Tuesday, April 16, 2024
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Will flying ever be green? | Air transport


On 16 December 2021, a group of men dressed in the sober, branded casual wear of the Silicon Valley startup gathered on the asphalt at an airstrip outside Salinas, California. In front of them stood a black shiny capsule on three spindly legs, which resembled the offspring of a suppository and a golf trolley, with a V-tail like a humpback whale. Its single cross-span wing had four banks of three rotor blades – six at the front and six at the back – which made the sound of a loud hairdryer. As the spectators bobbed nervously from foot to foot, the machine rose into the air, tipped a bow, and hovered for 10 seconds or so before coming gently to earth. Everyone cheered and clapped and exchanged slightly standoffish hugs. Back in the headquarters of Archer Aviation in Palo Alto, watching events on a huge screen, the rest of the company’s employees were on their feet, whooping and whistling.

It was the first test flight for Maker, Archer Aviation’s version of a new kind of aircraft called an electric vertical takeoff and landing vehicle. This masterpiece of nomenclature should on no account be attempted when drunk; its acronym, eVTOL, is also hard to get your mouth around; and consensus is lacking over whether the “e” should be capitalised. The bet that significant numbers of investors are making is that eVTOLs, if that is what they continue to be called, will be big. Three months before the test flight, Archer had merged with a special purpose acquisition company, or Spac, also known as a blank-cheque company.

From swapping engines on an old Dornier aircraft to turning someone’s smelly running shoes into fuel, it cannot be argued that sustainable aviation is especially glamorous. eVTOLs are the exception to that rule. Consider all that bespoke composite bodywork and fly-by-wire technology; the way Maker’s rotors sit flat, like adorable baby helicopters, for takeoff and landing, but tilt for forward flight; the tantalising promise of full automation. There is something about pure electric that appeals to the antiseptic, unsooty aesthetic of our age. If you peer into the workings of a Maker, you’ll see a neatly stowed battery pack and some cables; the cabin gives off the smell of a sanitised rental car.

A Joby Aviation air taxi outside the New York stock exchange, 2021.
A Joby Aviation air taxi outside the New York stock exchange, 2021. Photograph: Andrew Kelly/Reuters

Following the first successful test flight, Archer’s CEO Adam Goldstein’s next task is to guide Maker through certification with the Federal Aviation Administration, a process that can take years and costs hundreds of millions of dollars. It also means preparing for mass production (Archer has entered a partnership with Stellantis, one of the world’s biggest carmakers), identifying routes and takeoff sites in cooperation with municipal authorities, and preparing ordinary people for what may be a turning point in their flying lives – the moment when a plane stops trying to be a train, running scheduled services from point to point and packing in large numbers of people, and becomes a taxi on demand.

Hundreds of companies have entered the well-capitalised world of urban air mobility, which will over the next few years be shaken down to a few dozen genuine contenders. Joby, one of Archer’s California rivals, and the German company Lilium have also given themselves a head start through mergers with blank-check companies. In January 2022, Boeing put $450m into Wisk Aero, an aviation startup backed by Google’s co-founder Larry Page. Then there is Airbus’s urban air mobility programme, CityAirbus NextGen, and the Israeli startup Urban Aeronautics, whose small-format CityHawk is propelled by enclosed rotors and, according to a delightful company statement, “has more in common with the birds who nest upon the rooftops of skyscrapers than with nearly every other eVTOL prototype in existence”. But will this investment lead the industry to its declared objective of net-zero flight?


In 2018, CO2 emissions from civil aviation represented about 2.4% of total global emissions caused by human activity. But the impact of aviation on the climate goes beyond CO2, which may constitute little more than a third of the sector’s contribution to climate change. Aircraft also release nitrogen oxides, oxidised sulphur, water vapour and contrail cirrus – artificial clouds produced when water vapour condenses around soot from the plane’s exhaust at high altitude. This temporarily increases the amount of heat that is trapped in the atmosphere. All of these affect the climate; their combined effect is to warm it – in the case of contrail cirrus, much more intensively, if more briefly, than CO2. Released at high altitudes, aviation emissions have between two and four times the impact of comparable ground-source emissions.

