There is a cruel irony that one of the most visible parts of aviation’s impact on the ­environment has been one of the hardest to pin down in terms of its exact nature and the action required to mitigate its climate effect.

Contrails form the biggest part of the bucket of non-carbon dioxide (CO2) environmental impacts resulting from ­aviation, alongside nitrogen oxide, sulphur oxide, and soot particles.

But while contrails can often be seen with the naked eye, the science behind their formation and environmental impact is far less clear.

“The formation, persistence, and spread of contrails into cirrus clouds are subject to significant uncertainties,” writes KPMG, in a wide-ranging report on aviation’s non-CO2 impacts commissioned by the UK Department for Transport and co-produced with Cranfield University and SATAVIA (now part of GE Aerospace unit Aerospace Carbon Solutions).

“Factors such as the exact composition of aircraft exhaust, atmospheric humidity, and ambient temperature play critical roles in contrail development,” it adds.

Contrails are formed when an aircraft passes through Ice ­Super Saturated Regions (ISSRs), where water ­droplets emitted from the engine freeze. Around one in 20 flights is estimated to generate persistent ­contrails, which can last for several hours.

Persistent contrails can develop into cirrus clouds, which have a warming or cooling effect on the planet – either trapping hot air in, or reflecting sunlight away. While there is broadly consensus that there is a net warming effect from contrails, particularly at night when there is no sun to reflect heat away, agreement is harder to find on the exact impact and timeframe over which that should be considered.

SCIENTIFIC PICTURE

Speaking during a panel debate on the issue of ­contrails during the Royal Aeronautical Society (RAeS) Sustainability Conference in June, Mark Stettler, professor of transport and environment at Imperial College London, said that despite the ­uncertainties there is a clear scientific picture.

“It is true that there are remaining and persisting uncertainties in the overall magnitude of the ­effect from contrails,” he says. “But it’s also true the ­uncertainty bar – even if you take the lowest point on that – is still a significant part of aviation’s ­climate impact.

“The opportunity is very significant,” he adds. “If we were to be able to avoid forming contrails then we would get rid of [not only] a very significant proportion of aviation’s climate impact, but also a very significant proportion of the earth anthropogenic impact on the climate.”

While CO2 effects have an accumulated impact, the short lifespan of contrails means the warming effect would be wiped out if they could be avoided.

Targets are in place to reach net-zero carbon emissions by 2050, along with European mandates on sustainable aviation fuel (SAF) use, but so far there is limited regulation in place for non-CO2 emissions.

However, since the start of this year, a requirement for airlines to report their non-CO2 emissions was added to the EU’s Emissions Trading System (ETS). Initially mandated on all flights within the European Economic Area (EEA) – as well as Switzerland and the UK – airlines must provide the data under the Monitor, Report & Verify programme.

That will be expanded to flights from the EEA to third countries from 2027, although airlines can ­p­rovide that data on a voluntary basis before then.

That regulators in Europe are looking at non-CO2 emissions is unsurprising, given the EU’s track record of pushing environmental legislation and since Europe has relatively high levels of contrail formation.

Airline bodies though flag the risk of regulating without further understanding of the issues.

In a position paper, IATA says including non-CO2 provisions in the EU ETS or other such schemes is “premature”. It argues that in the absence of “accurate measurements and commercially available solutions”, to do so risks creating market distortions, adding operational complexity, reducing connectivity and producing negative climate-related trade-offs.

DEEPER UNDERSTANDING

IATA, which has been calling for “urgent action” to deepen understanding into contrails and their impact, is partnering with various stakeholders to research how to monitor and report non-CO2 emissions.

One moving part in the analysis is understanding what improvements in aircraft and engine technology, and a move to different fuel types like SAF, mean for non-CO2 emissions.

Industry partners have come together on several projects that have included research to better understand the impact of SAF on contrail formation. These include Boeing’s second Explorer Ecodemonstrator, a 737 Max 10 for United Airlines, in late 2023, and the ECLIF3 and VOLCAN flight test projects on Airbus narrowbody and A350 aircraft respectively.

Airbus says indications are there is an up to 25% reduction in ice-crystal formation from using SAF, but more research is needed. It has now teamed with engine manufacturer Rolls-Royce, German aerospace research centre DLR, SAF specialist Neste, and several universities on the EU-funded PACIFIC project. This will further study the impact of fuels and the engine cycle on non-CO2 emissions.

The airframer, which remains an advocate for hydrogen power in the long term, has used a hydrogen-powered modified glider to study contrails produced by these propulsion systems. It hopes to report data from the Airbus UpNext Blue Condor project this year.

Other recent contrail research initiatives include by GE Aerospace, which was a partner on both the Boeing Ecodemonstrator and the ECLIF and VOLCAN Airbus projects, last November teaming with NASA and the DLR on the contrail optical depth experiment (CODEX) project. These flights involved NASA’s Gulfstream III research aircraft trailing GE’s 747 flying testbed and scanning its wake with LiDAR sensors.

“Teams were able to collect 3D imaging on ­contrails from all four CF6 engines using LiDAR at a ­variety of distances, as well as data on the surrounding ­atmospheric conditions to assist in calibrating ­predictive models,” says GE.

“More detailed LiDAR imaging helps improve ­understanding of how contrails form and behave over time. Also, the optical depth can be used to estimate their net warming effect,” it says. “When combined with the detailed atmospheric data gathered by onboard sensors and radio drops, the accuracy of contrail predictive models is improved.”

GE adds that learnings from CODEX are being applied to new combustion and engine technologies in its RISE programme to reduce non-CO2 emissions.

