short-lived climate factors” make a decisive difference in terms of mode comparisons:

Air travel results in a lower temperature change per passenger-kilometer than car travel on the long run; the integrated radiative forcing of air travel is on short- to medium time horizons much higher than for car travel. Per passenger-hour traveled however, aviation’s climate impact is a factor 6 to 47 higher than the impact from car travel.

The primary reason for this dramatic climate impact is that the contrails and clouds produced by a plane’s waste water vapor are thought to have a pronounced short-term effect on climate, but those effects are short-lived. It is worth pointing out, however, that the precise effects of contrails and clouds produced this way are not fully understood yet by scientists, and impacts may depend on aircraft altitudes.

Finally, the energy intensity of flying, while still by no means green-friendly, has fallen by about a quarter over the past decade and has outpaced the declines for driving. According to recently published figures from the FAA , in 2012 the energy intensity gap was 3,193 BTU /passenger mile for driving, compared to 2,654 BTU/passenger mile for flying. Energy intensity for airplanes is “now significantly lower than automobiles.” (As noted below, these figures are the subject of some dispute).

Still, critics contend that aviation fuel efficiency gains have begun to tail off or even flatline over the past few years despite efforts such as United Airlines’s experimenting with biofuels and with energy-saving technologies such as FAA’s NextGen  advanced GPS-based transportation system. Those initiatives are expected to help with flight efficiency and reduce emissions.

Seat-spinning research

One of the few researchers trying to make a straight, consistent comparison across the U.S transportation sector is  Michael Sivak of the University of Michigan Transportation Research Institute. In working papers released over the past two years, Sivak has attempted to overturn the conventional wisdom: His main recent finding is that the average energy intensity of driving is about twice that of flying, a conclusion based on the current average on-road fuel economy of cars, pick-up trucks, SUVs, and vans (21.6 mpg).

How can that be?

First, Sivak asserts that the way some government energy intensity figures have been produced involves some inconsistencies, namely that different carrier groups for fuel consumed and passenger miles flown are used, and that estimates include cargo operations. Correcting for these, he arrives at 2,033 BTU per passenger mile for airline travel in 2012. (He uses the comparative figure 4,211 BTU per passenger mile for cars, a number derived from the federal Research and Innovative Technology Administration, part of the U.S. Department of Transportation.)

This new transportation reality also comes down to the fact, Sivak says, that cars increasingly have only the driver in them — no or few passengers. The result is that associated energy intensity and greenhouse gas emissions are very high, making air travel look like a comparatively sound alternative.

The University of Oslo team has published similar findings : “With only passenger in the car, corresponding to 20-25% [potential] occupancy, the climate impact is at the level of an average air trip.”

Sivak notes that the current average loads for each transportation mode are 1.38 persons in vehicles and 84% of seats occupied in planes. The big emissions volume produced by air travel has, by contrast, been spread out over more and more people, as airplane occupancy rates have gone up over the past four decades.

For the traveler with no choice but to go, Sivak has two pieces of advice. The first relates to the fuel economy of the vehicle: “To the extent that most families have more than one vehicle, [if you drive] take the most fuel-efficient vehicle for your trip: Drive the family car and leave the pickup truck, SUV, or van at home.” That can substantially change the calculus in flying versus driving.

In addition, Sivak suggests carpooling, as having three or more person in a vehicle “completely” changes the comparison with flying. Indeed, with a few people in the car, “driving becomes less energy intensive than flying (even after taking into account the increased weight that the vehicle needs to carry).”

Of course, finding a fellow passenger or two for a last-minute, long-distance road trip is no mean trick. Sivak puts it this way: “In other words, for a long business trip, driving solo is worse than flying, while for a long family vacation, driving is better than flying.”

Since publication of Sivak’s latest findings in April, Dan Rutherford and Irene Kwan of the International Council on Clean Transportation have critiqued the analysis a bit and warned that averaging statistics across entire transportation modes and sectors may not allow for apples-to-apples comparisons between realistic scenarios in which flying and driving both may be viable options (the Detroit-to-Raleigh case, or Phoenix to Dallas). Others in the environmental community also took issue with the analysis.

Still, Rutherford and Kwan concede that Sivak’s relative position has some merit.

