Climate strategy put to the test – reducing contrails

To this end, the researchers are combining satellite data, aircraft and ground-based measurements, advanced climate models and a new contrail prediction system. The project will analyse 400 scheduled flights, selected from those expected to produce as few contrails as possible. DLR is also currently conducting measurement flights with its Falcon 20E research aircraft, which deliberately follows TUIfly commercial aircraft during contrail-observation flights over Germany and Austria.

Unexpected challenges

To test the effectiveness of these countermeasures under real-world conditions, DLR is conducting specific measurement flights in collaboration with the German airline TUIfly and the Austrian flight planning services provider FLIGHTKEYS. The principle is simple: whenever possible, flights are routed outside the regions where contrails form.

In practice, however, it becomes clear how complex this can be. Delays and weather changes make it difficult to stick precisely to ideal, planned routes. And sometimes an alternative route also means a few more flight kilometres – and thus more carbon dioxide (CO₂) emitted. The task here is to develop and test robust processes and solutions under these constraints.

In addition to flight planning, A4CLIMATE is investigating how new engines and alternative fuels affect the formation of contrails. One particularly exciting possibility is the fact that some 'lean-burn' engines emit extremely low levels of soot – and soot particles are key nuclei for ice crystal formation. However, it is still unclear whether less soot automatically means fewer contrails.

To test this, DLR's Falcon 20E research aircraft is currently trailing TUIfly passenger aircraft equipped with innovative, low-soot lean-burn engines. The flights deliberately pass through regions that are prone to contrails. Measurements are taken directly in the atmosphere and under real conditions to determine the properties of the contrails produced by these new engines: "We want to understand how much global warming can be reduced when aircraft are more modern and smarter," explains project manager Christiane Voigt from the DLR Institute of Atmospheric Physics.

For the roughly two-week flight campaign, typical flight routes from Germany to Egypt are being flown. The Falcon 20E, operated by the DLR flight facility in Oberpfaffenhofen, will fly approximately ten kilometres behind selected TUIfly flights. The team measures the resulting contrails using highly precise instrumentation. Researchers at the DLR Institute of Atmospheric Physics are investigating in particular how soot and volatile particles in the exhaust plume change over a period of up to 30 minutes and how they influence contrails. The measurement data are used to further improve engine and contrail model simulations and to refine weather forecasts.

Climate-compatible flying – but realistic

Contrails form at high altitudes when hot exhaust gases meet very cold, humid air. These inconspicuous lines in the sky turn into ice clouds that trap heat in the atmosphere and contribute to global warming. Their climate impact is comparable to the total CO₂ emitted by aviation. The EU therefore aims to systematically monitor these non-CO₂ effects.

A4CLIMATE is providing the first comprehensive dataset on how contrails can be avoided, reduced or made less damaging to the climate. The project is intended to establish a scientific basis for new EU regulations and to equip airlines with practical tools.