Leeham News and Analysis
There's more to real news than a news release.
Leeham News and Analysis
- Bjorn’s Corner: Air Transport’s route to 2050. Part 18. April 18, 2025
- Natilus: Composite BWB, entry-into-service by 2029, 570 on order April 17, 2025
- China bans Boeing, US parts imports in tariff war April 15, 2025
- Boeing welcomes COMAC competition as Asia-Pacific demand soars April 15, 2025
- How good is the C929? April 14, 2025
Bjorn’s Corner: Air Transport’s route to 2050. Part 15.
March 21, 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.
After covering alternative propulsion concepts to lower CO2 and NOx emissions, we now study air transport’s non-CO2 effects on global warming. Of these, contrails have the largest impact.
Contrails form when aircraft gas turbine engines emit soot particles into low-temperature water vapor-saturated areas in the atmosphere. The soot particles form condensation nuclei, and the developed droplets freeze to ice crystals that form contrails.
Figure 1. The net Radiative Forcing of flights during 2019. Source: The report “Global aviation contrail climate effects from 2019 to 2021” from 2024.
Warming Contrails
Not all contrails contribute to Global Warming. Most contrails don’t last long, with only a few lasting so long that they convert to persistent Cirrus clouds, which change the amount of heat radiation into and out from the earth. Of all contrails, about 2% contribute 80% of the warming effect shown in Figure 1.
The figure shows the net Radiative Forcing, RF (i.e., the climate warming effect) from contrails. The color denotes the net RF in mW/m2 shown on the scale at the bottom of the figure. Observe that the study puts the major areas of warming contrails over Europe, the North Atlantic, and the eastern USA. These are the areas where the conditions for persistent contrails that convert to Cirrus are most common.
Recent research indicates that Global Warming from contrails can be larger than from CO2 (some reports say 60% from Contrails, 35% from CO2 and the rest from NOx and other emissions). Therefore, there has been a lot of interest in recent years in research into contrails’ formation, effects, and how to avoid warming contrails.
The importance of contrails for Global Warming is good news for Air Transport, as mitigating actions are much easier to achieve than the hard struggle to avoid CO2 emission by changing to SAF or new propulsion concepts.
Recent research has constructed methods to predict the parts of the atmosphere where there is a risk of warming contrail formation, using satellite information of the atmosphere combined with weather data. This information has then been used to verify that contrails form in these areas and that aircraft that change their trajectory avoid creating contrails.
Figure 2 shows how such avoidance can be made by changing the altitude of an airliner.
Figure 2. The principal of contrail avoidance. Source: Leeham Co.
The black line is the typical altitude profile of an airliner’s mission. Once the climb is finished at 33,000ft (FL330), the aircraft cruises for an hour at this altitude. When the fuel load has reduced so that a higher altitude is optimal, the aircraft cruise climbs to 35,000ft (FL350), where it stays until the Top Of Decent point is reached and the aircraft descends toward the destination.
The contrail prediction system has pinpointed a risk of warming contrails around 34,000 to 38,000ft along the cruise leg. Therefore, the aircraft avoids the cruise climb and stays at the original cruise altitude of 33,000ft. The non-optimal cruise altitude for the last part of the mission will result in an increased fuel burn.
Many trials have been made since 2021, when the first contrail avoidance trials started in Europe. Results indicate that contrail avoidance has a highly positive effect on Global Warming from air transport. We will look at some recent trials flown in Europe, where the positive impact of the contract avoidance outweighs the extra CO2 generated by a higher fuel burn by a factor of over 10 to one.
The low-hanging fruit in Global Warming mitigation
There is a lot of research and testing still to be done to get definitive knowledge on the scope of Global Warming from contrails, how to predict warming contrail areas reliably, what warming contrail avoidance costs in terms of extra fuel consumption, and how air traffic control shall handle needed route changes.
However, one has learned so far that warming contrail avoidance can be much easier to implement than CO2 emission avoidance and that small route changes can have a substantial climate effect. This is what makes the ongoing research and trials so interesting.
Very interesting that there is apparently an optimal altitude for reducing or avoiding contrail formation. Thanks for
this article.
