Green Aviation: Three Contrail Mitigation Developments To Watch
Making aviation green is a complex challenge. From carbon offsets that counteract aviation’s emissions without addressing them directly to engineering and introducing an entirely new and far cleaner production process for jet fuel, multiple initiatives are taking hold, but a single “silver bullet” remains elusive. There is, nevertheless, one promising initiative that appears to have had accelerating potential in recent years – contrail prediction, mitigation and avoidance.
It is often said that small hinges swing big doors, and learning how to effectively manage contrails could produce benefits in several ways. As aviation sustainability firm 4AIR explains in a recent white paper, contrails trap heat that would otherwise be radiated into space back towards earth, warming the environment. Additionally, contrails can evolve into cirrus cloud systems, multiplying the warming effect and persisting for far greater lengths of time than the contrails themselves. 4AIR points out that a 2022 Intergovernmental Panel on Climate Change (IPCC) report estimates contrails alone could account for 35% of aviation’s climate impact.
Amid all the various efforts being leveraged by the aviation industry to reduce its environmental impact, contrail management could prove to be the solution that provides the greatest gains with the least effort. Here are three emerging developments that appear to have great potential to reduce aviation’s environmental footprint through contrail mitigation.
SUSTAINABLE AVIATION FUEL (SAF) MAY BE THE KEY TO CONTRAIL MITIGATION
For the first time, Sustainable Aviation Fuel (SAF) has been shown to reduce a commercial aircraft’s production of soot particles and ice crystals, both of which are building blocks for contrails. The study, conducted by Airbus, Rolls-Royce, German Aerospace Center DLR, and SAF producer Neste utilized a chase plane to evaluate the emissions from both engines of an Airbus A350 that was utilizing 100% SAF.
In the study, researchers found that ice crystal production was 56% lower than that of exhaust fueled by standard Jet-A fuel. As contrails are largely made up of ice crystals, this significantly reduces one of the key ingredients to contrail formation. It is also some of the first data that suggest SAF can provide a significant reduction in non-CO2 effects.
However, reducing one of the building blocks of contrails does not reduce their effect on the atmosphere’s energy balance in a linear manner. Nevertheless, the team conducted further research using climate model simulations. These simulations estimated that lower ice crystal production is likely to reduce radiative forcing by at least 26% compared with emissions produced by the burning of Jet-A.
Thus far, the environmental benefit of SAF has been widely considered to be limited to a broad life-cycle reduction rather than a reduction in actual emissions. Just as recycled paper towels are more environmentally friendly due to the greener production process but perform identically to standard paper towels, SAF has historically been viewed in a similar manner. This study is notable in that it demonstrates a reduction in non-CO2 tailpipe emissions that build upon the known reduction in life-cycle emissions.
As promising as the study results are, significant roadblocks remain. One of the most significant is the inability of many legacy aircraft to utilize a 100% SAF blend. Critically, 100% SAF lacks aromatics – hydrocarbons that cause the synthetic O-rings utilized in older aircraft to swell, thus providing an effective seal. Without these aromatics, the O-rings do not swell, and thus, do not provide an effective seal.
To ensure aircraft old and new can utilize SAF, it is traditionally blended with standard Jet-A, typically in a 30/70 blend. Until a solution is found – perhaps in the form of non-hydrocarbon aromatics or perhaps by offering 100% SAF alongside existing fuels with its own infrastructure – the benefits of 100% SAF blends remain exclusive to studies and not attainable in widespread use.
ARTIFICIAL INTELLIGENCE IS EMERGING AS ANOTHER PATH TO MINIMIZE THE FORMATION OF CONTRAILS
Thus far, a thorough understanding of contrail formation and prediction has been hampered by challenges that are largely informational in nature. Atmospheric data, flight data, and contrail occurrence records are spread across multiple platforms in many different forms. Accurate data have been similarly scattered. Like spoken languages, complexities exist that preclude a smooth, seamless sharing of information..
Fortunately, the advent of artificial intelligence (AI) has presented a way forward. AI enables the analysis of massive amounts of unlike data and the blending of diverse formats into an intuitive, usable form. Advanced machine learning algorithms can process the raw data itself to improve accuracy.
With regard to contrail mitigation specifically, a number of tasks can be handled more quickly, accurately, and broadly than through the coordination of multiple siloed analysts. Widespread contrail detection can be automated with aft-facing cameras aboard aircraft. AI can collect and compile the resulting inflow of data and then cross-reference it with the specific atmospheric conditions that were present during the formation of each contrail.
