Contrails Present Challenge And Opportunity For Business Aviation

In an August press release announcing the results of a study into the effects of aircraft operational differences on the persistence of contrails, Imperial College London boldly declared, “Private jets [are] the worst offenders.” That is not good news for a business aviation industry already struggling with public perceptions of its sustainability.

But all is not lost. In May, a study of contrails by aviation sustainability solutions provider 4AIR found that small altitude adjustments to business aviation flights could substantially reduce their contrail impact on the environment without CO₂ trade-offs.

Contrails form through the mixing of warm, moist engine exhaust with the surrounding colder air. In ice-supersaturated regions of the atmosphere, ice crystals in the contrail can grow, and from there the contrail can persist, spread and merge with other contrails to produce aircraft-induced cirrus clouds.

• Altitude is key to mitigating persistent contrails
• European air traffic control constraints create difficulty but also an opening

During the day, the cirrus reflects sunlight and has a cooling effect on the Earth’s climate. But at night, such clouds trap the Earth’s heat and have a warming effect. The prevailing theory is that contrails have an overall warming effect and a climate impact greater than that of aviation’s CO₂ emissions.

But the scale of this warming is highly uncertain. Across aviation, multiple studies and tests are underway to understand contrails, quantify their climate effect and mitigate their impact. Most of this work is focused on commercial aircraft but applies equally to business jets.

The Imperial College study set out to quantify the factors controlling contrails by matching air traffic data to satellite imagery to isolate the role of aircraft type in contrail properties and evolution. Based on 64,000 cases, the study observed that more efficient aircraft form longer-lived contrails more frequently. This is primarily driven by an increase in flight altitude, the researchers said. In addition, the study found that business jets produced longer-lived contrails, despite their smaller engines’ lower fuel flow, as they fly at higher altitudes.

Imperial College used imagery from the NASA-NOAA GOES-16 weather satellite to identify contrails over the Western North Atlantic using a convolution neural network. These were matched with aircraft position reports from the FAA’s traffic flow management system and to aircraft-engine combinations using flight numbers. The resulting dataset included older airliners flying at about flight level 370 (FL370)—37,000-ft. pressure altitude—as well as modern airliners flying at around FL400 and business jets above FL410.

The researchers concluded that more efficient aircraft generate contrails with longer satellite-detectable lifetimes because they fly higher, where the ambient air temperature is colder. They also noted that the aircraft in the region studied usually fly in the troposphere rather than in the drier stratosphere, as is common farther north, likely increasing the frequency of persistent contrails.

Where Imperial College chose to focus is important, as results for the subtropical Atlantic region studied are not necessarily true for where most business jets fly, 4AIR President Kennedy Ricci says. Another factor is the altitude capability of most business jets—up to 51,000 ft. “If you fly even higher, you do not make contrails at all,” he states. “That is lost in the [Imperial College] study.”

The contrail study performed by 4AIR used publicly available automatic dependent surveillance-broadcast (ADS-B) data from almost 16,900 business jet flights involving 34 aircraft of seven different Bombardier, Embraer and Gulfstream types, 11 based in Europe and 23 in the U.S. The aircraft were not limited to flying in those regions, and the data included many global trips.

4AIR’s study did not match the flights to contrails observed in satellite imagery, as Imperial College’s did. Instead, researchers used a contrail prediction model based on historical weather data. “While the model is a leading model for predicting contrails, it does not include any observational data to confirm its results, and models inherently will be imperfect and may be inaccurate,” the study cautions.

Fleetwide, 4AIR found the aircraft spent about 1.1% of flight time in contrail-forming regions, varying based on aircraft location. Those based in Europe spent more time in regions where contrails form, an average of about 2.1% of flight time, versus 0.7% for U.S.-based aircraft.

The 4AIR report states that the relative difference in time flying in contrail-prone regions is likely related to cruise altitude. “Contrail-forming regions typically emerge between FL330-430, so aircraft that had more flights that reached cruise altitudes above that range correlated with lower flight time spent in contrail-forming regions,” the study says. “We didn’t see anything above 43,000 ft.,” Ricci adds.

