Contrails or ‘condensation trails’ are line-shaped clouds which can be seen in certain altitudes in the skies behind airborne aircraft. These linear clouds are formed by water vapor in combination with soot and sulfur gases that come out of jet engine exhaust.
Source: U.S. Environmental Protection Agency: ‘Aircraft Contrails Factsheet’, September 2000
Q: The sustainability debate has permeated the aviation industry, sparking discussions about genuine efforts versus greenwashing. In the midst of this, what role can pilots play in combating climate change?
European pilots recognise climate change as a monumental challenge, and we are committed to the European Green Deal. Recognising aviation’s integral role as part of the transportation infrastructure, pilots are keen to contribute to transforming aviation toward climate neutrality.
Q: There's a lot of talk about reducing CO2 emissions, but recently, there’s been a buzz about non-CO2 emissions reduction. What does this entail, particularly in terms of contrails?
This means that we need to start talking about the climate effect of contrails. Aircraft influence the temperature of the earth’s atmosphere by emitting CO2 but there are also a number of the so-called "non-CO2 effects" of aviation. Non-CO2 effects include persistent contrail formation.
Contrails reflect radiation, cooling the Earth during the day but potentially warming it at night. A recent study by David Lee1 suggests that despite their cooling effect, the combined total effect could potentially be 2-3 times more than the total aircraft CO2 emissions. This, if confirmed, represents a major additional challenge for the aviation sector to reach climate neutrality.
Avoiding the formation of persistent contrails seems to be a straightforward solution for aviation to halve its climate impact but, unfortunately, it comes with large scientific, operational, and logistic challenges. That is why all involved parties must take part in this discussion, pilots included.
Q: How can we prevent contrail formation?
Contrails form in moisture-supersaturated regions. If aircraft fly through such regions2, water droplets may form on the soot that is part of the exhaust plume under certain conditions. These droplets then freeze into ice crystals and grow from the surrounding moisture that deposits on these crystals, which then form a contrail. Altering flight paths either vertically or horizontally to avoid passing such a region can help avoid persistent contrail formation.
Reducing soot, by decreasing aromatics in fuel, could be a solution. Contrail formation and their lifetime also depends on the number of soot particles in the exhaust. When less soot is present, fewer droplets can form. However, since the same amount of ambient and exhaust moisture needs to be distributed onto fewer particles, the ice crystals generally will be larger and heavier and will therefore sink faster, which makes contrail lifetime shorter. Since lifetime is one of the main factors influencing the warming/cooling effect of contrails we can change this by reducing the amount of soot in the exhaust. This is achieved by reducing the aromatics content in the fuel by blending in more aromatics-free SAF. Flights, which are expected to have a large impact could be fuelled with a higher blending ratio than low-impact flights. This, however, would lead to high logistic challenges for fuel providers at airports that need additional infrastructure.
Q: Speaking of challenges, what other obstacles hinder immediate action?
One of the major scientific challenges in this topic revolves around accurately predicting where and when persistent contrails will form. Several ongoing research projects on securing higher weather forecasting accuracy are underway. However, the results are scientifically not conclusive. This leads to a problem when altering a flight route to avoid contrails turns out to be based on an erroneous prediction: more fuel is burned (on average) for the deviation to avoid contrails, which would not have formed anyway. This means that the negative effect on climate was larger in this case than if no deviating action had taken place. Therefore, the prediction methods should be very reliable before more fuel is burned and more CO2 is emitted in the benefit of preventing persistent contrail formation.
The challenges, however, are not only scientific, but also operational. Contrail-conscious airlines use flight planning software to obtain routings and levels where their flights will not produce contrails. But those airlines operate in a densely populated airspace. For example, the vast majority of air traffic in Europe occupies flight levels between FL300 and FL400, precisely the altitudes where contrails would form. If a contrail-conscious airline avoids a contrail formation, this level will quickly be occupied by some other airline... Contrail avoidance will only prove effective if most airlines participate.
There is also a role for ATC to play: being aware of the position of the so-called Ice Super Saturated Regions (ISSRs) in order to prevent aircraft flying through those areas. This is why a collective effort or a regulatory initiative to address the problem in a systemic manner, will be more beneficial.
Finally, future research projects, such as hydrogen, could also be a potential solution to reduce non CO2 effects. Obviously, this solution relies on future technologies that are currently not available.
Q: How can pilots contribute to contrail avoidance efforts?
Pilots are operational experts. We already have detailed information required for each flight: weight, weather, route, airspace constraints, that we routinely use to optimise the operation and reduce our fuel burn. We also have expertise in adapting to last-minute changes (which should remain as a last resort solution with most of changes ideally happening during the strategic and pre-tactical flight planning phases). With appropriate tools, we can optimise flight plans, reduce and avoid contrails. If there are tactical interventions in-flight, pilots ensure the safety aspect is always respected.
Therefore, high contrail-awareness among pilots is essential for a successful and efficient contrail avoidance scheme.
Q: What recommendations do pilots have for the way forward in contrail avoidance?
Contrail avoidance may be seen by some as the ‘low hanging fruit’ in reducing aviation’s climate impact. This is why we can’t emphasize enough on the importance of not having a hasty approach. It is crucial to avoid making aviation’s climate impact worse by implementing solutions that are not scientifically agreed on. These are the top 5 recommendations from pilots:
- Improve Weather Forecasting: Reliable ISSR prediction and agreed-upon tools for detection of contrail formation are essential before action is taken.
- Develop Climate Functions: Functions accounting for all aspects of contrail formation and avoidance are needed to minimize overall climate impact.
- Ensure ‘No-Regret’ Policies: Hastily implemented solutions might increase fuel consumption without effective contrail avoidance, harming the environment.
- Quantify Jet Fuel Impacts: Understanding how jet fuel quality affects contrail formation is vital.
- Incorporate Flexible Operations: Develop tools for operators and ANSPs allowing safe, strategic contrail avoidance without compromising flight safety.
1 The contribution of global aviation to anthropogenic climate forcing for 2000 to 2018 | Elsevier Enhanced Reader
2 This supersaturation is measured in regard to an ice surface and therefore these regions are called Ice Super Saturated Regions ISSRs
Interview with Florian Kuderna, Captain A320, flying for Austrian Airlines. He is a member of the Austrian Cockpit Association (ACA) & a member of the Environment Working Group at ECA. In parallel, he is also a master student in the department of atmospheric aerosol physics at the University of Vienna.