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Polluting the sky

How can we clean up air travel?

Picture: Getty

I worked in aviation for more than three decades and I’ve found myself feeling guilty for pointing out that, even if we all stopped flying tomorrow, it would only shave 2% off total global greenhouse emissions. I had also been getting increasingly frustrated by sanctimonious people telling me they’d given up flying but then hopping on “no-fly” cruises. This is the most polluting form of transport on the globe, using the lowest-quality diesel to push scarcely streamlined floating blocks of flats through the water.

I was also impatient with people who said they were off to catch a high-speed train, as though more and more train journeys equated to less and less pollution – oblivious to the carbon cost of building the concrete infrastructure, or the fact that a high-speed train’s energy consumption rises almost exponentially with speed.

Then a colleague, the highly respected Ireland-based aviation consultant, Patrick Edmond, pulled me up on this, saying: “It’s our responsibility to clean up our act, just like any other industry.”

Chastened, I jumped aboard a flight of discovery and realised that, in fact, aviation does have a decent story to tell. But part of its problem has been that – while advances in engine and airframe technology have cut fuel consumption and CO2 emissions – these gains were, until Covid at least, swallowed up by the continued growth in air travel.

Last October, the industry body, IATA, set its Fly Net Zero goal of becoming carbon-neutral by 2050, matching Europe’s own Destination 2050 target. At the Farnborough Air Show in July, the UK government announced that domestic aviation and the country’s airports would now be required to achieve net zero by 2040. But, scratch below the surface, and it quickly becomes clear that each segment of commercial aviation raises quite different challenges and, while we may soon be able to make carbon-neutral island hops in parts of Europe, achieving that goal becomes increasingly difficult as both aircraft size and flight times increase.

At the heart of the conundrum is the fact that aviation fuel, kerosene (paraffin), is actually very efficient. It has low mass and the aircraft it drives gets lighter and lighter as the journey progresses. Hydrogen, the “obvious” clean alternative, fills three times the volume of kerosene. That extra volume has to go somewhere and, in aviation, more volume means more drag and more drag means more fuel is needed. And hydrogen, in its most compact liquid form, also has to be stored at temperatures close to absolute zero, presenting a range of additional challenges.

While a lot of investors are making extravagant claims for both electric and hydrogen flight, those with a longer background offer a more sober assessment. Even so, at the smaller end of the commercial aviation scale, positive results may be very close indeed.

The Scottish airline Loganair, which operates a large number of subsidised services in the Highlands and Islands, has been working with Cranfield Aerospace Solutions on a hydrogen fuel cell concept, under which propellers would be turned by an electric motor. Loganair’s head of sustainability, Andy Smith, explains that the partnership has moved away from the idea of battery flight because a battery of suitable size and weight will currently only offer about 45 minutes of power.

Although some of Loganair’s flights are very short and include the “world’s shortest scheduled flight” of less than a minute – between Westray and Papa Westray, in the Orkney Islands – aviation safety regulations require that a scheduled flight must be able to divert to an alternative airfield (most likely equipped with full navigation aids) in the event of bad weather. In the Orkney Islands this currently means that aircraft have to carry a reserve of between 90 and 120 minutes’ worth of fuel – way outside the scope of battery operation.

“We are still working with a number of different technology partners to figure out what the art of the possible is,” says Smith, while reflecting that – with so many brains now applied to the challenge – a better electric solution may yet arrive. One day.

In the meantime, Project Fresson, with Cranfield, expects to have a 10-seat British-built Britten Norman Islander aircraft, retro-fitted with hydrogen fuel cells running the electric motors that turn propellers, in commercial service “in three or four years”. “We mean to be the first to put this type of aircraft in service,” says Smith.

Jenny Kavanagh is chief strategy officer in the Cranfield team working with Loganair, Isles of Scilly Skybus and Jersey-based Blue Islands, and is under no illusions as to the complexity of the challenges faced. “You need to bear in mind that there are no regulations for hydrogen or electric-propulsion aircraft and these can’t be created overnight,” she says. “We are talking to the Civil Aviation Authority almost every day.”

She sees hydrogen fuel cells as offering the best solution in regional aircraft of up to about 70 seats, certainly in the short to medium term. “We started at a size of aircraft for which the technology is available today.” That means aviation following where buses, cars, trains, even Formula 1, are already leading. “It’s not starting from scratch, but you have to prove it [the technology] at the small level and scale it up from there.”

The difficulty is that the market for a brand-new small aircraft type running on hydrogen fuel cells is unlikely to be large enough to repay development costs and so the focus is on converting pre-existing models – though these may not be as well suited to the new power source as a brand new design would be.

The Norwegian regional airline, Widerøe, is taking a longer-term view and has extended the life of its Canadian-built Dash 8 aircraft, currently operating its public service network to remote communities, so as to buy time for an all-electric solution to be developed, having put on hold earlier plans to replace its fleet with newer types using existing technology.

Widerøe has been working with Rolls Royce to explore a range of different technologies that might be applicable to different sizes of operation. These may include moving away from runways altogether and operating electric vertical take-off aircraft in some markets, which would even enable it to extend services to new communities. In the meantime, Widerøe and Rolls-Royce have teamed up with the Italian manufacturer Tecnam to create the all-electric 11-seat Tecnam P-Volt, which it is hoped could enter service as soon as 2026 on very short routes.

