Boeing intends to use a 737 Max 10 to evaluate the environmental effects of burning sustainable aviation fuel (SAF), including its impact on contrail formation.

The US airframer will undertake the project with assistance from NASA and United Airlines, which intends eventually to take delivery of the Max 10 used for the SAF trials, Boeing said on 12 October.


Source: Boeing

The Max is one of two technology test aircraft Boeing calls ecoDemonstrator Explorers – aircraft focussed on short-term specific projects. Its regular ecoDemonstrator aircraft typically assess a range of technolgies over a longer period.

“The researchers aim to understand how advanced fuels, engine combustor designs and other technologies may reduce atmospheric warming,” Boeing says.

“For example, tests will assess how SAF affects the characteristics of contrails. While their full impact is not yet understood, some research has suggested certain contrails can trap heat in the atmosphere.”

The aviation industry is banking on using more SAF to reduce its carbon output in the face of intense social and governmental pressure.

Burning SAF produces as little as 15% of the “lifecycle” carbon dioxide emissions produced by burning conventional fossil-based fuel, according to the fuel’s proponents, although other sources say SAF’s CO2-cutting benefits might be significantly less than advertised.

Researchers also think contrails, which develop when the water vapour in jet engine exhaust cools and forms ice crystals, also cause atmospheric warming.

NASA has concluded that burning SAF produces less soot than burning traditional jet fuel, in turn resulting in fewer contrail-forming ice crystals.

During the flight tests, the 737 Max 10 will carry 100% SAF in one fuel tank and conventional jet fuel in the other. The team will alternate which fuel the engines burn.

NASA’s Douglas DC-8 “Airborne Science Laboratory” will follow the 737 Max 10 and “measure emissions produced by each type of fuel and contrail ice particles”, Boeing says. Images of the contrails will also be taken by NASA satellites.

Partner United Airlines has also been an eager proponent of using SAF to reduce its carbon output.

Other programme participants include the Federal Aviation Administration, which is helping fund the effort, engine partner GE Aerospace and Germany’s DLR aerospace research centre, which will assist with instrumentation. GE Aerospace co-owns CFM International, which supplies the 737 Max’s Leap-1B engines.

Despite industry promotion, SAF’s feasibility as a commercial fuel remains unproven, as does its environmental benefits. The fuel costs two to nine times more than fossil-based jet fuel, hindering widespread adoption, and SAF production remains minuscule, accounting for less than 0.1% of jet fuel used in the USA last year, according to reports from The Royal Society and the US Government Accountability Office.

The Royal Society says various industry and government groups use different methods of calculating SAF’s net carbon benefit, with significantly differing results. Some results show that few SAF fuels “hit the renewable energy directive target”.