Analysis of in-flight emissions from an Airbus A350-900 using 100% sustainable aviation fuel indicates a reduction of 35% in soot particle numbers, and a halving of contrail ice-crystal formation per mass of burned fuel.
Contrails can form when water vapour condenses onto soot particles from engine emissions, and freezes, under certain temperature and humidity conditions.
The ECLIF3 study – designed to examine climate impact of alternative fuels – used a Dassault Falcon 20 chase aircraft to measure emissions from the A350’s Rolls-Royce Trent XWB-84 engines.
Previous observations of contrails from sustainable fuel have been based on blends with conventional fuel, leaving open the question of effects from using 100% sustainable fuel.
Figures in the ECLIF3 study were derived from measurements which took place over the Mediterranean Sea, west of Corsica and Sardinia, in April 2021.
The A350 flew north-south racetrack patterns at 35,000ft while burning 100% conventional Jet A-1 and the sustainable HEFA-SPK fuel sequentially in both engines, with contrails respectively probed for about 15min and 10min.
This flight was identified as having “suitable conditions for a side-by-side comparison” of contrail properties from the two fuels, says the study.
It found that, while the soot particle emissions were reduced by 35%, the ice particle numbers fell by a larger proportion of 56%.
The study has tried to understand the stronger reduction in ice crystals compared with the soot reduction, and suggests this might be due to lower sulphur content of the Neste-supplied HEFA-SPK fuel – which is a synthetic kerosene derived from hydro-processed esters and fatty acids.
German aerospace laboratory DLR, a partner in the study, carried out global climate simulations which, based on the 2018 fleet, estimate that the use of 100% sustainable aviation fuel could reduce contrail radiative forcing impact by 26% in addition to cutting carbon dioxide emissions.
“We already knew that sustainable aviation fuels could reduce the carbon footprint of aviation,” says Airbus head of research and technology Mark Bentall.
“We now know that [sustainable fuel] 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.”
Optimising flights to avoid contrails by using contrail-prediction models to modify routes is a potential mitigation strategy, the study says. But routing flights for contrail avoidance could lead to increased fuel-burn, and climate-optimised routes might not be feasible for air traffic control reasons.
“Results from the ECLIF3 study confirm a significantly lower climate impact when using 100% sustainable aviation fuel due to the lack of aromatics in Neste’s SAF used,” says Neste vice-president of renewable aviation business Alexander Kueper.
He says it also provides data supporting use of such fuels at higher concentrations than the current 50% blend limit.
Compositions of Jet A-1 and sustainable aviation fuel are variable in terms of their hydrogen, aromatic, and sulphur contents, says the study, and this should be taken into account when deciding on aviation climate strategies.
“Cleaner jet fuel with a naturally – or artificially achieved – low aromatic and naphthalene content as well as a low sulphur content could reduce the contrail impact on climate,” it adds.