“If you are driving alone in a vehicle that gets 40 mpg or less – somewhat close to Professor Sivak’s average car trip in the U.S. including commuting, shopping, etc. – flying may be more efficient,” Rutherford and Kwan note. “If you have one or more additional passengers, driving is typically more efficient unless you are in a large vehicle.” And if you are driving a hybrid with a bunch of passengers, they contend, you are “four to five times as efficient as a plane over a similar distance.” In that scenario, they say, you even beat taking a Greyhound.

Wider Factors and Enduring Truths

In 2009, Mikhail V Chester and Arpad Horvath of the University of California, Berkeley, published an influential analysis arguing that any assessment of passenger transportation impact needs to also include infrastructure and life cycle emissions – from maintenance of roads and airports to the manufacture of planes, trains and automobiles, along with other machines and physical structures that support particular transport modes.

Maintaining the nation’s roads takes massive effort and expense, with substantial associated emissions, whereas the carbon footprint of airports is comparatively small.

In any case, it points to the slippery and complex nature of these sorts of comparisons. Adding the options of trains and coach buses, too, makes any mode choice between air and ground all the more complicated.

But at the end of the day, all of the statistics and numbers point to an enduring reality in the twenty-first century: Getting in your hybrid with friends for a classic road trip beats a free mini-bag of peanuts or pretzels and a cramped seat squeezed between two strangers anytime.

John Wihbey

John Wihbey, a writer, educator, and researcher, is an assistant professor of journalism at Northeastern University and a correspondent for Boston Globe Ideas. Previously, he was an assistant director... More by John Wihbey

Short-haul flying redefined: The promise of regional air mobility

Why drive to your neighboring city or region when you can fly? Over the past 30 years or so, the standard response has been because driving is cheaper, more convenient, and probably faster door to door. That may not be true for much longer, however. Advances in aerospace technology, new attitudes about travel, and a growing ecosystem of established players and startups could drive a resurgence in regional air mobility.

About the authors

This article is a collaborative effort by Lukas Brink, Ryan Brown, Sarina Carter, Axel Esqué , Benjamin Meigs, and Robin Riedel  representing views from McKinsey’s Aerospace and Defense practice.

In 2019, air travel accounted for just 4 percent of all journeys between 150 and 800 kilometers in the European Union and 8 percent in the United States, with most of these flights involving large commercial aircraft (Exhibit 1). 1 Eurostat transportation statistics, March 20, 2023; Flightradar24 database, March 20, 2023; National Household Transportation survey, March 20, 2023. The business of operating smaller regional aircraft is challenging; this market has substantially declined in recent years despite strong demand for air travel overall. Since 2004, flights on aircraft with six to 50 seats declined from 16 percent of available seat kilometers (ASK) on regional routes to just 4 percent in 2019. 2 Cirium Diio Mi database, March 20, 2023; commercial aircraft, 150 to 800 kilometer trips. This decline has led to small airports losing service, hurting access to rural communities. 3 Liz Crampton, “Rural America dips into its wallet as airlines drop service,” Politico , April 26, 2023.

These numbers may soon change, however, because of four converging megatrends that might spur demand for regional flights and make the economics more attractive—significant technology advances, a greater focus on sustainability, growing frustration with road and airport congestion, and the emergence of mobility-as-a-service. These trends could fuel a new aviation paradigm, termed regional air mobility (RAM), which could lead to a resurgence in short-range flight.

RAM brings together new aviation technologies and existing small airport infrastructure into a transportation model that is more equitable, more economical, and more environmentally friendly for air travel over short distances, compared to today’s status quo. If these changes materialize, the total addressable market (TAM) for small regional flights globally could be $75 billion to $115 billion by 2035, representing 300 to 700 million passengers annually.

The emergence of the RAM market is not a foregone conclusion. It will require several critical enablers: a seamless customer experience, more mature aircraft technology, public acceptance, and new energy infrastructure at small airports, as well as a substantial increase in the regional fleet size.

The rise of a new regional transportation model

We define RAM as the transportation of passengers and goods by air over about 150 to 800 kilometers on five- to 50-passenger aircraft (or the equivalent size for cargo), primarily using smaller regional airports.

