Aviation Project A4Climate: How Can Contrails be Avoided?

17 partners from research and industry are investigating within the A4Climate project how the environmental impact of aviation on the climate can be minimized and contrails avoided. The DLR's Falcon 20E research aircraft is being used to capture and measure contrails exactly where they form.

Contrails form at high altitudes when hot aircraft exhaust meets very cold, moist air. The white lines in the sky then become ice clouds, trapping heat in the atmosphere and contributing to global warming. The German Weather Service, the Institutul National de Cercetare-Dezvoltare Aerospatiala "Elie Carafoli"—Incas Bucuresti, the Max Planck Institute for Chemistry, Imperial College of Science Technology and Medicine, Johannes Gutenberg University Mainz, Johann Wolfgang Goethe University Frankfurt am Main, the University of Leeds, the University of Reading, Flightkeys, To70, PNO Innovation Germany, Sopra Steria Group, Tuifly, ETH Zurich, Contrails.org, and Eurocontrol, under the coordination of the German Aerospace Center (DLR), are investigating ways to effectively avoid them. The partners use intelligent flight routes, innovative engine technology, and alternative fuels. For this purpose, they combine satellite data, aircraft and ground measurements, advanced climate models, and a new contrail prediction system. Over the course of the project, 400 commercial flights that aim to avoid contrail formation are being studied. Currently, the DLR is also conducting measurement flights with its research aircraft Falcon 20E, which deliberately follows TUIfly commercial aircraft during contrail observation flights over Germany and Austria.

Testing Measures Under Real Conditions

To test the effectiveness of the measures under real conditions, the DLR is conducting specific measurement flights in collaboration with the German airline TUIfly and the Austrian aviation company Flightkeys. The principle is simple: when possible, flights avoid regions where contrails form. What sounds simple in theory is quite complex in practice—ranging from delays to weather changes, making it not always possible to follow the ideally planned routes exactly. Sometimes a detour also means slightly more flight kilometers—and thus higher CO₂ emissions. The challenge is to develop and test robust processes and solutions under these conditions. To this end, the project team is working on a fully automated data pipeline that provides real-time route recommendations and instant feedback on whether a flight can be planned in a more climate-friendly way. Satellites will later verify whether the strategy actually results in fewer contrails in practice. The net climate balance is also being calculated using models.

Less Soot Equals Fewer Contrails?

In addition to flight planning, the research project is investigating how new engines and alternative fuels affect contrail formation. An important detail: some lean-burn engines emit little soot—and soot particles are key starting points for ice crystals. However, it is still unclear whether less soot automatically means fewer contrails. To test this, the DLR research aircraft Falcon 20E is currently accompanying TUIfly passenger planes equipped with the low-soot lean-burn engines. These flights intentionally pass through regions conducive to contrail formation. This allows measurements to be taken directly in the atmosphere and under real conditions to determine the properties of the contrails produced by these new engines.

For the approximately two-week flight tests, typical routes from Germany to Egypt are being flown. The Falcon 20E, operated by the DLR Flight Experiments Facility in Oberpfaffenhofen, flies about ten kilometers behind selected TUIfly flights. The resulting contrails are measured with precise instrumentation. The DLR Institute for Atmospheric Physics is specifically investigating how soot and volatile particles in the exhaust plume change over a period of up to 30 minutes and influence the contrails. The measurement data is being used to further improve engine and contrail model simulations and to refine weather forecasts. (se)

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