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Eco Travel: First Flights Using Sustainable Fuels

Updated: Feb 23, 2022

Milly Letcher reports on the supposed breakthrough in sustainably aviation using biofuels.

United Airlines has become the first airline to launch a passenger-carrying flight using ‘100% sustainable aviation fuel’ (SAF).

On 1 December, United Airlines flew 100 passengers from Chicago to Washington DC in a Boeing 737 using ‘100% pure SAF’ produced by partnering company World Energy. Despite previous test flights by Rolls Royce, UA says this is the first flight to transport people using sustainable aviation or SAF.

Aviation is now responsible for 2-3% of all global carbon emissions, and the IATA (International Transport Association) predicts that the number of air passengers will double in the next 20 years. This puts considerable pressure on airlines to drastically reduce their carbon footprint, especially following COP26 which reinforced the need to halve emissions by 2030 and achieve net-zero by mid-century.

“There is no other human activity pushing individual emission levels as fast and as high as air travel”. - Dr Stefan Gössling, Lund University

What is Sustainable Aviation Fuel?

United argue that SAF will reduce greenhouse gas emissions by 80% on a lifecycle basis compared to standard jet fuel. The fuel used in the Boeing 737 flight was produced by World Energy and developed by Virent – oil giant Marathon Petroleum’s subsidiary company. According to Marathon, the bio-based fuel was produced using corn sugar, and combined SAF with synthesised aromatic kerosene (SAK) made from renewable plant sugars.

Pure SAF can’t be used as jet fuel because it lacks the compounds (known as aromatics) which are required to meet today’s jet fuel specifications. In the past, hybrid blends of conventional and sustainable aviation fuel were used. United claim that the combination of SAK and SAF produces a fuel that can compete with conventional jet fuel without having to blend biofuels with petroleum.

However, the Boeing 737 aircraft did not technically fly using 100% SAF. Only one of the two engines onboard used SAF (50% as per the Federal Aviation Authority regulations), while the other ran using petroleum-based jet fuel which United stated was to "further prove there are no operational differences between the two". Critics are now branding UA’s claim that the flight was ‘100% sustainable’ as an attempt at greenwashing.

Are Biofuels the Future?

Just as with conventional fuels, SAF releases carbon dioxide when it is burned inside a plane’s engine. But unlike conventional jet fuel, the CO2 released has previously been absorbed by the plant as part of the normal growing process, so in theory, net CO2 emissions should be a lot lower than fossil-based fuels.

However, CO2 is not the only aviation emission – nitrogen oxides (NOx) and water vapour are also produced and have global warming potential. It has also been argued that the emissions emitted in the process of actually growing the corn for fuel are equivalent to those emitted by conventional jet fuel.

There are still flaws behind biofuels that must be addressed before they are adopted at-scale.


But how sustainable and scalable is this idea in the long term? With the number of flights predicted to increase dramatically in the next few years, can we grow enough feedstock needed to produce SAF to match these high demands?

According to the IATA, only 1% of the global demand for jet fuel was met using SAF in 2019. With a finite supply of land, allocating spaces for growing crops for SAF production reduces the land available for food and puts a strain on water resources.

For fuels produced from waste products like cooking fats and oils, there is also doubt about limited availability. Then there is the cost of SAF to consider. According to IHS Markit, "SAF prices are currently about five times higher than prices for conventional jet fuel".

The Future of Flying

With growing pressure on airlines to make drastic cuts in emissions, it’s easy to see why they are looking for solutions such as SAF. But despite the claims by United and others, there are no easy answers. Ultimately, reducing the impact of aviation may come down to flying less.

As Cait Hewitt, policy director of the UK non-profit the Aviation Environment Federation, put it recently:

“Given how far we are from knowing how to decarbonise flying, we really need to be downsizing aviation demand.”

This might even mean giving up flying altogether. But if that’s not an option for you right now, FlyGrn provides tips on how to fly as sustainably as possible.


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