RAM is enabled by a broad range of modern technologies, such as green propulsion, digitization, and autonomy, which will reduce costs, boost reliability, and improve customer experience. This article focuses on the passenger market, although regional cargo flights also offer interesting opportunities. RAM is adjacent to the more widely discussed urban air mobility (UAM), but is different in a few important ways. UAM is focused on shorter, intra-urban distances below 150 km and involves primarily electric vertical takeoff and landing (eVTOL) aircraft. RAM will primarily use runways, generally at smaller regional airports. It typically will not require new landing sites in or near dense urban cores, while UAM will. Similar to UAM, many RAM aircraft will require electric charging infrastructure. However, RAM will likely also include hydrogen-fueled aircraft, as well as hybrid aircraft, some of which can leverage existing ground infrastructure. Taken together, these factors could enable RAM to gain traction sooner than UAM.

The RAM market is already taking shape. More than 50 companies are developing battery-electric, hybrid, and hydrogen powertrains; new and retrofitted aircraft designs; advanced avionics; operations and booking platforms; and other important enablers of the RAM ecosystem. More than $1 billion has been invested in these RAM start-ups to date and the first retrofitted aircraft are slated to enter service in the mid-2020s. Simultaneously, an ecosystem of operators, consisting mainly of established airlines and regionally focused start-ups, is coming together to drive the industry forward.

A departure from recent trends in regional travel

The potential growth of RAM represents a departure from historical declines in the market for regional flights on small aircraft. Although air travel for trips between 150 and 800 kilometers has increased in recent years, with ASKs rising by 55 percent from 2004 to 2019, much of this growth was driven by low-cost carriers using larger aircraft. For aircraft with six to 50 seats, ASKs declined by almost 60 percent in the same timeframe. 4 Cirium Diio Mi database, March 20, 2023. In 2019, passenger revenues for air trips between 150 and 800 kilometers were almost $50 billion, but only 11 percent (or $5 billion) came from flights on aircraft with six to 50 seats, including non-scheduled flights and business jets. 5 FlightRadar24 database, March 20, 2023; MarketIS database, March 20, 2023. Airlines have gradually shifted toward bigger aircraft and have consolidated operations at larger airports. A shortage of pilots has also contributed to the phaseout of small regional aircraft from airline operations.

In tandem with the decline in regional traffic, production of small aircraft has slowed, with the exception of private aircraft. Deliveries of passenger aircraft with six to 50 seats (for commercial, non-scheduled, and business or private use) peaked at about 1,500 in 2008. While the global fleet of six- to 50-seat passenger aircraft grew from 21,500 aircraft in 1990 to 41,400 in 2022, the growth was exclusively driven by business or private aircraft. Excluding business or private aircraft, the fleet has declined from 6,100 aircraft in 2008 to 4,100 today. New deliveries averaged 1,000 aircraft per year from 2016 through 2022, of which 98 percent were business or private aircraft. 6 Cirium fleets analyzer database, March 20, 2023; includes commercial regional jets and turboprops, non-scheduled aircraft, and business or private aircraft. The decline in production, combined with the increasing age of the small regional fleet, may compel regional operators to invest in green propulsion aircraft rather than extending the life of existing aircraft.

Policy and local community factors have also negatively affected legacy regional air travel. In Europe, the number of Public Service Obligation routes declined from 290 in 2013 to 176 today. 7 “Public services obligation,” Transport Themes, European Commission, September 2019; “Definition of public service obligations potential in new EU member states,” Transport Problems, European Commission, March 2017. Similarly, US government Essential Air Service subsidies cover 175 airports today, down from 400 in 1980. 8 Bella Richards, “What is the USA’s essential air service program?” Simple Flying, September 11, 2022; Logan R. Leyer, Evolution of essential air service program 1978 –2012 , Southern Illinois University, Carbondale, August 2013. In addition, concerns about noise and emissions have stymied some attempts to increase flights at regional airports. 9 Kevin Antliff et al., Regional air mobility , NASA, April 2021. Quieter, more environmentally and community friendly RAM aircraft could begin to reverse these headwinds.

