Condensation trails, or contrails, are ice clouds that form as a result of the mixing of cold, humid air with aircraft engine exhaust plumes. They influence Earth's energy equilibrium by altering global cloudiness and interacting with both incoming solar radiation and outgoing thermal radiation. Contrails, especially at night, have been shown to result in net positive radiative forcing (RF), thereby contributing to climate change. Studies indicate that contrails and contrail cirrus may be the largest contributor to aviationattributable RF, potentially exceeding contributions of aviation carbon dioxide (CO2) emissions.
Reductions in contrail RF could be achieved by contrail avoidance strategies which should reduce aviation-attributable warming. The strategies would reroute traffic in the planning phase and eventually adjust flight levels in-flight so that flights do not cross the contrail prone areas, either laterally or vertically. Mitigation strategies very often need to be offset against additional fuel burn and hence slightly increased CO2 emissions.
Realistically, it needs to be shown how many warming contrails can be avoided to minimise the climate effect and the feasible reduction of contrails still needs detailed research. Areas of research include determining how well meteorological conditions can be forecasted in advance, and with sufficient quality, so that the air traffic management system can react with precision to propose vertical deviations. Another research area is to determine whether these forecasts and calculations should take place at the airline planning stage – by orchestration with, for example, the Network Manager (NM) – or with tactical vertical deviations by ATC, or pilots acting in-flight, or all of these. Further work is also needed to understand the impact of contrail avoidance measures on the air traffic system, potential costs and which systems and procedures are impacted and will need investment.
Since 2020, MUAC has participated in the research programme for contrail prevention and has been investigating operational contrail mitigation.
MUAC, in collaboration with the German Aerospace Center (DLR) as a scientific partner, conducted the first large-scale operational mitigation trial throughout most of 2021. The trials were carried out during the night across all MUAC airspace. Satellite images confirmed that contrails can be effectively prevented through relatively simple measures. However, the lessons learned from this world-first operational trial highlighted several areas for improvement, including the need for enhanced weather predictions, more automated decision-making processes, better tool integration, requirements for capacity handling as well as climate indicators for success.
In 2023, MUAC conducted the first large-scale realtime simulation, comparing two operational concepts: mitigation by airlines through flight planning around contrail areas and tactical vertical clearances. The insights are that ATCOs prefer planned against tactical contrail avoidance. Capacity will be impacted when blocking flight level bands at moderate traffic and this becomes impossible at high traffic loads.
In 2024 the German parliamentarian task force AKKL (ArbeitsKreis Klimaneutrale Luftfahrt - Climateneutral Aviation Working Group) decided to conduct an operational live trial with all German airlines with the objective of at least 100 contrail avoiding flights. Airlines decided on contrail avoidance routes based on new weather predictions including state-of-the-art climate functions and support from their automated flight planning systems. MUAC participated and conceived the operational procedure for dispatching the contrail flight plans amongst NM and other air navigation service providers (ANSPs). The trial was conclusive; the airlines gained knowledge and insights into the difficulties and costs of contrail avoidance. MUAC is looking to implement a standard procedure for these kinds of contrail avoidance flights.
From 2023 onwards MUAC developed a ground camera system to monitor the accuracy of weather forecasts and contrail mitigation. For the first time, contrail detection in imagery was successfully achieved using machine learning, improving previous research that applied computer vision. A complete small monitoring system, based on several cameras, was developed, marking another world-first event. Currently, image detection is being improved with multiple classifications of contrail ages, which will help to understand the evolution of contrails and, hopefully, enhance decision-making processes.
Further trials are ongoing. The next significant step is testing contrail prediction tools and satellite image recognition from Google, both based on convolutional neural networks (CNN) which utilises a special form of Artificial Intelligence (AI) that can learn to recognise patterns and features in images. This will involve another up-scaled trial over several months.
The MUAC contrail programme has a number of key threads.
First, there is the development of technical systems and operational procedures that prepare operations for contrail mitigation. The technical systems are developed to support operational staff in their decision-making whether, when, where and how contrail mitigation can be implemented. The technical system ingests various traffic data sources correlating with multiple weather forecasts. It proposes the best areas and time slots to supervisory staff, and provides specific advice to air traffic controllers.
Then there is the implementation of live and background trials, which are continuing.
For example, the ongoing “Google trial” uses predictions and detections based on satellite imagery and machine learning, which is very different from traditional weather forecasts. Google heard about the MUAC trial in 2021 and asked how it might be able to support. This led to a conversation as to whether the analysis of satellite imagery could be accomplished in an automated way to get quick feedback about successful contrail mitigations. This was the starting point of a fruitful partnership. Google has since developed contrail detection from satellite images and predictions of contrail areas with machine learning, which are being used in flight trials this year, with some selected flights during daytime and then a switch to night trials during summer and autumn 2025.
So-called ‘silent’ trials – studies that are conducted by observing, but not interacting with the live environment – will also take place as part of the CICONIA research programme led by AIRBUS. The consortium is also working on the concept of an integrated and collaborative decision-process within Network Manager operations, including demand-capacity balancing concepts for contrail mitigation.
The MUAC Contrail Server programme, with all its decision support and camera systems, is also underway. Major progress is being made at MUAC to automate the decision support function with a new set of tools that decide on the significance of the meteorological situation to automatically support the work of operational capacity managers, supervisory staff and ATCOs. It comprises developments and research for the camera system with the contrail detections and machine learning.
MUAC is also working on developing contrail mitigation measure key performance indicators with international working groups.
It is of paramount importance that operational staff be involved from the very beginning in the “design” phase of the trial. To do so, a Human Factor Analysis (HFA) with representatives from the operations room is performed to address issues such as roles and responsibilities, procedures, “human in system” operations, training needs and so on.
HFA conclusions are then used to build systems and procedures along with organising the training and briefing packages required for operational staff to feel confident in the changes induced by the contrail prevention research trial.
Finally, an important aspect of the integration into daily operations is the operational support and followup by the project team once the trial has started.
One of the big challenges associated with contrail prevention research is uncertainty. It is true to say that the scientific knowledge regarding the non-CO2 effects of aviation has advanced but there remain significant uncertainties in forecasting contrail formation and assessing their climate impact. This means that it is difficult at this stage to define any meaningful tradeoff between reducing contrails and increasing fuel consumption.
With a focus being naturally put on efficiency, operational actors need some degree of confidence in the benefits that altering aircraft trajectory for contrail prevention will bring. Another potential challenge will be the impact on the workload and the sector capacity. Should contrail prevention measures require large portions of airspace being vacated for a significant amount of time, the impact on air traffic controllers’ workload and on sector capacity will need to be assessed.
With much work still to be done, MUAC is dedicated to supporting and advancing research into the impact of contrails and to sharing the science-based findings with the industry.
