Contrail Avoidance Goes Airspace-Wide in North Atlantic Trial This Winter

After airline-by-airline tests, controllers over the eastern North Atlantic will adjust the altitude of selected flights this winter to see whether contrail avoidance can work across an entire airspace.

Air traffic controllers will make small altitude changes to selected flights over the eastern North Atlantic this winter in Operation Blue Skies, a trial testing whether contrail avoidance works across an entire airspace.

The Google UK-led program aims to extend avoidance beyond individual airline experiments by building forecasts into air traffic control. British air navigation service provider NATS will coordinate the maneuvers in Shanwick oceanic airspace, which Google describes as the eastern half of the North Atlantic corridor.

“Operation Blue Skies is an airspace-scale trial,” Paul Hodgson, a technical program manager at Google, told Aviation Week in an interview published Thursday.

Why the trial runs through air traffic control

Contrails form when water vapor in aircraft exhaust mixes with cold upper-atmosphere air and freezes into ice crystals, according to the Met Office, Britain’s national weather service and a project partner. Some disappear quickly. Others persist for hours and spread. They can reflect sunlight and also trap outgoing heat, and the balance shifts with time, place and atmospheric conditions.

The warming effect is concentrated. A 2024 peer-reviewed study led by Roger Teoh estimated that about 2.7% of flights produced 80% of modeled global annual contrail energy forcing in 2019. Google cites modeling that places about 5% of global contrail warming in Shanwick.

Most earlier trials relied on airlines requesting changes one flight at a time. Contrails.org, which designs the trial parameters, says the unresolved question is whether many aircraft sharing the same airspace can avoid contrails in a coordinated way.

“We want to understand whether we can have all aircraft or most of the aircraft avoid a given particularly sensitive contrail region, which therefore has to be operated by air traffic control, and ensure that the findings that we have on an individual aircraft case apply at airspace scale,” Hodgson said.

Contrails.org also expects satellite imagery to provide a clearer observational signal of whether the maneuvers work.

How the trial will run

NATS plans roughly 20 to 40 test days each winter in 2026-27 and 2027-28, scheduled during lower-traffic periods, according to its Aug. 18 announcement. Controllers will issue altitude adjustments through established air traffic control (ATC) communication channels within standard operations. NATS also handles safety assessment and controller training.

Google estimates that around 10,000 flights will pass through Shanwick during trial hours each year, with only a small percentage slightly deviated, according to a company blog post by Hodgson and senior program manager Chaim Langermann.

The planned sequence has three steps. Forecasts flag contrail-sensitive regions, controllers coordinate minor altitude changes on selected test days, and researchers evaluate the resulting contrails and their climate impact.

“At Google, we have an [artificial intelligence]-based forecast that helps airlines and airspace managers see these contrail-sensitive areas for avoidance maneuvers and take contrail mitigation action to lower aviation’s climate impact,” Kemal Armada, a product manager at Google, told Aviation Week.

The Aerospace Technology Institute describes Operation Blue Skies as a £5 million, 30-month program, including £2.65 million from the U.K. Department for Transport. Google UK is contributing £1.4 million in in-kind support.

Google UK develops the artificial intelligence (AI) forecasts and the satellite and machine-learning verification. Contrails.org assesses and integrates the forecasts. Imperial College London and the University of Cambridge will evaluate outcomes and verify climate impact, while the Met Office, a subcontractor to Contrails.org, develops forecasting capability.

“What makes Operation Blue Skies unique is that it’s an oceanic trial,” Armada said.

Exact operating dates have not been publicly announced. Aviation Week reported a first phase running from November to March, but partner announcements reviewed for this story give no start or end dates. Contrails.org describes two four-month winter windows and identifies roughly November through February as the region’s strongest contrail-warming season.

Single-airline trials set the stage

“We have shown with American Airlines and with Cathay Pacific that the forecast, the technology, works,” Armada said.

In a March 6 preprint that has not completed journal peer review and that Google describes as its own study, researchers reported an 11.6% reduction in contrail formation rate among 1,232 flights eligible for avoidance, compared with control flights. The 112 flights that flew the avoidance plan showed a 62.0% lower formation rate. The researchers found no statistically significant difference in fuel use between the groups, a result that does not rule out a fuel cost. Google has described the work as part of a 2,400-flight American Airlines transatlantic trial.

Cathay Pacific said Sept. 6 that an initial operational trial with Google began in late 2025. More than 100 flights were targeted and more than 80 followed avoidance routes, with Google estimating about 40% less contrail warming for those flights. The airline cited airspace and payload restrictions among the reasons not every targeted flight took part, and said a larger second phase would follow across Asia and transpacific operations.

Armada said the Cathay work is meant to show where avoidance drops off.

“It’s like a funnel: Not every flight creates a contrail. Not all created contrails are persistent. Not all persistent contrails are easily avoidable, and so on.

“We need to understand the shape of the funnel to be able to design a much larger trial,” he said.

Air France said Sept. 17 that it worked with French startups Estuaire and Klima on nearly 4,300 North Atlantic flights between February and April 2026. The airline said 245 flights were identified for alternative-path assessment and 130 were rerouted. It reported an 8% reduction in contrail-related climate impact across the trial’s scope, a 57% reduction among rerouted flights and a 36% reduction in overall climate impact for those rerouted flights.

Air France noted that any flight-path change requires air traffic control approval.

The airline figures are estimates reported by the carriers or their research partners. They measure different outcomes and are not Operation Blue Skies results. Cathay said further research is needed before individual-flight results can establish benefits across airlines and airspaces.

Climate claims and open questions

“We are able to avoid these contrail-sensitive areas in a very cost-effective manner. If you think about the available tools we have to decarbonize aviation, contrail avoidance is one of the few that’s available within this decade,” Armada said.

Contrail avoidance targets warming caused by contrails rather than carbon dioxide emissions. The advocacy group Transport & Environment, or T&E, said Sept. 22 that contrail mitigation must accompany cuts in carbon dioxide, or CO2, emissions. The group called for research focused on humidity forecasting and satellite-based contrail verification.

T&E also asked the European Union to extend non-CO2 monitoring to all flights in 2027 and backed proposed free EU Emissions Trading System allowances for contrail avoidance.

Steven Barrett, Regius Professor of Engineering at the University of Cambridge, a participating institution, said in a program announcement: “Contrail avoidance could drastically reduce the global climate impact of aviation, at minimal cost to passengers and companies.”

The science still carries uncertainty. The Teoh study identified humidity correction and assumptions about engine particle emissions as important sources of sensitivity in contrail models.

Partner materials reviewed for this story include no completed Blue Skies results, measured climate benefit, fuel penalty, controller workload findings or numerical success threshold. Announcements and trade coverage reviewed as of Oct. 15 contained no assessment from an analyst outside the consortium and no public response from an airline association or air traffic controllers’ union.

Armada said the trials are meant to expose practical problems. “We obviously continuously work on improving the forecast and the technology, but doing these trials helps understand some of the operational frictions and the things that we need to iron out so that this technology can be scaled broadly,” he said.

Hodgson said the attention is spreading. “Projects such as Operation Blue Skies and the Cathay trial raise the profile of this,” he said.