Although the regional market has declined, key infrastructure remains. There are thousands of regional airports worldwide, most of them underutilized. In Europe, 50 percent of people live within a 30-minute drive of a regional airport, compared with 40 percent for a commercial airport. In the United States, 90 percent of people live within a 30-minute drive of a regional airport, compared with 60 percent for a commercial airport. 10 Future mobility , “ Right in your backyard: Regional airports are an accessible and underused resource for future air mobility ,” blog entry by Leonardo Blanchik, Benedikt Kloss, and Robin Riedel, McKinsey, May 26, 2022. This existing capacity of underutilized airports could support a growing RAM market with less investment and ramp-up time than other new modes of transport.

Four megatrends and rising investment

While the regional air travel market has been in decline over the last 30 years, it could be taking off again, thanks to four global megatrends:

• Technological advances. Innovations in propulsion and aircraft design and manufacturing, combined with next-generation guidance, navigation, and control, could drive down operating costs and make small aircraft more competitive. 11 Regional air mobility , April 2021. In the longer term, technological advances may even enable autonomous aircraft, which would further lower costs, as labor typically accounts for 20 to 30 percent of small aircraft operating costs. Some OEMs are developing novel propulsion powertrains that can be retrofitted into existing aircraft, a more straightforward path to market. Beyond aircraft improvements, innovations such as modern flight planning and navigation systems, cloud services, digital tools for fleet and network planning, and predictive maintenance are also enabling smoother and lower-cost operations. For example, the increasing use of precision GPS approaches, rather than traditional instrument landing systems (ILS), will allow smaller airports to receive reliable air service without the need for costly navigation infrastructure. 12 Localizer performance with vertical guidance; Fred Simonds, “ILS on the block,” IFR , January 29, 2020.
• The importance of sustainability. Governments and the public are increasingly focused on sustainability; their concerns will help shape the future of the aviation industry, which is responsible for approximately 4 percent of anthropogenic global warming. 13 Special report on aviation and global atmosphere , IPCC; Liz Kimbrough, “How much does air travel warm the planet? New study gives figure,” Mongabay , April 6, 2022. New initiatives and policy frameworks seek to lower the climate impact of aviation , including regulatory mandates for emissions reductions and sustainable fuel blending. 14 “Fact sheet: EU and US policy approaches to advance SAF production,” IATA, December 1, 2022. These trends demonstrate an opportunity for growth in sustainable regional air travel. Almost half of travelers recently surveyed believe there are not enough sustainable options and 87 percent want to travel in a more sustainable way. 15 Sustainable travel report 2021, Booking.com, June 2021; Sustainable travel study , Expedia Group, April 2022. One-third of travelers rank emissions as their number one aviation concern, ahead of others such as noise pollution and excess tourism, and over 25 percent are willing to pay at least 5 percent more for carbon-neutral tickets. 16 McKinsey research based on passenger survey, July 2021.
• Road and airport congestion. The priority for most travelers is to get from point A to B quickly and easily. But on the road, the typical driver in the United States loses 51 hours annually due to congestion; in the United Kingdom, drivers lose 80 hours annually. In 39 percent of US metro areas and 42 percent of European metro areas, traffic was worse in 2022 than it was before the COVID-19 pandemic. 17 “2022 Global traffic scorecard,” INRIX, 2022. Major airports are once again congested, delays are common, and system meltdowns regularly make headlines. 18 Jon Brodkin, “FAA outage that grounded flights blamed on old tech and damaged database file,” Ars Technica, January 12, 2023. More than 190,000 flights were cancelled in the United States alone in 2022. 19 US Bureau of Transportation statistics, 2022. Compounding the problem, large airports have little room to expand. More than 200 major airports worldwide—handling 43 percent of the world’s passengers—are capacity constrained and routinely limit landing slots. 20 “Fact sheet: Worldwide airport slots,” November 2022. These factors, exacerbated by the consolidation of flights at major hubs, make for a frustrating and delay-prone travel experience. These trends underscore the unmet demand for a fast transportation mode that avoids congested roads and major airports.
• The rise of mobility as a service. The past ten years have seen a rise in public appetite for mobility as a service, enabling people to search and book multiple types of transport in one place, reducing the need for individuals to own vehicles and allowing the sharing and thus better utilization of transportation assets. Consumer spending on shared ground mobility is forecast to grow to between $500 billion and $1 trillion by 2030. Over $100 billion has been invested in shared ground mobility companies since 2010. 21 “ Shared mobility: Sustainable cities, shared destinies ,” McKinsey, January 5, 2023. Consumers have shown they are willing to try alternative forms of mobility if they offer good value, convenience, and accessibility through a user-friendly app. The growth of this ecosystem will allow RAM operators to streamline the first and last mile of transportation, reducing consumer pain points related to wait times and overall transit times. In addition, declining personal car ownership will likely increase interest in RAM.