There also has been ongoing research and trials around contrail avoidance, particularly through ­altering flight altitudes.

Eurocontrol’s Maastricht Upper Area Control Centre (MUAC), which manages the upper airspace over ­Belgium, Luxembourg, the Netherlands and ­northwest Germany, was the first to carry out a ­large-scale operational contrail mitigation trial in 2021, conducted during the night across all MUAC airspace in collaboration with the DLR.

The effort concluded that contrails could be prevented by taking practical measures such as adjusting the flight level of an aircraft, and highlighted that further research was necessary in areas including enhanced weather predictions and more automated ­decision-making processes.

MUAC has since conducted the first large-scale, ­real-time contrail prevention simulation comparing two operational concepts: mitigation by airlines through flight planning around contrail areas; and tactical vertical clearances by air traffic controllers.

“While both options are possible, the preferred option is pre-planned contrail avoidance. If contrail avoidance is to be carried out during periods of high traffic, additional capacity will be required,” MUAC says.

It has also developed a ground camera system to monitor the effectiveness of weather forecasts in ­relation to contrail development and is partnering with Google to utilise this satellite imagery and ­artificial intelligence to predict regions where ­contrails may develop.

“This information is being used as part of an ­operational flight trial, allowing us to provide tactical clearances for contrail mitigation at night during summer and autumn 2025,” MUAC says.

Some of MUAC’s work has been done as part of the CICONIA programme, a three-year SESAR Joint Undertaking project looking into mitigating the impact of aviation’s non-CO2 emissions. The wide-ranging programme has 15 participants, including Airbus, Boeing, Air France, Swiss International Air Lines and UK air traffic management body NATS.

“We’ve got a number of validation exercises [that] are ongoing. We are starting to see some really good results from that,” Sian Andrews, SESAR environmental lead at NATS, said during the RAeS Sustainability Conference.

MITIGATION OPPORTUNITIES

Much of the initial opportunity for contrail mitigation is on quieter periods, as there is more scope to make alterations. There is a particular focus on night flights, given that this is a quieter time for flight activity, and that it fits with the bigger environmental opportunity of mitigating contrails given there is no cooling effect from reflection at night.

“What we are looking at, as we have developed the concept of operations within CICONIA, is a kind of step one, where are saying when it is a bit easier, when the traffic density is a bit lower, what can we do? And we think we can do quite a bit, we can get some significant results, and we are maturing that,” says Andrews.

Work is also continuing on what can be done when traffic levels are busier, while Andrews points to the wider importance of addressing contrails over the North Atlantic in particular.

“The North Atlantic is a very strong case for contrail mitigation, partly because it is very ­susceptible to ISSRs and the amount of traffic that is passing over the North Atlantic, especially at night, is quite high,” she says.

French carrier Amelia has been working on contrail avoidance with Thales and Breakthrough Energy Contrails for the last year and has already delivered notable environmental savings.

“Just focusing on some Amelia flights for a few months, we have saved several hundred tonnes of equivalent CO2 through the contrails we have avoided generating,” explains Julien Lopez, head of green operations within Thales’s avionics division.

Amelia has been working with Thales since 2022 on first evaluating its wider environmental impact, and since last summer on contrail avoidance. After initially demonstrating that contrail mitigation could work on Amelia’s scheduled service between Paris and Valladolid in northern Spain, Thales subsequently automated the process and Amelia has since February been deploying it progressively across its flights.

“Now it is fully automatic other than there is still a manual dispatcher of every flight-plan optimisation we propose,” explains Lopez.

As of mid-May, contrail avoidance had been implemented on around 50 out of roughly 1,700 Amelia flights assessed. That is in line with wider data on the number of flights causing persistent contrails.

“Contrails are generated by a very small portion of flights, so you can modify only a small ­portion of flights and have a very significant impact on ­contrails and on the whole climate impact of ­aviation,” notes Lopez.

CLEAR BENEFIT

One of the challenges of changing flight plans is the additional fuel, and therefore CO2 emissions, ­incurred in the new routing. “Usually, based on all the mitigation we have made, extra fuel was in the range of less than 1%,” he says. “So, for the about 700t of CO2 equivalent saved, we have generated only 0.3t of extra CO2.”

Validation work is carried out after a flight, as the model is re-run using actual weather data rather than the forecast. “On average we lose about 10% of the benefits because of the lack of precision of the forecast data,” Lopez explains. “But when you consider the gain we can make, that is still very consistent.

“Clearly the next step is to move from a flight-­per-flight basis with a single airline to an airspace ­approach in a co-ordinated way with air traffic ­management and airlines,” he says.

This is where Thales’s work on another SESAR Joint Undertaking project, CONCERTO, has a role to play. The initiative, which covers nine countries in northern Europe and involves 23 partners, aims to develop solutions to reduce CO2 emissions and introduce the management of non-CO2 impacts into daily operations.

Lopez says that CONCERTO has, based on a ­large-scale simulation, validated an ­operational concept to make contrail avoidance feasible at an airspace level. “We have demonstrated that by only re-routing a very small subset of those flights we could have a very significant impact. It is very promising,” Lopez says, noting the results will be published shortly.

This body of activity within the industry, despite the various uncertainties that remain, is enough to ­encourage those seeking action on aviation’s non-CO2 impact that momentum is increasing.

“We have been talking about it for a long time,” noted Tim Johnson, director of UK NGO Aviation Environment Federation during a panel debate at the Airbus Summit in late March.

“It feels different this time. It feels like the ­momentum is not just within Europe, it seems to be global. The interest has spread to ICAO. I don’t think this time we’re going to go back, having spiked the ­public’s interest and having these operational ­responses and trials.