“The non-optimal cruise altitude for the last part of the mission will result in an increased fuel burn.”
How big a fuel penalty would typically need to be paid?
It very much depends on the amount of avoidance, but typically, it is a few percent. I will give examples from actual trials in a future Corner.
IMU:
The contrail forming region is not static.
with global warming creating more water vapor mass
that rises adiabatic higher until dry the critical height should move upward.
FG: “Airbus brings contrails down to ground level through new study”
“Airbus is taking a down-to-earth approach for its latest study into aviation’s non-CO2 impacts, unveiling plans to replicate the conditions found at cruise altitudes and generate contrails at ground level.
“Running until June 2028, the EU-funded project, called PACIFIC, sees Airbus head a consortium that includes engine supplier Rolls-Royce, German aerospace research centre DLR, fuel supplier Neste, and universities in Finland, Germany and the UK.”
“Recent studies – including the Airbus-backed VOLCAN and ECLIF3 projects – have suggested that sustainable aviation fuel (SAF) can cut contrail production by up to 25% due to lower emissions of soot particles.
“While more data is needed to validate those potential benefits researchers face a challenge to replicate the results obtained through flight testing.”
Airbus and its partners in the three-year study will assess how the composition of different fuels and the combustion process affects soot emissions and contrail formation.
“You can easily measure emissions on the ground – it is much easier than in flight – but it is harder to replicate the temperature, pressure and humidity conditions you find at altitude,” says Thomas Viguier, Airbus innovation and technology specialist.
“To do that, Airbus will use a cold chamber – described by Bentall, half-jokingly, as a “big fridge” – supplied by project partner the University of Helsinki.
“This will be located to the rear of one Rolls-Royce Trent XWB engine on an A350 test aircraft and will cool the exhaust stream and “set the conditions so the water vapour emitted will freeze and generate ice crystals”, says Viguier.”
“Starting with 10 different fuels, including Jet A-1, the team will run a series of combustion tests, both in the laboratory and on test rigs, to evaluate the soot particles produced by each and how different engine power levels affect that.
“It will then select three fuels to take forward to the ground-test phase, examining how different fuel compositions and engine settings influence contrail formation and properties.”
https://www.flightglobal.com/air-transport/airbus-brings-contrails-down-to-ground-level-through-new-study/162395.article
It would seem that the highest contrail being over Europe & eastern US is also the densist aviation area. I’m making an assumption here. Is it just altitude? Are there other areas of the globe with the same aviation density and altitude?
The “Global aviation contrail climate effects from 2019 to 2021” study gives the answers. It has to do with the density of the traffic, but for the North Atlantic also that there is persistent high-altitude cruise there, whereas for the US and Europe, it’s a more mixed mission profile.
The convection streams in the atmosphere also play a role, making pole-closer positions more prone to contrails, whereas equator-closer locations are less prone to creating warming contrails.
From Flight Global
https://www.flightglobal.com/engines/mtu-changes-course-on-switch-project-as-wet-concept-proves-a-damp-squib/162402.article
New aircrafts with better wings with higher L/D one would assume they could fly economically at 50′ ft (FL50) and hopefully fly over contrails forming altitudes. Like the G800 51 000 ft max cruise altitude capability.
At what point do safety regulations start to have an effect? In the event of cabin pressure loss, could an aircraft descend safely from FL50 in the time allowed?
It was OK for the Concorde:
“Concorde’s maximum cruising altitude was 60,000 ft (18,000 m) (though the typical altitude reached between London and New York was about 56,000 ft (17,000 m))”
The days after the British government texting everyone and telling them to go home immediately because of the Covid 19 pandemic seemed to have much bluer skies over southern England.Does anyone actually measure this?How much would have been due to other forms of pollution?
It does seem to have interested some scientists,but I can’t find anything definitive.This is a bit like the one degree cooler in the day,one degree warmer at night on the 10/9/2001( British style)that seems to have now been debunked and no one can make up their minds
A came across this interesting website, which tries to calculate which recent flight paths lead to warming or cooling contrails:
https://map.contrails.org/
If you aggregate this data over year, you probably get the visual shown in the article.
However the specific impact (or lack thereof) for an individual flight is very varied.