Spurred on by these four megatrends, investment in RAM is rising. Since 2015, over $1.1 billion in investment has been disclosed for this space, out of the over $16 billion invested in future air mobility as a whole. This does not include R&D investments by incumbent aerospace firms that have not been publicly disclosed.

More than 50 companies are developing battery-electric, hybrid, or hydrogen aircraft , or powertrains for retrofit, with 2025 seen as the earliest potential date for entry into service. Companies that are developing powertrains include Ampaire, MagniX, Pratt & Whitney, Rolls-Royce Electric, VerdeGo, and ZeroAvia. Other players are working on new aircraft designs using those technologies, including Airbus, Electra.aero, Embraer, Eviation, and Heart Aerospace. Yet another set is working on highly augmented flight controls and autonomy, including Boeing, Merlin Labs, Reliable Robotics, Sikorsky, Thales, and Xwing. Operators that are actively engaged in this market include Air Canada, Air New Zealand, Surf Air Mobility, United Airlines, and Widerøe Zero. There are more than 4,700 RAM aircraft or powertrains on order, option, or subject to letters of intent, with a total value of $38 billion.

A potential multi-billion-dollar market in the making

While the RAM market has momentum, estimates of its potential value vary widely. Our modeling suggests a TAM for RAM passenger revenue of $75 billion to $115 billion by 2035 (Exhibit 2). 22 Global passenger RAM market defined as flights for point-to-point trips of 150 to 800 kilometers operated by five- to 50-seat aircraft. This includes $40 billion to $65 billion from travelers switching away from ground or marine travel, $18 billion to $20 billion from passengers who take RAM flights instead of commercial flights on larger aircraft, and $12 billion to $25 billion from stimulated demand—that is, travelers taking trips they would not otherwise have taken, due to the convenience and availability of RAM. Our estimate of TAM includes roughly $5 billion in revenue for existing point-to-point flights 150 to 800 kilometers in length, operated by aircraft with five to 50 seats.

To estimate the TAM, we used anonymized cell phone location data, which show where people travel, in combination with estimates on aircraft operating costs, operational metrics, and route-level time savings calculations. Our model assumes that demand for travel will increase in line with historical growth and that travelers will switch to RAM from ground or marine travel, or from commercial air travel on larger aircraft, if the economics and time savings merit it.

Our model estimated global RAM market size and aircraft needs under two scenarios. The low-end scenario is based on a modest operating cost reduction against current small aircraft economics, while the high-end scenario reflects a more significant operating cost reduction, driven by semi-autonomous operations. Both TAM scenarios are unconstrained—in other words, they assume that everything that is needed to enable the market will “go right” and that the industry can meet demand by producing the required aircraft and establishing needed airport infrastructure, which will not be a trivial task. As a point of comparison, the estimated 2035 TAM for RAM is 8 to 13 percent the size of the 2019 global commercial airline market. 23 “Fact sheet: Industry statistics,” December 2022.

For the RAM market to reach its full potential, we expect 18,000 new or retrofitted RAM aircraft will be required by 2035 in the low-end scenario and 36,000 in the high-end scenario. 24 Estimate is based on a market size model and assumes that all new or retrofitted aircraft produced between 2025 and 2035 will be in service in 2035. Achieving this level of production will require significant investment in OEM and supply chain capacity. More than 3,500 new or retrofitted RAM aircraft will need to be produced annually between 2025 and 2035 in the high-end scenario. This number is more than twice the approximately 1,500 regional or private aircraft built in 2008, the previous production peak.

What needs to happen for the RAM market to take off?