If we take these additional impacts into account, we see that aviation represents about 3.5% of the warming impact caused by humans. That compares with around 6% for the cement sector and 17% for road transport. But cement is in virtually every road and building that’s made, and cars and trucks are vital to billions. Aviation barely counts as a mass activity, and it’s rarely an essential one. The majority of people who take planes do so not for vital work or family reasons but in order to have fun at the other end. Aviation has a strong claim to be the most damaging leisure activity around.

A long car journey of, say, 11 hours, with breaks for fuel and other necessities, might cover 650 miles (1,045km). In a full car, the amount of fuel burned by each passenger is under 14kg. In the same 11 hours in an airliner, you’ve gone from Paris to San Francisco, a distance of 5,500 miles (8,850km), and the average amount of fuel burned by each passenger exceeds 300kg. And that’s assuming the plane is full. If it’s half-empty, the per-person burn is much higher.

As a global sector, transport lags behind global climate targets. Within transport, aviation is even further behind, sharing a subset (with shipping) of spectacular underachievers. The environmental crisis has made the principle of “polluter pays” a fact of life for many economic sectors. Whether it is through carbon pricing for power plants, fines for farmers who pollute rivers or visitor taxes to mitigate the effects of overtourism, the idea that the cost of environmental degradation must be borne by those who create it has been widely, if grudgingly, accepted. Not, however, by the aviation industry. The customer whom aviation treats the best, whom the airlines garland with their most egregious toadying, the frequent flyer, is the worst polluter of all. And the most logical way of reducing aviation’s contribution to global warming, which is for people to fly less, is the one that the airlines – and their political allies – steadfastly refuse to entertain.

Passengers queue for check-in at Schiphol airport in Amsterdam.
Passengers queue for check-in at Schiphol airport in Amsterdam. Photograph: Robin Utrecht /AFP/Getty Images

The aviation industry revels in its own exceptionalism, which is indivisible from its sense of entitlement and is reflected in the bizarre privileges it enjoys. To this day aviation has no equivalent of the tax that, when you drive a car in the UK, for example, accounts for more than a third of the price you pay at the petrol pump. Nor is VAT levied on international air tickets. Like shipping – another sector that scorns national jurisdictions – cross-border aviation is absent from the 2015 Paris agreement on climate change, in part because of the difficulty of assigning responsibility for the emissions of international flights in which a carrier from one country flies from a second country to a third.

In 2016, the International Civil Aviation Organization agreed to the outline of a carbon offsetting and reduction scheme for international aviation, or Corsia, which aimed to stabilise emissions at 2020 levels. Under the scheme, airlines buy offsets – planting trees, investing in solar farms or distributing low-emission stoves – in order to atone for any excess growth in carbon emissions above an agreed limit. Carbon offsetting is hated by environmental groups, which estimate that a lot of the emission reductions that take place under the scheme would have happened anyway. Even after it becomes mandatory, in 2027 – at present, India, China and Russia are among the countries that haven’t joined – it won’t include domestic flights, which produce more than a third of the industry’s emissions.

Unlike many other sectors, aviation’s decarbonisation efforts are barely out of the starting blocks. Of the technologies vying to dominate the hypothetical future of “green aviation”, it’s unclear which stand the greatest chance of success. Advocates of hydrogen-powered flight point out that this wondrous element packs much carbon-free energy; but hydrogen also takes up a lot of room and will require big, and expensive, changes to aircraft design as well as infrastructure on the ground. Proponents of a kind of sustainable aviation fuel (SAF) called e-fuel, which involves sucking carbon out of the air and fusing it with hydrogen, consider biofuels to be almost as polluting as the hydrocarbons they are designed to replace; and yet President Joe Biden’s US administration has put these same fuels, made from crops grown across the country, at the heart of its plans to ramp up SAF production to 3bn gallons a year by 2030. (Last year that figure was a mere 60m.) Some see a place for carbon capture and sequestration – that is, filtering off carbon dioxide produced by industry before it is released into the atmosphere, and burying it. Others regard it as an expensive irrelevance. All sides unite to deride electrification – all except those investors who have poured billions into electric flight.

