Reducing aviation’s carbon and carbon-equivalent warming emissions is a crucial part of aviation’s goal to decarbonise the industry and achieve net zero emissions by 2050. Non-emissions warming contributions, including the creation of high-level contrails, the introduction of particulate matter to the atmosphere, and the effect that aerosolised emissions have on high-level cloud formation are also critical to aviation’s future sustainability, and thus our planet.
At one basic level, the simple amount of carbon and other warming gases emitted needs to drop to as close to zero as possible. Beyond the direct effects of the emissions by weight, aircraft emit at altitude, they affect clouds, and they create contrails that need careful study — especially when it comes to the benefits of using more sustainable fuels in contemporary jet kerosene engines.
To learn more, we sat down with senior policy fellow Jeff Overton from the Environmental and Energy Study Institute, a US-based sustainability nonprofit formed in 1984 by a bipartisan group of members of congress.
Beyond direct emissions of carbon and other greenhouse gases, aviation changes the atmosphere by adding clouds, both by the direct formation of contrails and by the indirect aerosol effect on clouds, which increases overall cloudiness and changes the properties of high-level cirrus clouds.
In essence, Overton tells us, “the ambient conditions of sufficiently cold temperature and atmospheric water and ice supersaturation — with the particulates present from aircraft exhaust or ambient aerosols to form the nucleus of ice crystals — can produce lingering contrails. These contrails may expand into cirrus clouds that can reflect UV rays in daylight and trap infrared rays day and night with a net positive climate warming effect.”
In particular, thin high clouds like contrails can have a more significant impact on the outgoing long-wave radiation from the Earth, creating a high-atmosphere warming effect. The science on this is complicated, but understandable.
“Contrails form in the wake of aircraft if water saturation is reached or surpassed during the mixing of exhaust and environmental air,” according to a 2018 article in the journal Climate and Atmospheric Science: “The vast majority of contrail ice crystals form when emitted soot particles and ambient aerosol particles entrained into the plume activate to droplets in water supersaturated conditions and freeze subsequently by homogeneous nucleation.”
The ultimate effect is still uncertain, says industry association ATAG in its Waypoint 2050 report [PDF]. “The impact of contrails (and the hazy cirrus clouds they sometimes generate as they dissipate) on climate change is complex and still includes large uncertainties, despite advances in research,” ATAG writes.
Yet recent research suggests this could be as much as two thirds of the total climate impact of aviation. “66% of the aviation net equivalent radiative forcing (ERF) in 2018 is due to non-CO2 aviation climate forcings,” states a 2021 article in the journal Aerospace.
Clearly, reducing the formation of contrails and aviation-induced cloudiness will have a substantial impact on aviation’s overall contribution to climate change.
Contrail mitigation is possible, but there are tradeoffs — and requirements
Particulate matter is a key element in the formation of contrails, and newer more sustainable aviation fuels that have neither sulphur nor aromatic hydrocarbons mean a markedly reduced particulate matter emission at altitude. Hydrogen, too, reduces particulate emission, with hydrogen combustion’s primary emission being water. Both fuel types will thus reduce contrail formation, but the indirect aerosol effect will require substantial further study.
In addition, EESI’s Overton notes, “hybrid-electric propulsion combines electric motors with combustion engines to reduce liquid fuel consumption. It is possible to use sustainable aviation fuel with electric power to realise larger fuel savings and lower emissions.”
Aircraft can avoid creating contrails in some circumstances. This might be by avoiding areas with super-saturated cold air in which contrails are likely to form, either laterally (flying around them) or vertically (flying above or below them).
A new generation of digitalised technologies can help here, Overton says. “Digital technologies contribute to the operational fuel efficiency of commercial aviation in flight, for fuel planning, and in aircraft operation. Dispatchers typically file flight plans with the flight controlling agency, for example the Federal Aviation Administration, FAA, in the United States. There are several digital platforms available for this purpose.”
Improvements to digital flight planning software and platforms — as well as onboard flight management systems — to include contrail reduction as an element in flight operations efficiency will be a key goal as, Overton explains, “the onboard flight management system, programmed for a particular cost index, will calculate the most efficient cruise altitude and speed, and indicate when the aircraft weight will permit climb to a higher cruise altitude, often improving the efficiency en route.”
It will be critical to ensure that the flight management system and flight planning systems accurately calculate the balance of efficiencies when determining a flight path. Fortunately, this looks to be within the realms of the current capabilities of the computers used for flight planning and management — both those embedded within the flight deck avionics and used externally, such as at flight planning or dispatching computers, or within the flight deck using iPads or other tablets.
In essence, says the 2021 Aerospace article, what is needed is “a multi-objective and multi-phase 4D trajectory optimisation tool”, and it proposes a methodology that allows its creation. More widely, this climate mitigation-oriented flight planning must take into account that fuel-optimal trajectories are not necessarily the ones that minimise environmental impact or global warming potential.
Among other requirements, pre-requisites include advanced, reliable and near-live meteorological data — as well as the inflight Internet connectivity, flight planning systems, air traffic management provision, and crew resource to do so. Calculations as to whether substantial lateral diversions (burning extra fuel through flying extra distance) or vertical diversions (burning extra fuel through flying at less efficient altitudes) have a greater or lesser impact than contrail formation.
Combining these new technologies and systems, and ensuring that provision is made within the regulatory and air traffic management frameworks in which they sit, will be vital.
Author John Walton
Published 6th October 2022
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