Many factors will influence the growth and ultimate size of the RAM market (Exhibit 3). Take trip distance—a passenger would see more time savings compared to driving for a flight that is 800 kilometers, rather than a 150-kilometer flight. Likewise, the population density of a given geographic area must be high enough to ensure a steady stream of passengers. Choices on aircraft capacity and scheduling also play a role, as do environmental constraints, the flexibility of RAM in meeting passengers’ needs, and public acceptance. All of these categories depend on multiple variables. Public acceptance, for instance, will partly depend on guaranteeing RAM’s safety.

At a macro level, four enablers will be particularly important for the RAM market’s growth:

• A seamless end-to-end customer experience. In a recent McKinsey survey of potential RAM passengers, respondents stated that time savings would be the primary reason for switching to RAM, followed by convenience and flexibility. Passengers who might forego their cars and take RAM flights will need transparent information on timing, a simple booking process, good access to airports, an efficient security and boarding process, frequent or on-demand flights, and first mile/last-mile integration. Avoiding the “hassle factor” is critical (Exhibit 4). For instance, travelers may have to switch from RAM to another transportation mode, and some people might prefer to drive the entire trip to avoid such shifts. RAM operators could mitigate the hassle by coordinating flights with last-mile transport, so a car would be waiting when an aircraft lands, minimizing mode switching time and boosting predictability.
• Continuous technological advances. Aviation has significant energy needs, but a battery, by weight, holds just one-fiftieth of the energy of an equivalent unit of jet fuel. 25 “Sustainable aviation fuel,” US Department of Energy, May 2023; Terry Persun, “Advancing battery technology for modern innovators,” American Society of Mechanical Engineers, May 2021. Battery energy density will need to at least double today’s density for the RAM market to meet its full potential. Similarly, hydrogen fuel cells are at an early stage of maturity and further advances will be critical to RAM’s growth. 26 Target True Zero: Unlocking sustainable battery and hydrogen-powered flight , World Economic Forum, July 2022. Hybrid powertrains are nearer to commercialization and will play an important role. Improvements in composites, especially thermoplastics, will be needed to enable high-rate, low-cost production of lightweight airframes. OEMs are likely to employ advanced simulation and AI-enabled digital engineering to iterate and optimize designs quickly. All of these technologies must be extensively tested and certified for use in aircraft—which, in some cases, may require regulators to develop or update certification standards and regulations. Beyond the aircraft themselves, new air traffic management systems will be required to govern the higher volume of small aircraft in crowded airspaces, including some that may eventually be autonomous.
• Increased public acceptance. Passengers must be willing to fly on aircraft with new propulsion technology and ultimately on autonomous or semi-autonomous aircraft. Simultaneously, industry players may need to overcome local resistance to increased traffic at small airports, especially near urban areas. RAM aircraft have advantages here: Beyond reduced emissions, small electric aircraft create smaller noise footprints and have flight profiles that mitigate noise compared to conventional regional aircraft. Still, technical improvements alone will not sway hearts and minds. OEMs and other industry champions will need to engage with communities, collaborate with travel and local development groups, and respond appropriately to public concerns. Successful RAM players will be able to articulate the significant benefits to communities and travelers to overcome inherent skepticism of new technology.
• Airport and energy infrastructure to support the new fleet. For the RAM market to truly take off, airports will require hydrogen and electric charging infrastructure. A typical regional airport serving 200,000 passengers annually could require $6 million in investment for charging or fueling. For electric aircraft, it would require 15 to 30 megawatts of peak onsite electrical power for charging. To fuel hydrogen planes, the typical airport will need 500 to 1,500 tons of hydrogen annually. The cost of green hydrogen fuel—which must be from low- or zero-emissions sources—will need to fall significantly for the economics to work. Additionally, hydrogen must be produced in sufficient volumes to serve the market globally. While RAM benefits from its ability to use existing airport infrastructure, energy infrastructure will require significant investment for the market to scale. 27 Target true zero: Delivering infrastructure for battery- and hydrogen-powered flight , World Economic Forum, April 20, 2023.

Overall, the trends suggest that RAM’s time has come. The world may soon discover this revived form of air travel, which is more sustainable and capitalizes on underutilized infrastructure that we already have today. Re-energizing short-haul air travel will help increase equity for rural communities and stimulate economies beyond major metropolitan hubs. Material uncertainties remain about RAM’s future, but a discernible path forward exists to support strategic decision making—and to create value in a market whose time has come.