But here is the silver lining. Thousands of startups and new units within companies are pouring resources into technologies that have made flying green their objective. There’s a chance that the cumbersome, needy, petulant, change-averse behemoth that is modern aviation is starting to rediscover the fearlessness of the first aviators, and that by finally shouldering its responsibilities it will set itself on a path to eliminating the greenhouse gases it has spent recent decades so blithely emitting.


It was in 2016 that eVTOLs first started receiving attention, when Uber Elevate, the ride-hailing and food-delivery platform’s urban air mobility division, published a research paper that presented small aircraft running on renewable electricity as part of a broader endeavour to decarbonise society. Uber Elevate also declared that, through automobile-style mass manufacturing, such aircraft would become “an affordable form of daily transportation for the masses, even less expensive than owning a car”.

Another surge of hype came in 2019, when Morgan Stanley predicted that by 2040 the market for autonomous urban aircraft could be worth $1.5tn, a forecast that made its way into investor pitches. Morgan Stanley later revised this figure down to a mere $1tn, but stood by its assertion that eVTOLs could have as dramatic an effect on transport as cars did in the early 20th century and commercial airliners after the second world war. “Radical changes to transportation modality,” the investment house noted, “don’t so much ‘cannibalise’ the current/prevailing form of transport … as totally reinvent and rescale the size of the market itself, frequently by orders of magnitude.”

A model of Uber’s electric vertical take-off and landing vehicle (eVTOL) concept flying taxi.
A model of Uber’s electric vertical take-off and landing vehicle concept flying taxi. Photograph: Robyn Beck/AFP/Getty Images

Archer will sell direct to airlines – as it did in 2021 when United placed an order for $1bn worth of Archer planes, with an option for half as many again. Its second aim is to become a ride-sharing platform; “taking” an Archer will mean using the app to book a seat in a vehicle leaving in 20 minutes from near your home, a commute that – miraculously – won’t require you to sit in traffic for hours.

Goldstein showed me a 2020 clip of Jay Merkle, the aviation federation official in charge of certifying eVTOLs, saying: “Probably the biggest question I get … is, ‘Is this real? Are they really happening?’ Yes, this is more than just hype … we have at least six aircraft well along in their type certification, which is the first step in introducing the new aircraft into operation.” In 2021, Archer had another round of certification approved, taking it closer again to full commercial certification.


There’s something endlessly fascinating about America’s record of contributing so handsomely to the climate crisis and then regarding it as a business opportunity with a workaround. The atmosphere at Archer certainly isn’t moody and apocalyptic, as it can be at some of the more introspective European startups I have visited. It’s possibly the effect of the California sun, but the idea that you can create a new category of aviation using components from around the world and that this might actually be good for the climate has been internalised without demur.

The idea that no problem of human origin is so great that it cannot be overcome, and that this is a splendid thing because it engages every last atom of human ingenuity and profit-lust, has been known to bring out the cynic in me. (Surely better to avoid creating the problem in the first place?) But meeting Geoff Bower, Archer’s unfeasibly youthful chief engineer, who comes to work in a Tesla Model Y and wrote his Stanford doctoral thesis on the way albatrosses fly over the ocean without flapping (they borrow energy from the atmosphere), I get the feeling that the problem isn’t about abstractions and guilt. It’s about physics.

The problem is taking off and landing. The bigger the area covered by your rotors – the disc area – the less power you need in order to take off, but the more you need in order to cruise, because the drag is higher. The balance that Archer tries to strike between disc area and drag involves tilting Maker’s front six rotors in cruise and relying on its fixed wings to provide lift in horizontal flight.

