Fewer contrails: flight trial shows value of targeted detours

Air France tested over the North Atlantic whether the climate impact of its flights can be reduced with different routes. Between February and April 2026, almost 4,300 commercial flights were examined in the European research project CICONIA. For 245 of them, those involved calculated alternatives; on 130, the aircraft actually changed their path. The airline published the results in September; the European air traffic research programme SESAR presented them on 2 October.

For the rerouted flights, the calculated climate impact was 36 percent lower overall. The share attributable to contrails fell by 57 percent. Across all flights examined, however, the reduction in contrail impact was 8 percent. These figures describe calculated climate effects, not a measured change in the Earth's temperature.

A few flights cause a large share of the impact

Within the trial, 25 percent of the contrail climate impact came from just 3 percent of flights. This concentration explains why targeted interventions are of interest: airlines would not have to plan a detour for every flight in order to achieve a measurable contribution. Which connections qualify, however, depends on the atmospheric conditions at the time. A blanket new standard route cannot be derived from the results.

Behind an aircraft, water vapour freezes onto small particles, including soot. In sufficiently cold and humid air, long-lasting contrails can form and spread into thin clouds. According to the Federal Office of Civil Aviation, these clouds have a warming effect on average. Their effect varies with the time of day, the weather and the flight path. Not every visible trail has the same climate balance.

That distinguishes the short-lived effects from carbon dioxide, which remains in the atmosphere over very long periods. Besides contrails, nitrogen oxides and other emissions also influence the climate. Anyone who knows only a flight's fuel consumption therefore cannot reliably read off its total climate impact.

Flight planning needs precise weather data

According to FOCA, a change of flight altitude by just a few hundred metres can already help to avoid sensitive layers of air. This requires precise information on temperature and humidity at high altitude. An area that is poorly forecast can reduce the benefit of an avoidance manoeuvre. If an aircraft leaves its most fuel-efficient altitude, additional CO2 also arises from the higher fuel consumption.

In scheduled operations, traffic management comes on top. A crew cannot change its route on the basis of a climate calculation alone; air traffic control must approve the adjustment. SESAR explicitly names safety and efficiency as conditions for broader application. The trial thus provides experience from real flights, but not yet proof that the same percentage figures can be transferred to the entire international route network.