Lukas Brink is a consultant in McKinsey’s New York office, Ryan Brown is a consultant in the Seattle office, Sarina Carter is a consultant in the Waltham office, Axel Esqué is a partner in the Paris office, Benjamin Meigs is an associate partner in the New Jersey office, and Robin Riedel is a partner in the San Francisco office.

The authors would like to thank Andrea Cornell, Guenter Fuchs, Tore Johnston, Adam Mitchell, and Michael Saposnik for their contributions to this article.

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Is this the end of short-haul flights? How sustainability is shaping the future of air travel

From fuel levies to a ban on short-haul flights, sustainability shake-ups in the airline industry are set to change where you can travel by plane — and how much it will cost.

This January, Air France became the first airline to introduce biofuel surcharges, with other airlines looking set to follow suit. Furthermore, a recent ruling in the carrier’s home country now requires all aircraft refuelling within France to do so using sustainable aviation fuel (SAF), which is far more expensive than traditional aviation fuel, meaning potentially higher airfares for passengers as a result. Meanwhile, in a bid to cut carbon emissions, governments across Europe are beginning to rethink their short-haul flight networks. We look at what all of this might mean for passengers.   What’s happening with aviation fuel? As of 1 January 2022, France requires the fuel mix of all airlines refuelling in the country to be at least 1% SAF, with this figure set to increase to 2% in 2025 and 5% in 2030. The EU is likely to introduce a blending mandate in 2025 that will help the airline industry in its stated aim of becoming carbon-neutral by 2050. In the meantime, Sweden has announced plans to become the first country to charge a landing and take-off fee for older, less fuel-efficient aircraft. How this cost will be passed on to passengers, however, remains to be seen.

Will passengers be affected? Air France’s announcement will mean that passengers pay between €1-12 (85p-£10), depending on flight duration and cabin class. Partner airline KLM and subsidiary Transavia will also implement surcharges in efforts to offset the more expensive SAF. All of this is driven by costs: SAF is between four and eight times more expensive than traditional fuel (which accounts for up to 30% of an airline’s costs) but allows airlines to cut carbon emissions by 75% compared with conventional kerosene jet fuel. Air France has said it expects the costs of SAF — largely made from used cooking oil as well as agricultural waste — to drop as more European countries start producing it.     And how will this affect flight routes? As part of efforts to reduce airline carbon emissions, a growing number of European nations are proposing to follow France’s example: last April, the country banned short-haul domestic flights on routes where comparable train journeys of up to two-and-a-half hours exist. Services affected included flights between Paris and Nantes, and Lyon and Bordeaux. Similar bans are being considered in Spain, Germany and Scandinavian countries. During the pandemic, Austrian Airlines secured a government bailout on condition it ditched domestic flights where a train journey of under three hours was available.     Will this be enough to cut emissions?   For many environmentalists, these bans don’t go far enough. According to last October’s Greenpeace report Get On Track, the French ban will result in less than a 1% reduction in carbon emissions for the country’s air transport sector. John Hyland, the EU spokesperson for the group said: “The EU and European governments, France included, should ban all short flights when passengers can use less polluting transport like rail or bus.” A third of the busiest short-haul flights in Europe have train alternatives of under six hours, according to research by think tank OBC Transeuropa, which has called on European governments to ban short-haul flights in favour of accessible rail travel for all.

And what does this mean for long-haul flights? Banning short-haul flights won’t resolve aviation’s bigger problem: long haul. According to European air traffic management body Eurocontrol, long-haul flights account for 6% of all the continent’s flights but produced a disproportionate 52% of emissions — making the implementation of biofuel an increasingly burning issue.

Biofuel facts

What's it made of? SAF is largely composed of used cooking oil and forestry and agricultural waste.

Cutting carbon: SAF emits 75% less carbon than traditional aviation fuel.

Concerns: The EU Court of Auditors has voiced concerns over imported ‘virgin’ oils (including palm oil) being passed off as ‘used’, fearing deforestation. It’s called for tougher regulation and monitoring.

Published in the April 2022 issue of   National Geographic Traveller (UK)

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• SUSTAINABILITY
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• CARBON FOOTPRINT
• AIR POLLUTION

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