By Bjorn Fehrm

April 4, 2025, ©. Leeham News: We do a Corner series about the state of developments to improve the emission situation for Air Transport. We try to understand why development has been slow.

We now examine the non-CO2 effects of Air Transport that contribute to global warming. Of these, contrails have the largest impact.

Contrails form when aircraft gas turbine engines emit soot particles into water vapor-saturated areas with low temperatures, where the soot particles act as condensation nuclei and the droplets freeze into ice crystals.

The report from Lee et al. (2021) lists non-CO2 effects as more important for Global Warming from Air Transport than CO2 emissions from burning hydrocarbon fuels, Figure 1.

Figure 1. A summary of the CO2 and non-CO2 Effective Radiative Forcing (ERF) contributions from Air Transport. Source: The report “The contribution of global aviation to anthropogenic climate forcing for 2000-2018” by Lee et al. (2021).

Contrails Avoidance Trials

The German Arbeitskreis klimaneutrale Luftfahrt (AkkL) recently conducted a contrail avoidance trial with airliners from Lufthansa, TUIfly, Condor, and DHL, with support from the Deutsches Zentrum für Luft- und Raumfahrt (DLR, German equivalent of NASA) and other partners for the prediction of contrail areas and flight planning (Figure 2).

Figure 2. Partners in the Arbeitskreis klimaneutrale Luftfahrt (AkkL) flight trials to avoid warming contrails. Source: AkkL.

In total, more than 100 contrail avoidance flights were made. Feedback will be provided to the airline and its pilots for the flights. A typical flight by a TUIfly aircraft is shown in Figure 3.

The feedback from the DLR shows the original flight path (left graph), with its yellow and red colours indicating where it would have passed through warming contrail areas, and the actual flight path in green.

Figure 3. A feedback analysis of a TUIfly flight during the trials. Source: DLR.

The right graph shows how the flight used altitude changes to avoid the contrail areas. The evaluation showed that the warming by contrails was avoided on the planned climate optimized and the actually flown flight path compared to the originally planned flight. Hence, the flight had a significantly reduced warming effect.

Figure 3 shows just one flight and the evaluation of the impact of the changed flight path. At a contrails conference last week, TUIfly presented the results of the third week in February, when TUIfly aimed to avoid as many warming contrails as possible on flights from and to Germany between February 17 and 23.

TUIfly flew 303 flights this week, and of the 177 that would have generated warming contrails, 84 were specifically planned for diversion to avoid doing so.

The net gain in CO2e (the reduction in CO2e from not generating warming contrails, minus the extra CO2 generated from burning 2.1% more fuel due to the change paths) calculated by TUIfly’s flight planning system and research partners was 15,664 tonnes.

The 15,664 tonnes of CO2e represents the CO2 emission equivalent of 5,000 tonnes of Jet fuel burnt. As the typical domestic flight of 800nm for the A320/737 MAX 8 has a fuel burn of 4 tonnes, this represents a warming effect improvement of the CO2 emission from 1,250 flights.

Conclusions

The science around warming contrails is complicated and not yet fully accepted/verified by all parties. However, the evidence from made trials shows that there is enormous potential to reduce the Global Warming effect from Air Transport, and that the measures that need to be taken are modest, both in terms of effort and the time required for change.

The contrail avoidance flights were made on a winter schedule, when air traffic is lower, and ATC has an easier task of handling requests for changes to flight plans and reroute requests from pilots. It will be tougher during the summer schedule, when the skies are saturated with traffic.

We will conclude the series about contrails next week, when we will delve into more detail about the changes required on the airline side, including their dispatch, weather, and flight planning tools, as well as air traffic control, to reduce warming contrail generation from Air Transport.