New Study Finds Lean-Burn Engines Don’t Reduce Aircraft Contrail Formation

April 17, 2026

This image shows DLR's Falcon 20E research aircraft and the Airbus A321neo during joint flight tests to investigate emissions from lean-burn engines. The measurement flights took place as part of the NEOFUELS/VOLCAN project in 2023. (Image: DLR)

The latest findings in atmospheric research show that less soot does not automatically mean fewer contrail ice crystals. Instead, small volatile particles play a crucial role in contrail formation at low soot emission levels.

This has been demonstrated in measurement flights conducted by the German Aerospace Center (DLR) in collaboration with Airbus and CFM International in spring 2023. The findings of the research team were published earlier this month in the journal Nature.

Markus Fischer, DLR Divisional Board Member for Aeronautics, said measurement flight produced a unique dataset that is helping to improve both engine and climate models.

"Such test campaigns are crucial to improve the understanding and modelling of contrails as a function of the various drivers, and to inform our future technological and operational mitigation choices. This is another example of excellent collaboration between research centres and industry which is key to progressing quickly on this complex topic,” said Mark Bentall, Airbus Head of R&T Program.

The view from the cockpit shows DLR’s Falcon 20E research aircraft measuring the contrails of the Airbus A321neo flying ahead. On the left side of the image, the nose mast of the Falcon 20E for wind measurements is visible. (Image: DLR)

Understanding Contrail Formation

Contrail cirrus clouds are a large contributor to aviation's climate impact. These ice clouds form at cruising altitude when hot exhaust gases meet very cold, humid air. Particles from engine exhaust then act as nuclei for ice crystals. Until now, soot particles were thought to control ice crystal numbers in contrails. The new findings mark important progress in contrail formation theory and help to shape the future development of engines and fuel composition.

Modern lean-burn engines significantly reduce soot emissions, as ground tests have shown. This might have suggested that fewer ice crystals would form, thus reducing their climate impact. However, contrails from lean-burn engines had, until now, never been measured in flight. To address this knowledge gap, researchers from DLR and Johannes Gutenberg University Mainz (JGU) joined forces with Airbus, CFM International and modelling teams from the University at Albany and French research institute ONERA (Office national d’Etudes et de Recherches Aérospatiales).

As part of the NEOFUELS/VOLCAN measurement campaign, they conducted the first measurement flights of emissions from a lean-burn engine and the resulting contrails. During chase flights behind an Airbus A321neo equipped with CFM LEAP-1A engines, the DLR Falcon 20E research aircraft measured the exhaust gases and particles under cruise flight conditions. The study also investigated the previously unknown roles of fuel sulphur, organic matter and lubrication oils for contrail formation in the low-soot regime.

Challenging Chase Maneuvers

High-speed chase maneuvers were carried out at an altitude of ten kilometers over restricted airspace above the Mediterranean and the Atlantic. In 15 total flights, the Falcon 20E repeatedly sampled the exhaust plume of the passenger aircraft at distances of 40 to 250 meters (131 – 820 feet). The Falcon also intercepted the fully developed contrails of the A321neo several kilometers downstream. Modified engine control settings by CFM International enabled defined lean- and rich-burn operation, allowing researchers to compare emissions and contrail properties at different soot emission levels. The engines were also operated with test fuels containing varying levels of sulphur and aromatics. The DLR Flight Experiments facility has operated the Falcon 20E for 30 years – its decades of experience were essential for the demanding deployment in this challenging research mission.

The ice particle sensors mounted beneath the wings enable the number and size of ice crystals to be measured. Here: a researcher cleans the mirror of the measuring instrument. (Image: DLR)

Results from flight measurements

The results show that for this engine type, lean-burn engine operations reduced soot emissions by three orders of magnitude compared to rich-burn conditions. Contrail ice crystal numbers remained high, far exceeding the number of measured soot particles. This indicates that soot alone cannot explain contrail formation under low-soot conditions typical of lean-burn engines. Instead, in these tests, researchers observed large numbers of liquid volatile particles forming in the cooling exhaust plume, with concentrations comparable to the measured number of contrail ice crystals.

"The defining moment came when the initial data revealed no soot – but plenty of contrail ice crystals," said Christiane Voigt, Scientific Lead of the project at DLR and JGU. "It immediately became clear that advancing our understanding of contrail formation will be essential for shaping the technological future of aviation."

New findings for climate models

Fuels with lower sulphur content reduced the number of contrail ice crystals, and microphysical simulations by the University at Albany and ONERA successfully reproduced the trends from the observations. The results also show that for ultra-low-sulphur fuels and at low soot emission levels, volatile organic compounds and lubrication oil vapors become increasingly important for new particle formation.

These findings extend the classical theory of contrail formation by introducing additional pathways involving volatile sulphate, organic and lubrication oil particles. Because most contrail climate models do not yet include ice formation on liquid particles, they may underestimate the climate impact from contrails. Incorporating these processes will be essential for improving projections of aviation’s climate impact.

Technologies for Climate-Compatible Aviation

Future mitigation strategies could consider fuel composition alongside combustion design and oil venting architectures. Current emission standards regulate gases and non-volatile particles. The new findings indicate that minimizing volatile particles can also be an important pathway to reduce the climate impact of contrails. Although fuel sulphur content is limited to a maximum of 0.3 percent by mass – while typical levels today are approximately 0.046 percent – further reductions may be needed to significantly limit contrail ice formation. Optimizing lubrication oil venting systems could provide an additional engineering lever for engine developers.

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