Flight tests in Europe have shown that long-lasting contrails can be avoided by slightly changing flight altitudes.
The flights were conducted as part of a project run by research agency the German Aerospace Center with ATC organisation EUROCONTROL Maastricht Upper Area Control Centre (MUAC).
The avoidance procedure that was tested could help reduce the climate impact of aviation and the results have been published in the journal ‘Meteorologische Zeitschrift‘ (in English).
In 2021, there was considerably less air traffic than in previous years due to the COVID-19 pandemic. A research team from the DLR Institute of Atmospheric Physics and MUAC used this situation as a basis for analyses of contrails in the upper airspace over north-west Germany and the Benelux countries.
Th eprocedure was used every alternate day, when the weather forecast for the flight altitudes used by regular air traffic suggested that long-lasting contrails would form. The flights were diverted up or down by 2000ft (660m).
The researchers then used satellite images to check if the long-lasting contrails had formed. The flights on the days when air traffic was not subject to the procedure served as a reference. The research team was able to show that long-lasting contrails occurred less frequently.
Avoiding the climate impact of contrails
Aircraft usually emit at altitudes where their emissions have a different effect than on the ground and have a particularly large climate impact. The non-carbon-dioxide effects of air transport include, in particular, contrails and contrail cirrus – that is, ice clouds.
The non-carbon-dioxide effects of aviation can have both a warming and a cooling effect, with the warming effect predominating. As they only have short atmospheric lifetimes compared to carbon dioxide, the effects are not evenly distributed in the atmosphere. Their effect on the climate therefore depends heavily on numerous parameters – geographical location, altitude, time of emission, position of the Sun and weather conditions.
This opens up the possibility of reducing the climate impact of aviation by choosing suitable flight routes and altitudes (referred to as climate-optimized flight trajectories). However, the use of climate-optimized flight trajectories generally leads to increased carbon dioxide emissions. As such, the optimised flight trajectories must be selected in such a way that the overall climate impact of the flight in question is reduced.
Important prerequisites for the possible introduction of such climate-optimized flight trajectories include that the climate impact of individual flights must be able to be predicted so reliably by the weather services that a diversion of air traffic actually leads to a reduction in the climate impact.
The climate impact of non-carbon-dioxide effects must also be integrated into the operational tools and processes for planning flight trajectories. This requires a system that calculates the climate impact of a flight with sufficient accuracy within the time available for flight planning.
In addition, when flights are rerouted in higher airspace, it must be ensured that authorized air traffic can continue to be handled safely, in an orderly way and without delays, because taking climate aspects into account when selecting trajectories can lead to capacity bottlenecks in the airspace.
In contrast to the other non-carbon-dioxide effects, in the case of long-lasting contrails it is possible to control whether more or fewer contrails are formed. This requires sophisticated statistical methods. The DLR and EUROCONTROL/MUAC team used this to prove that the avoidance of long-lasting contrails actually works in real air traffic. This is a successful step on the way to climate-friendly air transport.