While estimates of the extent to which the use of sustainable aviation fuel (SAF) reduces CO2 emissions in aircraft can vary, aerospace manufacturers generally cite an 80 percent lifecycle carbon emissions benefit for 100 percent SAF. Scientists know less about how the contrails produced by SAF emissions differ from those generated by kerosene, however, prompting Boeing to partner with United Airlines and NASA to study the phenomenon with its latest EcoDemonstrator aircraft.
Contrails, or the persistent condensation trails produced when airplanes fly through cold, humid air, can trap heat in the atmosphere, some of the latest research suggests. To learn more, Boeing in early October began flying a CFM Leap-1B-powered 737 Max 10 destined for United Airlines on 100 percent sustainable aviation fuel (SAF) and conventional fuel in separate tanks, alternating the fuels during testing. Studying multiple fuels with the same engine allows researchers to differentiate the emissions reductions resulting from advanced engine technology from those resulting from advanced jet fuel chemistry.
Although the sample of results from last month’s testing remains too small to draw any conclusions, researchers from Boeing, NASA, GE Aerospace, and German aerospace research center DLR expressed optimism over the observations from the first three weeks of trials. Speaking late last month from Boeing offices in Seattle, DLR research team lead Christiane Voigt explained some of the science behind SAF’s effect on the production of contrails.
“These sustainable aviation fuels have a lower aromatic content and these are precursors to the soot particle emissions and the soot particles are the precursors for the ice crystals and contrails,” she noted. “Science tells us that warming from contrails might be as large as the warming from CO2 emissions.”
NASA’s DC-8 Airborne Science Lab flies behind the 737 to measure emissions produced by each type of fuel and contrail ice particles. Based at NASA’s Armstrong Flight Research Center, the DC-8 incorporates a suite of sensors and data systems adaptable to specific missions or instruments. Nicki Reid, operations engineer/DC-8 mission manager for the NASA Airborne Science Program, explained that the DC-8 flies between four and five miles behind the EcoDemonstrator while the research team alternates from one fuel to another in the Max 10.
“We’ll fly through for maybe 20 or 30 minutes sampling on jet-A, and then as we’re coming back through, we’ll make a turn and fly through that exact same position in the air so that we’ll have the same humidity, the same altitude, we’ll have the engines running at the same power and we’ll direct the Boeing team to switch to the SAF so that we can determine the actual difference between both of those fuels,” she said.
The most recent EcoDemonstrator flights build upon testing in 2021 and 2022 on a 737 Max 9 and a 777-200ER, respectively. Analysis of the resulting data confirmed that the use of paraffinic SAF yields “dramatic” particle emission reductions, the extent to which depending on the chemical composition of the SAF used.
“We know from ground measurement tests that we reduced soot emissions when we go with paraffinic fuel relative to higher aromatic fuels,” explained Boeing emissions technical fellow Steve Baughcum. “And because it was a synthetic fuel and never really had any sulfur in the feedstocks, it’s a low-sulfur fuel for us so that we expect both to reduce soot emissions and the volatile aerosols that are being produced by the aircraft. So we can basically explore how those reductions change the contrail properties that we measure in the aircraft.”
Funded by the FAA’s Ascent Center of Excellence, the SAF for the most recent studies comes from World Energy in California. While Boeing has set a target date of 2030 to begin delivering aircraft compatible with 100 percent SAF, company vice president of product development Mike Sinnett explained that the transition will require examination of the long-term effects of using the fuel on each aircraft component that touches fuel.
Sinnett added that researchers know that running 100 percent SAF in aircraft that have had long-term exposure to jet-A could result in seal damage. SAF lacks the aromatics contained in jet-A, which helps seals swell to prevent leakage. Although they know running 100 percent SAF on a new airplane not exposed to conventional jet fuel will not affect the seals, the long-term effects on the entire fuel system and other parts of the airplane such as the wings and engines need further study.
“The other issue is the density differences change how you gauge the fuel,” noted Sinnett. “We account for that in the flight test environment, but certainly over the longer term we’ll have to make sure that our fuel gauging systems or fuel quantity systems can account for a much broader range of densities that we see today in conventional jet fuel.”