But there’s no getting away from the fact that, of all the fuel technologies currently in development, electric batteries offer by far the least power for the most weight. A kilogram of petrol holds 13 kilowatt-hours (the amount of energy used per hour) of energy. A kilogram of lithium-ion battery holds not even 0.3 kilowatt-hours.

Archer Aviation’s Maker eVTOL aircraft, a personal electric vertical take-off and landing demonstrator aircraft.
Archer Aviation’s Maker eVTOL aircraft, a personal electric vertical take-off and landing demonstrator aircraft. Photograph: Archer Aviation Inc/AFP/Getty Images

That we’re even able to contemplate electric-powered aircraft is thanks to Elon Musk, whose Tesla Inc has done more than any other entity, public or private, to stimulate improvements in the lithium-ion battery. That puny-sounding 0.3 kilowatt-hours is in fact five times more energy than the old lead-acid battery could muster. Electric motors are between two and three times more efficient than combustion engines. And batteries can be recharged.

Although flying an electric plane produces no old-school CO2, nitrogen oxide, or water vapour, that doesn’t mean they are entirely good for the climate. The lifecycle climate impact of eVTOLs depends on other things. The first of these is the impact of the components that go into them. This can be alleviated through recycling: batteries continue to have terrestrial applications long after they are past their shelf life in the air, and in the future eVTOLs may be built of thermoplastics that can be remoulded many times. But, more than anything else, the sustainability or otherwise of all-electric craft depends on where the electricity that charges their batteries comes from.

A given technology’s carbon intensity is a measure of emissions of CO2 or its equivalent over its entire life – including materials, construction, use, demolition and disposal, referred to as lifecycle – compared with the megawatts of energy expended. Just how far electric aviation must progress before it becomes truly climate friendly was illustrated in a 2019 assessment by environmental and aviation scientists in the UK and the US. Basing their calculations on the 2015 average US grid CO2 intensity, the scientists found that all-electric aircraft would have a lifecycle carbon intensity 20% higher than that of their modern jet engine counterparts, though taking into account non-CO2 impacts (including nitrogen oxides and contrail cirrus) brought that figure down by around 30%.

The International Energy Agency has forecast that the world’s electric power demand will near-double between 2010 and 2040 (without accounting for aviation or ground transportation electrification), but that emissions from generating electric power will drop by only about 5% due to continued use of coal. In India, taking an electric train can be more damaging to the climate than flying. On the other hand, if you’re running a plane using electricity from the Brazilian grid, which has a high share of renewables, your lifecycle emissions are likely to be lower than they are in China, where most electricity comes from coal. Nothing is truly climate friendly unless everything about it is climate friendly. And this, naturally, isn’t something that a plane manufacturer can guarantee.


Is the eVTOL a commercially viable product or a genuine weapon against climate change? That was the question that nagged at me as I made my way north from Salinas, between polytunnels and vines and neat rows of lettuce and broccoli, back to San Francisco and the airport and home. A few weeks later, when I spoke to Richard Aboulafia, an aviation analyst at AeroDynamic Advisory, he pointed out that the aviation technologies that can be reproduced at scale are often those that are least good for the environment. He described eVTOLs as this “bizarre circus sideshow … in which everybody has decided that the thing that should be most funded is the thing that does the least good”.

Decarbonisation surely doesn’t mean creating new demand for emissions, no matter how cleverly their impact is reduced. It means replacement – getting rid of what is costly to the climate and putting in its place something less damaging, or, better still, not damaging at all. And Aboulafia is right to question whether this is what eVTOLs achieve. The money and ingenuity that are going into the sector are indeed stupendous, but the only way that all these resources will help the climate in the wider sense – that’s to say, lessen the effects of the wrecking ball that is currently smashing its way through our weather – is for electric flight to assume heavier loads over longer distances and enter the grownup world of aviation proper.