Andreas Aks, chief executive of Widerøe Zero, says: “I believe there are opportunities where, if you wanted to extend the range, battery power could be combined with other technologies.”

The airline and Rolls-Royce are also working with the Brazilian manufacturer Embraer to examine options for decarbonising operations in the 50- to 100-seat segment of the market, exploring a variety of propulsion methods, including all-electric, hydrogen fuel cell and hydrogen-fuelled gas turbines.

However, none of these projects addresses the elephant in the aviation room: the very large jets flying very long distances.

The Cranfield-based Aerospace Technology Institute, backed by the UK government, has developed three concepts in its FlyZero project, which is looking at the potential for liquid hydrogen-fuelled aircraft, ranging from regional up to “mid-size” market segments. While hydrogen fuel cells would turn propellers on the smaller aircraft, crucially FlyZero’s modelling maintains that an aircraft the size of today’s Boeing 767 could serve trans-world routes with just one stop.

At Airbus, the pan-European manufacturer which is arguably now the world leader, the aspiration is to bring more sustainable aircraft to market comfortably before 2050 and has sharpened its focus on the up-to-200-seats market segment in its ZEROe project. For aircraft of up to 100 seats and a range of 1,150 miles or more, the design concept uses hydrogen-fuelled gas turbines, turning propellers but also generating electricity to augment the power. Two design concepts address the 2,300-plus miles range requirement for larger aircraft, using a hybrid hydrogen-powered turbofan (“jet”) engine, again also generating additional electric power. A second turbofan design concept is a more radical departure from today’s airliner, with a “blended-wing body” to provide more flexible solutions for storing the liquid hydrogen.

As Dr Mark Bentall, head of Airbus’s research and technology programme, stresses: “Right now our focus is on the propulsion fuel systems because we need to ‘secure’ these now if we are looking at introducing something in the middle of the next decade.

“You have to completely redesign the fuel system – a step change from liquid to gaseous, which is one of the things we will have to learn how to do.” Not only is there the issue of the safe storage of liquid hydrogen on the aircraft and how it is then piped to the engines, but also how the fuel can best be transported to the airport. Should it already be liquefied, or will airports need to develop infrastructure so the gas can be liquefied there?

Airbus is also cooperating with American engine manufacturer CFM on optimising engines to burn hydrogen and has adapted an Airbus A380 as a test-bed for hydrogen engines. “We have to marry all of that up together to bring a total solution to the market,” says Bentall.

“What’s important is that we are working with partner companies so it’s not just all Airbus resources. There are new skills we need to develop so it’s important to ensure that we have brought our engineers up to speed on new skills like the management of cryogenics [handling materials at ultra-low temperatures].”

The goal is to be able to make a decision about which fuel and technology systems to adopt so as to be ready to begin design and development work by the middle of this decade.

To achieve carbon goals, the hydrogen fuel will have to be “green” or produced by electrolysis. Not “grey hydrogen”, which is derived from natural gas and therefore fossil fuel reliant.

Airbus has also done a lot of work on the introduction of Sustainable Aviation Fuel, or SAF, and sees this as the most practical option for limiting carbon emissions in the shorter term and for long-range flights by the largest aircraft. SAF can be manufactured from renewable crops or by recycling animal and vegetable fats and oils from the food chain. It is compatible with kerosene and its use is already proven.

On June 28, five flights converged on Lyon from cities across Europe, each 30% fuelled by SAF and achieving a carbon reduction of 27%, according to the European Regions Airline Association. Working with Eurocontrol, the pan-European airspace management body, they also used more fuel-efficient routes, including passing through airspace usually set aside for the military. Lyon Airport itself claims to be on target to achieve net zero CO2 emissions by 2026.

While widespread use of SAF would require the assembly of an infrastructure to both recover and then refine the waste oil and fat products, it is claimed that SAF can still achieve a life-cycle carbon reduction of about 80%.

Bentall says: “Our target is to make sure our aircraft are 100% SAF-capable by the end of this decade.”

As the industry edges towards carbon neutrality, airlines continue to offer their customers the opportunity to “offset” their carbon emissions via a range of largely unregulated and unratified forestation and other schemes, which Edmonds likens to the selling of religious indulgences in the Middle Ages to mitigate the penance for sins committed. “Offset is a complete Wild West at the moment,” he says, adding that, at one well-known carrier, passenger uptake of the scheme was just 1%.

“The other philosophical position is that some sectors are just harder to decarbonise than others and aviation is particularly difficult so it could be given a bit of a ‘free pass’. I think that’s a bit morally flabby.”

However, he notes that as the industry collectively sets Destination 2050 in its sights, the words “demand reduction” have entered the airline vocabulary. Given the growth in passenger numbers since the low-cost airline model first delivered seats on aeroplanes for less than those on trains and in cars, this has to be seen as a milestone. “For the first time airlines are talking about demand reduction: prices will go up and fewer people will travel,” he says.

Given that SAF remains significantly more expensive than kerosene and that decarbonisation will require the construction of completely new and costly infrastructures, perhaps such demand reduction will be an inevitable by-product.

Before returning to journalism, Stan Abbott worked for and with numerous airlines, both in-house and as a consultant, including Air France, Atlantic Airways (Faroe Islands), Luxair (Luxembourg), Widerøe (Norway), and Eastern Airways (UK)

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