By Zach Wichter
November 4, 2023

A science experiment in the sky attempts to unravel the mysteries of contrails

IN THE SKIES OVER MONTANA ― It was a six hour flight that officially didn't go anywhere, but could help usher in a new chapter of aviation sustainability.

A high altitude series of science experiments over the course of a few weeks in October tested different kinds of aviation fuels and studied the effects of contrails – those thin, wispy cloud-like lines you sometimes see behind planes. USA TODAY got a front-row seat to the cutting-edge research a few days before the mission concluded on Nov. 1.

The flights were carried out by NASA’s DC-8 flying laboratory out of the Armstrong Flight Research Center in California while it tailed a brand-new Boeing 737 MAX 10, which will eventually join United Airlines’ fleet.

USA TODAY joined one of the DC-8 flights from Paine Field, north of Seattle, at the invitation of NASA, Boeing and United and saw a promising glimpse of what eco-conscious travelers might be able to expect in the future, if the research delivers on its promise.

“We think from the modelers that the contrails have a bigger climate impact today than all the aviation CO2 that’s been emitted in the last 100 years,” Rich Moore, principal investigator from NASA Langley said before the flight took off. “We’ve accumulated a certain amount of CO2 in the atmosphere and that’s a greenhouse gas, so there’s some warming associated with that … (But) the modelers are telling us that those clouds are also having a warming effect overall and that the effect is bigger.” 

How do contrails affect the environment?

It’s not totally clear, and that’s part of why this research is important. Scientists do have a basic understanding that contrails (short for condensation trails, which are formed partly around the tiny soot particles that jet engines emit), like clouds, trap heat and reflect it back toward the Earth’s surface, but the extent and severity of the contrail effect isn’t fully understood. The research is multi-faceted. It's looking at what conditions contribute to contrail formation and their composition, and that knowledge can be used to help better understand their effect on the climate as well as inform development of new aviation technologies.

What was the flight like?

NASA's DC-8 trailed Boeing's 737 MAX 10 and wove in and out of its wake for about six hours on Oct. 23. That allowed sensors onboard the flying lab to capture emissions data from the Boeing's engines, as well as compare that airflow to the surrounding environment. Scientists analyzed the emissions from the Boeing aircraft as it alternated between 100% sustainable aviation fuel and low-sulfur traditional petroleum-based type A jet fuel, with the help of an array of sensors onboard the DC-8.

Boeing's 737 MAX fleet uses a new generation of engines, the LEAP-1B, manufactured by CFM, which are highly efficient. The engines emit about 15% less carbon dioxide than earlier generations of 737 power plants according to SAFRAN, one of the companies in the CFM consortium

How are contrails being studied?

If an observer on the ground had been able to see through the cloud cover, they would have seen two planes circling for a few hours at 35,000 feet or so above Montana. Onboard, however, the passengers and cargoes they carried are the vanguard of aviation’s more sustainable future.

“We want to be able to sample the emissions from the aircraft ahead of us in order to understand, at cruise conditions, what are the particles and gasses coming out of the engines,” Moore said. “We want to understand the effect of those particles on the contrails under a variety of different environmental conditions.”

A view inside the cabin of the NASA DC-8 Airborne Lab.    Zach Wichter/USA Today

The technology already seems to be achieving at least some of its goals according to the researchers.

“The lean burn combustors on the LEAP-1B are incredibly low-sooting,” Moore said, explaining that engine particle emissions are a crucial part of contrail formation. “It’s observable at cruise, the absence of soot particle emissions from this engine.” 

How can the industry reduce contrails?

Moore said that during another flight, a chase plane was able to capture photographs showing the DC-8, which first entered service in 1969, producing contrails, while the new Boeing 737, with its ultra-efficient CFM LEAP-1B engines and more streamlined design, had no such streamers behind it. Boeing and NASA did not make those photos available for publication.

A full analysis of the test flights’ results is still at least a few months away, but researchers onboard and other industry partners are excited about what they’ve seen so far.

“It is a super wonky and complicated science challenge but I’m excited for the amount we’re learning,” Lauren Riley, United Airlines’ chief sustainability officer, told USA TODAY. “The industry is committed to doing the right thing.”

According to Moore, sustainable aviation fuel combined with highly efficient engines made the formation of contrails less likely than with earlier aviation technologies.

It's complicated, though.

Contrails are more likely to form when airplane engines have higher emissions, but that's not the only factor involved in their formation. Environmental factors like humidity play a role, too. Cleaner jet fuels and more efficient engines make contrails less likely to form, but they can still show up even with the most cutting-edge airplane and engine technology.

The experts emphasized that cleaner fuels are an important goal for the industry's sustainability, and that they have an added benefit of making contrail formation less likely, which probably further reduces the warming effect of flying.

“We’re still seeing contrails under some conditions but we’re having to tease out the effects of the fuels and the environmental conditions and that will be something the team will be working on over the coming weeks and months.” Moore said. “We have our work cut out for us. The emissions are just that low, we’re measuring very small signals above the atmospheric background. The fact that it’s that hard is a good thing for this technology and these fuels going forward, even if it makes our job as scientists a little harder.”

Contrails are visible behind both NASA's  DC-8 Airborne Lab and Boeing's 737 MAX 10 ecoDemonstrator  during one of the test flights. Boeing

Will contrails research change passenger flights?

It probably won't mean anything noticeable to flyers right away, but the goal of this research is to make travel better for the environment. The overall aim of the airline industry is to be carbon neutral by 2050, but understanding contrails and their impact is part of a broader sustainability push by the aviation sector.

Many of those onboard the NASA flight said the research will inform the next generation of airplane technology and will help the industry move toward its sustainability goals.

“This is really allowing us to tune our technologies, to make the right technology choices to put out the products that are really maximized for sustainability, that are really best suited to where the market needs and wants to go,” David Ostdiek, a representative from GE, which is part of the CFM engine manufacturing consortium said during a post-flight briefing.

Longer term, this research will also likely contribute to better contrail forecasting tools and could lead to a better understanding of the tradeoffs between fuel efficient routings, which reduce CO2 emissions, and “clean” routings, which reduce contrail formation.

“How and where do contrails form but secondarily, what are the additional benefits of sustainable aviation fuels in mitigating (the formation and effect of contrails,)” Riley said. “There’s so much that’s still unknown and we have to take those steps forward rooted in science.” 

Zach Wichter is a travel reporter for USA TODAY based in New York. You can reach him at zwichter@usatoday.com