On Tuesday an Airbus A350XWB airliner, powered by two Rolls-Royce Trent XWB engines, took off from Blagnac Airport in Toulouse, in France. What was special about this flight was that the aircraft was using only sustainable aviation fuels (SAF). The airliner, registration F-WXWB, is one of Airbus’ own flight test aircraft and is designated by the company as a ‘flightlab’.

This flight, announced on Thursday, was the first in the Airbus-led Emission and Climate Impact of Alternative Fuels project. The other participants are Rolls-Royce, Finnish oil refining and alternative fuels company Neste and Germany’s DLR aerospace research centre.

Currently, all Airbus airliners are certified to fly with blends containing up to 50% SAF. But the practice of blending SAF with conventional jet fuel has meant that it has not been possible to measure the emissions of unblended SAF in actual flight. The purpose of this project is to fully measure the environmental impact of pure SAF emissions.

“SAF is one of the aviation industry’s best low-carbon solutions with an immediate impact on (carbon dioxide) emissions today,” highlighted Airbus New Energy Programme manager Steven Le Moing. “This research project will help us to better understand the impact of unblended SAF on the full scope of aircraft emissions, while supporting SAF’s future certification for blends that exceed today’s maximum of 50%.”

The SAF being used in the test programme is made from hydroprocessed esters and fatty acids (usually obtained from animal fat and used cooking oil). In all, the project will use 117 t of neat SAF, all of it produced in Europe from European feedstocks.

The initial flights will be clearance flights, to ensure that it is safe to fly the aircraft using only SAF. “The first flight went exceptionally well,” reported Airbus flight test engineer Emiliano Requena Esteban. “There’s no perceptible difference in engine behaviour between jet fuel and SAF. It’s very exciting for me to contribute to a project that participates in the decarbonisation of our skies.”

Once the aircraft is cleared for the test programme, the data-gathering flights will begin. This should happen next month. On these flights, the A350XWB will be followed by a chase plane, equipped with sensors to measure the SAF-produced emissions directly from the engine exhaust. The chase plane will fly only 50 m behind the flightlab. There will also be a parallel series of SAF-powered engine ground-run tests.

These tests will measure much more than carbon dioxide. They will seek to establish the full impact of all climate-relevant emissions from the burning of SAF. So they will also measure emissions of carbon monoxide, nitrogen dioxide, water vapour, soot, aerosol and sulphate aerosol particles and ice crystals (produced by jet engines under certain conditions and at high altitudes – these create contrails).

The first results from the test programme are expected later this year. However, more complete results will only become available next year.