What Boeing, Airbus and other plane makers are doing to clean up air travel

In Sweden they call it flygskam, or flying shame – the guilty feeling that you're helping to destroy the planet as you zip off to Spain for the weekend.

With the UN's International Civil Aviation Organization forecasting that aviation emissions could triple by 2050 from their 2015 level, some countries are taking action by outlawing short flights where rail is a viable alternative.

Now the clock is ticking for the aviation industry to decarbonise or face further restrictions on its growth. That means developing planes that run on electricity or clean fuels.

Ending carbon emissions from aviation is fraught with technical and economic hurdles, not least the unbending laws of physics.

Getting an 80-ton Airbus A320 jumbo jet off the ground requires huge amounts of energy: A typical aircraft holds about 20,000 litres of kerosene, almost 10 times as much fuel as an average car uses in a year.

Long-distance journeys are even more polluting: A flight from Frankfurt to New York on a Boeing 747 jumbo jet emits about the same amount of carbon dioxide as heating 440 German homes for a year (roughly 2000 kilograms per passenger).

While start-ups are working on batteries to lift smaller planes into the air, there's so far no viable alternative to combustion propulsion for long-haul jets.

Airbus, the world's biggest aircraft maker, is betting that hydrogen-powered planes could be a solution for zero-emissions flying. It says it will have a model for commercial use by the middle of next decade.

The company is converting the first A380 superjumbo that it ever built into a demonstrator aircraft, which is scheduled to perform flight tests of a hydrogen combustion engine mounted on its fuselage starting in 2026.

US rival Boeing is testing hydrogen fuel cells on its ScanEagle3 pilotless military drone. However, the company says that planes powered by hydrogen will be realistic only in the latter half of the century.

There's also no infrastructure on the ground at airports to support wide use of the fuel. Hydrogen also needs to be cooled and is flammable.

The typically long certification timelines for new aircraft and the advancements that still need to be made with hydrogen, including how to store it and generate enough energy for takeoff and landing, mean that airlines and aerospace companies need an interim solution to allow them to meet their public commitments of net zero carbon emissions by 2050.

Sustainable aviation fuels (SAF), an umbrella term for substitutes to fossil-based kerosene, have emerged as a bridge to zero-emissions aircraft.

One kind is derived either from biological ingredients such as waste cooking oils, animal fats, municipal waste or forest residues. The other is synthetic fuel made using electricity to split water into oxygen and hydrogen, and then combining the hydrogen with carbon extracted from the atmosphere.

Like kerosene, SAF spews carbon dioxide and other pollutants into the atmosphere. The emissions reduction stems from the fact that the materials being burned don't come from extracting fossil fuels sequestered for thousands of years.

In its push for net zero, the European Union wants all aircraft fuel to contain 2% SAF starting in 2025, rising to 63% by 2063. For now, annual global output would barely fuel up the global fleet for a few days.

Suppliers such as Finland's Neste Oyj are promising to ramp up production of SAF as demand grows. It currently uses waste cooking oils and animal fat waste to make SAF but is exploring other sources including municipal solid waste.

Since the burning of synthetic SAF emits exactly as much carbon dioxide as was used to produce it, it is considered net zero - meaning that no new carbon emissions are generated from its use.

Lufthansa estimates that SAF is three to four times as expensive as regular jet fuel, and industry executives say they'll need subsidies to help push the alternative fuel over its initial hump.

They exist, but they're small. Heart Aerospace, a Swedish start-up, is developing an electric plane called the ES-30 that can seat 30 passengers and has a fully electric, zero-emissions range of 200 kilometres (124 miles).

The company says the ES-30 will enter service in 2028. Meanwhile, Los Angeles-based Ampaire has converted an 11-passenger Cessna Grand Caravan into a hybrid model with both a conventional combustion engine and an electric motor.

Ampaire says the plane entered service in 2021 and can cut fuel consumption and greenhouse gas emissions by 50%. In all, the Roland Berger consultancy says, there are about 100 different electric aviation programs under development around the world.

The big problem is the size and weight of the batteries needed to power them. Current lithium-ion batteries store only a small fraction of the energy of an equal volume of liquid jet fuel. That makes them too inefficient for large or long-distance planes.

Also, unlike liquid fuel, their weight doesn't diminish as the flight progresses. Hybrid technology mitigates that by using both conventional and electric engines. Either they share the workload or the conventional engine charges the batteries; in both cases, emissions are lower and the batteries can be much smaller.

In some hybrid designs, electric engines are used to power the short but heavily polluting takeoffs and landings, while at cruising altitude, propulsion is provided by conventional jet engines designed to be more fuel-efficient midflight.

That depends. As with cars, the carbon footprint of a hybrid-electric plane depends on how much it uses electric power and on the source of the electricity.

A plane that charges its battery midair by burning jet fuel won't reduce emissions much. But all-electric designs and "parallel" hybrids charged before takeoff using electricity from renewable energy sources could make a real difference, as could hydrogen-powered planes.

Still, considering the lengthy certification timelines and long life cycles of aircraft, the total potential pollution reductions will be limited for the next few decades.

Small electric flying taxis are poised to take off first. EHang Air Mobility Group of China is developing electric vertical takeoff and landing (eVTOL) aircraft costing $300,000, anticipating strong demand from emergency responders, air taxi services and operators of tourist flights.

Germany's Volocopter GmbH, which is backed by Daimler, in 2019 opened a "Voloport" in Singapore for public test flights and expects to start commercial service as early as 2024. Other start-ups chasing the opportunity include US-based Zunum Aero, MagniX Technologies and the U.K.'s Vertical Aerospace.

Israeli start-up Eviation Aircraft conducted the first test flight of its nine-passenger electric plane last September. Small hybrid-electric planes ferrying 10 to 20 passengers could arrive by the middle of the next decade and bigger regional aircraft carrying many as 40 passengers around 2030, according to Stephane Cueille, chief technology officer at Safran, a top jet engine maker.

Bloomberg