Contrails with the greatest warming impact formed between FL350 and FL400. EU-based aircraft flew at cruise levels in these altitudes on 18.7% of flights compared with 4.4% for U.S.-based aircraft. “We have two parallel fleets in the U.S. and EU with the same types,” Ricci says. “The EU fleet more often flies low enough to create contrails, even on similar mission profiles. Most of that comes down to air traffic control [ATC] and operating rules, which means there is a bigger opportunity to mitigate.”

EU-based aircraft most frequently had cruise altitudes at FL380-420—43.1% of flights versus 8.1% for U.S.-based aircraft. The study found that 49.3% of the U.S.-based aircraft flights were flown above FL420, compared with 9.6% for EU-based aircraft. Because of that, U.S.-based aircraft spent less time in contrail-forming regions, even comparing similar types.

“The biggest climate impact comes from contrail persistence, or when we have a hot spot with lots of contrails that combine,” Ricci says. Contrails that persist and grow are much more impactful than those that quickly form and disappear. Longer-lasting contrails have more time to absorb heat leaving the Earth or to reflect sunlight back into space.

4AIR estimated that the contrails formed by flights in its dataset lasted about 2.5 hr. EU-based aircraft formed contrails with an estimated duration of 3 hr. compared with 2.2 hr. for those based in the U.S.—one factor in why the EU flights had a higher contrail impact per flight hour, the study says.

“We’ve started expanding the flights we are looking at and doing more forecasting to find out what are the real opportunities to mitigate contrails,” Kennedy says. “We found a lot are ATC-related, so we need to understand what is practical. How much a flight can climb or descend [to avoid forming contrails] is limited by the ability of ATC to handle the change. We need a more realistic assessment.”

One of the services 4AIR will provide for operators is compliance with the new monitoring, reporting and verification system for non-CO₂ effects—such as persistent contrails and nitrogen-oxide emissions—that will be operational beginning in 2025 under the EU Emissions Trading System.

Under this scheme, which covers only intra-EU air travel, operators will provide ADS-B data on their flights for the EU to run in its contrail prediction model. 4AIR uses the same model. “Once we start to measure, we can start to update the modeling,” Ricci says. “Over the next three years, we can build an understanding of what we can do [to avoid forming persistent contrails].”

The 4AIR study sought to identify “big hits”—flights with extremely high contrail impacts in proportion to the distance flown, often far exceeding their CO₂ effects. Although only 17.9% of flights in the overall dataset created contrails, the lion’s share of the overall contrail footprint calculated by the model came from flights meeting this “big hit” criterion, the study says.

A mere 17 flights (0.1%) accounted for more than 26% of the entire fleet’s contrail footprint over a year. About 50 (0.3%) of the almost 16,900 flights in the dataset accounted for 51% of the total contrail impact, and about 123 flights (0.73%) accounted for 75%.

“Had mitigation efforts been able to successfully avoid or minimize contrail exposure on just 50 flights over the course of the year, it would have avoided 51% of the entire sample’s contrail impact,” the study says.

“We do not need to mitigate every flight,” Ricci says. “This is low-hanging fruit. We do not need to sacrifice fuel just to know where the hot spots are. And we need to incorporate that at the operator level.” He notes that ForeFlight is developing a way for operators to visualize areas of possible contrail formation through its flight planning tool.

While the scale of the problem for business aviation is far smaller than it is for commercial aviation, contrails are highly visible, draw public attention and could add to the industry’s image issues with sustainability. “All aviation needs to focus on this,” Ricci says. “But for business aviation, we found so many opportunities to mitigate contrails.

“These are aircraft with ceilings above 45,000 ft.,” he continues. “The airline operating ceiling is right in the contrail formation zone. Business aviation has a greater opportunity to mitigate the problem. And we do not need to mitigate every flight. We can identify hot zones and times of day when other aircraft are in the area and also creating contrails, and mitigate just those flights.”