Sustainable aviation fuel (SAF) used in a recent Boeing ecoDemonstrator flight-test program produced “dramatically” reduced soot particles and contrails, according to preliminary findings shared by researchers.

Speaking at a media briefing earlier this month as the three-week contrail research campaign concluded, Richard Moore, a research physical scientist at NASA and principal investigator for emissions flight testing, said that while “we’re still working through the data” and more detailed findings would be released over the coming year, researchers saw a “dramatic difference” between emissions released when SAF was burned versus those produced when the fuel tank containing conventional kerosene was in use.

“The soot emissions from the SAF are dramatically and noticeably different than with the Jet A-1, and that’s a great result,” says Moore.

Christiane Voigt, head of the cloud physics department at the German Aerospace Center (DLR) — which supported the campaign alongside the US Federal Aviation Administration (FAA), GE Aerospace and United Airlines — adds: “We saw a reduction in particle emissions when we were using SAFs, and we also saw a reduction in the ice crystals and contrails. That’s really a great result from this campaign.”

The aim of the flight-test program was to test the effects of SAF on non-CO2 emissions, which are thought to account for as much as two-thirds of the aviation industry’s contribution to climate change. It involved using a Boeing ecoDemonstrator Explorer 737-10 powered by 100% SAF from California-based World Energy in one tank and conventional jet fuel in another. The flights were trailed by NASA’s DC-8 airborne science lab aircraft, which measured emissions produced by each fuel type.

Boeing ecoDemonstrator program technical lead Bill Griffin says that “very specific procedures” were put in place “to ensure we didn’t have any contamination” between the two types of fuel that could skew the results. The DC-8 flew four to five miles behind the 737-10, taking a 20-30 minute sampling of Jet A-1 emissions before turning back to fly through the same patch of air on SAF, to enable a like-for-like comparison.

One of the most “intriguing” observations made by the researchers, says Moore, was that the contrail produced by the 737-10 when it was burning SAF was visibly thinner and wispier than that produced by jet fuel.

While the initial results seem promising, much more research is needed to overcome uncertainties about the extent to which contrails have a warming effect and whether certain contrails could instead have a cooling effect by reflecting radiation from the sun back into space.

“Longer-term, what we’ll maybe need to do is take measurements from commercial airplanes to get a much broader database because we have limited data in situ at the altitudes where aircraft fly that you need for making accurate predictions,” says Boeing emissions technical fellow Steve Baughcum. “Previous published studies [show] that, on average, the models are pretty good but your ability to forecast a specific time and place and altitude is not as good. That’s an area that contributes to the uncertainty in doing something operational to avoid those regions.”

Moore agrees that more research on contrails is required, noting: “It’s a complicated story depending on where the contrail is, whether it’s day or night and whether it’s over water or sand. That’s one of the research topics we’re keenly interested in — understanding at the individual contrail level, is this a cloud that’s warming or is this a cloud that’s cooling, and do we need to do something about that?'”

From GE Aerospace’s perspective, the campaign provided a rare opportunity to test 100% SAF in flight — SAFs are currently permitted to be used up to a 50% blend — and this will help with future engine designs, according to Cassi Miller, a senior staff engineer at the engine manufacturer.

“We don’t have a lot of opportunity to understand emissions at altitude — we do most of our testing on the ground,” says Miller. “We’re really trying to look ahead to our next products and what do those combustors need to do, what does the engine need to be able to run on, and how do we design something that is going to be compatible with future SAFs.”

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All images credited to Boeing