Aviation Project A4Climate: How can Contrails be Avoided?

17 partners from research and industry are investigating within the A4Climate project how to minimize the impact of aviation on the climate and avoid contrails. The DLR's Falcon 20E research aircraft is being used, capturing and measuring contrails right where they are formed.

Contrails form at high altitudes when hot aircraft exhaust meets very cold, moist air. The white lines in the sky then turn into ice clouds, which trap heat in the atmosphere and contribute to global warming. The German Meteorological Service, the "Elie Carafoli" National Institute for Aerospace Research and Development—Incas Bucharest, the Max Planck Institute for Chemistry, Imperial College of Science Technology and Medicine, Johannes Gutenberg University Mainz, Johann Wolfgang Goethe University Frankfurt am Main, University of Leeds, University of Reading, Flightkeys, To70, PNO Innovation Germany, Sopra Steria Group, Tuifly, ETH Zurich, Contrails.org, and Eurocontrol are researching how to effectively prevent this under the leadership of the German Aerospace Center (DLR). The partners utilize intelligent flight routes, innovative engine technology, and alternative fuels. To achieve this, the partners combine satellite data, aircraft and ground-based measurements, modern climate models, and a new forecasting system for contrails. Over the course of the project, a total of 400 commercial flights that aim to generate as few contrails as possible are being studied. Currently, the DLR is also conducting measurement flights with its Falcon 20E research aircraft, which deliberately follows TUIfly aircraft during contrail observation flights over Germany and Austria.

Test measures under real conditions

To test the effectiveness of the measures under real conditions, the DLR is specifically conducting measurement flights together with the German airline TUIfly and the Austrian aviation company Flightkeys. The principle is simple: if possible, flights avoid regions where contrails form. What sounds simple in theory proves to be quite complex in practice—ranging from delays to weather changes, making it not always possible to adhere precisely to the ideally planned routes. And sometimes, an alternate route also means slightly more flight kilometers—and thus more CO₂ emissions. The challenge here is to develop and test robust procedures and solutions within these constraints. For this purpose, the project team is working on a fully automated data pipeline that provides route recommendations in real time and immediately gives feedback on whether a flight can be planned to be more climate-friendly. Satellites are later supposed to verify whether the strategy actually produces fewer contrails in reality. The net climate balance will be additionally calculated using models.

Less soot equals fewer contrails?

In addition to flight planning, the research project is investigating how new engines and alternative fuels change contrail formation. Important information: some lean-burn engines emit little soot—and soot particles are important 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 aircraft equipped with soot-free lean-burn engines. The flights deliberately pass through regions conducive to contrail formation. This allows direct measurements 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 travel routes from Germany to Egypt are flown. The Falcon 20E, operated by the DLR Flight Experiments Facility in Oberpfaffenhofen, flies about ten kilometers behind selected TUIfly flights. During this, the resulting contrails are measured with precise instrumentation. The DLR Institute for Atmospheric Physics is specifically studying how soot and volatile particles in the exhaust plume change over a period of up to 30 minutes and how they affect the contrails. The measurement data is used to further improve engine and contrail model simulations and to refine weather forecasts. (se)