A commercial airliner with a Shell refuelling tanker.
A commercial airliner with a Shell refuelling tanker. Photograph: Clynt Garnham Business/Alamy

If we accept that sustainable aviation fuels are currently the only pathway to zero-carbon long-distance flying, what about flights whose range is under 500 miles (805km)? Two electric strategies are possible solutions. The one that is closer to hand is hybrid electric. A hybrid engine involves burning fuel of one kind or another, and using that to charge and/or augment your batteries.

In June 2022, Rolls-Royce, the world’s second-largest aircraft engine manufacturer, unveiled the prototype of a new compact engine, a “turbogenerator”, which burns fuel to either turn propellers directly or charge batteries onboard, enabling aircraft to switch between power sources depending on the phase of the flight (and how much power remains in the batteries). The generator can be scaled to deliver power in the range of 500kW–1,200kW, handily capable of propelling single-aisle airliners well over the 150 or so miles that is currently the absolute upper limit of the most ambitious eVTOL. To reduce emissions while using such a configuration, the pilot could switch to battery power during especially carbon-intensive phases of flight, whether that be passing through ice-saturated parts of the atmosphere (to reduce contrails) or during takeoff and landing. But emissions there will be. They are unavoidable.

The holy grail isn’t hybrid, but full electrification, and this requires radically improved battery performance. Since the turn of the millennium, frenetic R&D has given lithium-ion batteries, on average, about 4% more specific energy every year. From the high-voltage lithium-ion batteries that are currently under development at Rolls-Royce, to the lithium-air batteries that Japan’s National Institute for Materials Science unveiled in 2022 and the “solid-state” battery that is being pursued by QuantumScape, a US startup, the critical element in electric aviation will carry on getting lighter and more powerful. The only question is how fast.

Night and day, rain or shine, in his laboratory at Pittsburgh’s Carnegie Mellon University, robots controlled by Venkat Viswanathan, one of the world’s leading battery experts, conduct experiments aimed at achieving breakthroughs in performance. In 2021, Viswanathan and his colleague Shashank Sripad wrote that “a battery-pack specific energy [density] of 800 wh/kg [watt hour per kg] could potentially be reached at around mid-century”. In other words, in around 2050, lithium-ion batteries will be capable of powering a single-aisle A320 or Boeing 737 the 600 nautical miles that constitute a regional mission. Furthermore, assuming strong continued progress, all-electric “aircraft with … a range of 1,200 nautical miles … could replace more than 80% of all aircraft departures” sometime in the second half of this century.

Capability on the workbench does not necessarily denote capability in the real world, however. As Alan Epstein, a former vice-president of technology and environment at Pratt & Whitney, and a professor emeritus at Massachusetts Institute of Technology, put it when we spoke in the summer of 2022: “You have to distinguish between range and mission length. For about 1,400 miles – the actual amount may vary considerably depending on the weather and traffic at the time of departure – the airlines routinely carry more than 50% more fuel on board than you need just to fly that distance.” This is because in case of unforeseen events, “you have to be able to divert to another airport and often … the only airport that you know is open is the one you took off from.” If your fuel is kerosene, with its high energy density, excess fuel can be carried without undue trouble. Translated into electric flight, however, the principle of excess capacity plays havoc with your finely calibrated trade-off between battery pack and distance. “So a 500-mile range airplane may only be able to fly 200- or 250-mile missions,” Epstein said.

Everyone likes an optimistic vision, and the truth is that no one actually knows how fast battery technology is going to develop. The two days I spent visiting Archer were glass-half-full days. Then, towards the end of my conversation with Bower, after he had patiently explained his formulae to me, I felt the familiar, unsettling tug of legacy aviation. “This technical prowess sounds very fine,” I said, “but the decarbonisation of the aviation industry can be completely upended by passenger numbers. Relatively few people in the developing world have been in an airplane, and understandably many would be interested in trying. What then?”

“That’s the long-term hard problem for aviation,” one of the world’s best aerospace engineers ruefully acknowledged. “I don’t know the answer.”

This is an edited extract from Flying Green: On the Frontiers of New Aviation by Christopher de Bellaigue, published by Columbia Global Reports

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