After these initial data are collected, compiled, and analyzed, they must be further produced into actionable directives for dispatchers and flight crews and packaged into an intuitive, usable format. With advanced AI, the information can be processed and translated in real-time, presenting flight crews with actionable recommendations to avoid contrail formation. As a comprehensive study by 4AIR demonstrated, small changes in altitudes and routings can produce a significant impact.
Barring strict legislation governing and requiring contrail mitigation, all of these efforts will come second to profitability in the eyes of airlines and operators. Presently, onboard Flight Management Systems (FMS) calculate a parameter called “cost index” that takes many factors, such as crew salaries and fuel costs, into account to determine the optimum balance between fuel consumption and flight time. The integration of AI with flight planning could, in theory, create an “environmental index” that determines a similar balance with regard to cost efficiency and contrail mitigation. Airlines and operators would then be able to utilize the resulting data to demonstrate their environmental efforts to stakeholders.
While it’s not feasible to force operators around the world to utilize contrail mitigation techniques, AI could foster widespread adaptation by making it easy for them to do so.
THE AVIATION INDUSTRY IS WORKING AROUND THE CLOCK ON NEW AND INNOVATIVE METHODS OF CONTRAIL MITIGATION
With efforts fueled by large-scale data interpretation and prediction algorithms well underway and SAF now beginning to take hold as a potential solution to minimize contrail formation, aircraft and engine manufacturers are exploring other experimental theories.
One such initiative by Rolls Royce has been revealed in the form of a pending patent that was published in February of this year. Titled “Method of contrail mitigation and aircraft having contrail mitigation functionality,” it describes a method of mitigating contrails produced by multi-engine jet aircraft.
The patent observes that aircraft tend to produce contrails most effectively when operating relatively modern, efficient engines at their most efficient settings. This correlates with previous studies that found modern turbofan engines are more apt to produce contrails than legacy turbojets. It goes on to suggest that if a multiengine aircraft could artificially reduce the efficiency of one or more engines in flight, those engines are likely to stop generating contrails.
The study suggests several methods of reducing efficiency, including activating engine bleeds—diverting high-pressure air away from the engine, as is done to power other aircraft systems. An analogy would be turning the air conditioning on in an automobile to force the engine to work harder. This would have the side effect of increased engine temperatures in the adjusted engines, which would then produce a reduction in contrail generation.
The method described inevitably increases fuel burn. This occurs not just on the adjusted engines but also on the non adjusted engines, as their thrust levels would have to be increased to compensate for the reduced efficiency of the adjusted engines and maintain the same total thrust generation.
It’s unclear just how much additional fuel consumption would be required for this method of contrail mitigation, but a cursory study conducted for this article using a three-engine Dassault Falcon 900LX simulator revealed promising results. During the exercise, the thrust of the Falcon’s center engine was reduced by 17% N1 to simulate an engine adjusted to decrease efficiency. Increasing the outboard engines to maintain the original cruise speed of .80 Mach resulted in a fuel burn increase of only 30 pounds per hour in total. These data suggest that the increased fuel burn on non-adjusted engines resulting from higher thrust settings could be minimal.
While the patent diagrams display a four-engine aircraft to illustrate the concept and emphasize the importance of maintaining symmetrical thrust, the wording acknowledges that the concept can be applied to any multiengine aircraft – including two-engine aircraft, provided the engines are mounted on the aircraft centerline and continue to produce centerline thrust after the split thrust conditions are introduced. The patent text does describe a variant in which asymmetric thrust would be utilized, but the increased drag from control surface inputs likely make this method a non-starter.
An additional passage in the Rolls-Royce patent application observes that it’s possible the engines set to an increased thrust level may actually reduce the number of soot particles produced by those engines, thus reducing the intensity of any contrails they might produce. Finally, the patent observes that any contrail mitigation taking place farther out on the wing, toward the wingtips, will be more effective due to the proximity of the engine exhaust to wingtip vortices.
Because the exhaust from these outboard engines is more thoroughly blended with ambient air by the vortices, the thickness of the contrails will be correspondingly greater, resulting in increased radiative forcing.
Perhaps the most significant unanswered question related to this concept is whether the reduction in contrails would produce enough of a benefit to outweigh the increase in greenhouse gases emitted from the adjusted, less-efficient engines. If the net benefit is minimal, the inherent increase in fuel burn and reduction in engine life might render the concept unfeasible for operational use, limiting it to a concept with potential for future development.
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