By Bjorn Fehrm

April 11, 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, Figure 1.

In the last Corner, we described encouraging results from airline flight trials with warming contrail avoidance. What is required to move from trials to warming contrail avoidance for regular flights?

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).

Warming Contrail Avoidance as Part of Regular Airline Operations

The graph of the TUYfly trial flight last week (Figure 2) shows that, compared with regular flights, warming contrail avoidance requires additional steps in flight planning and execution.

We shall emphasize that not all flights generate warming contrails. During TUIfly’s trial week in February, 84 of 303 flights from and to Germany had to change flight paths to avoid generating warming contrails.

Figure 2. An analysis of a TUIfly flight during the German trials. Source: DLR.

Flight Briefings and Planning

Today’s flights have a weather briefing that informs the pilots of expected weather on the route and forms the basis for the route flight planning (winds, temperatures, fronts). This briefing is usually then converted to a flight plan by a dedicated flight planner specialist in the airline called a dispatcher.

The weather briefing must be complemented with data about where the risk of warming contrails is high and what options are available horizontally and vertically to avoid such areas. Different research organizations currently generate such data. This work must progress from a trial basis to a permanent part of preflight weather briefings worldwide.

For this to happen, the research methods used today must be verified as an adequate, scaleable solution. Then, a reliable and rational method must be developed to make this part of standard weather briefings for airline operations. In addition, the flight planning systems used by the airlines must support inputting such data and the generation of optimized flight plans around risky contrail areas.

Air Traffic Mangament

The other important change needed to enable warming contrail avoidance is an Air Traffic Management (ATM) system that can handle the more volatile flight plans that airlines will issue as these avoid generating warming contrails.

The present ATM systems are highly stressed during the busier summer period. This is when more flexible route and altitude planning by the airlines will be the hardest to handle.

Present flight plans approved by ATM follow certain standard patterns developed over time to enable the traffic volumes between busy cities during summer.

To avoid warming contrails, flights must file different flight trajectories, sometimes both horizontally and vertically, as shown in Figure 2. The trails by German airlines during the first quarter were done in cooperation with ATM during the winter schedule. It gave the ATM the possibility to allow the more flexible flight planning needed. Allowing this for more airlines from more countries and at a larger period of the year will mean that ATM must upgrade to a more integrated and modern architecture.

For the US, this means moving faster into ATM NextGen. For Europe, it means stopping the national blocking of a Single European Sky (SES) and converting to the Digital European Sky, based on work done in the SESAR projects.