The results of a European study into the impact of a widebody jet flown with 100% sustainable aviation fuel show a substantial 56% reduction in the number of contrail ice crystals produced by unblended SAF compared to Jet A-1 fuel. The project, involving Airbus, Rolls-Royce, the German Aerospace Centre (DLR) and SAF producer Neste, was the third stage of the Emission and Climate Impact of Alternative Fuels (ECLIF) programme, with in-flight and ground emissions tests taking place in 2021. They worked together to quantify the reduction in engine soot particles produced by a Rolls-Royce powered Airbus A350 fuelled by Neste and trailed by a DLR research aircraft. In certain weather conditions, airborne vapour freezes around soot particles emitted by aircraft engines, creating cloudy canopies of ice crystals which can trap heat in the atmosphere.

Prior to the A350 test flights using 100% SAF – a campaign designated as ECLIF3 – airborne research was conducted in 2015 (ECLIF1) to characterise the emissions of synthetic fuels, followed in 2018 by flight tests with NASA (ECLIF2) to demonstrate that 50/50 blends of conventional jet fuel and SAF could reduce the climate damage caused by aircraft condensation trails.  

The ECLIF3 tests were conducted over the Mediterranean and southern France using the first Airbus A350 aircraft built by the airframer and powered by two Rolls-Royce XWB-84 engines. Conventional Jet A-1 was used as the reference fuel, while the comparative SAF was a mix of hydro-processed esters and fatty acids and synthetic paraffinic kerosene (HEFA-SPK).

In all conventional jet fuels, naturally-occurring aromatics serve as a vital sealant within the aircraft engine to help prevent fuel leaks. But this compound is slow-burning and emits soot particles which contribute to contrail crystals that can remain for several hours in cold, humid conditions at altitudes of eight to 12 kilometres, and can have a local warming or cooling impact depending on the position of the sun and underlying surface, with a warming effect predominating globally. Many types of SAF are free of aromatic compounds.

During the ECLIF3 programme, which involved multiple flights, the A350 testbed departed Toulouse Blagnac airport in France while DLR’s research jet, a Falcon 20-E, flew from the agency’s base in Oberpfaffenhofen, Germany, meeting at multiple points over the Mediterranean and southern France. The research aircraft was equipped with instruments to assess exhaust gases, volatile and non-volatile aerosol particles, and contrail ice particles produced by the A350, which it followed at various distances to capture data on emissions and condensation trails produced by both conventional Jet A-1 and Neste’s SAF.  

DLR used global climate model simulations to estimate how contrails could change the energy balance in the earth’s atmosphere, an effect known as radiative forcing. The tests revealed a 26% reduction in the overall climate impact of contrails when 100% SAF was used.

“The results from the ECLIF3 flight experiments show how the use of 100% SAF can help us to significantly reduce the climate-warming effect of contrails, in addition to lowering the carbon footprint of flying – a clear sign of the effectiveness of SAF towards climate-compatible aviation,” explained Markus Fischer, DLR Divisional Board Member for Aeronautics.

“We already knew that sustainable aviation fuels could reduce the carbon footprint of aviation,” added Mark Bentall, head of Research and Technology Programme, Airbus. “Thanks to the ECLIF studies, we now know that SAF can also reduce soot emissions and ice particulate formation that we see as contrails. This is a very encouraging result, based on science, which shows just how crucial sustainable aviation fuels are for decarbonising air transport.”

Alexander Kueper, Neste’s VP, Renewable Aviation Business, said SAF was recognised widely as a crucial means of mitigating the climate impacts of aviation. “The results from the ECLIF3 study confirm a significantly lower climate impact when using 100% SAF due to the lack of aromatics in Neste’s SAF used, and provide additional scientific data to support the use of SAF at higher concentrations than the currently approved 50%.”

And Rolls-Royce’s Director of Research and Technology, Alan Newby, said the use of SAF at higher blend ratios would be a key factor in enabling aviation to achieve its target of net zero CO2 emissions by 2050. “Not only did these tests show that our Trent XWB-84 engine can run on 100% SAF,” he said. “The results also show how additional value can be unlocked from SAF through reducing non-CO2 climate effects as well.”

The ECLIF3 research team says its programme is “the first in-situ evidence of the climate impact mitigation potential of using pure, 100% SAF on a commercial aircraft,” and has reported the findings of its tests in the Copernicus journal Atmospheric Chemistry and Physics (ACP) as part of a peer-reviewed scientific process. The ECLIF3 programme also includes members of the National Research Council of Canada and the University of Manchester.

Last year, Boeing, NASA and United Airlines conducted contrail research in the US using a Boeing 737 MAX twinjet with one engine powered by 100% SAF and the other by conventional jet fuel, and Virgin Atlantic partnered with Rolls Royce, Boeing, Air BP and Virent to perform a trans-Atlantic crossing using a Boeing 787 powered purely by SAF, with a blend of 88% recycled waste fats and oils and 12% sustainable aromatic kerosene. UK-based technology group SATAVIA also worked with 12 airlines over 65 flights to test its DECISIONX route optimisation software, which uses atmospheric modelling data to assist carriers in planning flight paths which avoid conditions in which contrails can form. The company said its tests avoided more than 2,200 tonnes of carbon dioxide equivalent (CO2e), or an average of more than 40 tonnes per flight, with little impact on aircraft fuel burn or flight distances.

Photo (S. Ramadier): Airbus A350 fuelled with 100% Neste SAF trailed by DLR’s Falcon 20-E research jet

Tony Harrington

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