This series of white papers concerning the sustainability of air transportation is published in the midst of the Covid-19 disease period, which has had a tremendous effect on the worldwide aviation sector. Some voices are rising to push governments and institutions to tackle, in the recovery efforts to address the economic crisis, the sustainability issue of this transport mode at the source and redefine its global rules. We see those debates as a great opportunity to condense and review the main challenges of the aviation sector regarding sustainability. This part gives an interesting look to the potential measures proposed by the industry and their potential effectiveness.

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Part 2: The air transport conundrum: How to decarbonise the sector ?

In our last White Paper, we focused on the environmental issues of the air transport sector and detailed the main challenges associated with its decarbonisation. As mentioned, the International Air Transport Association (IATA) – the benchmark trade association representing the world’s airlines – has set up three main targets in the fight for climate mitigation:

  • An average improvement in energy efficiency of 1.5% per year from 2009 to 2020,
  • A carbon-neutral growth starting from 2020, addressed by the CORSIA mechanism, and
  • A 50% reduction in emissions in 2050 compared to 2005.

This white paper is the second part of a tetralogy that dives into the sustainability issues of the air transport sector. It will go into more details regarding current and future technical developments that could be instrumental in allowing these objectives to be met. Carriers and manufacturers can work on two axes; firstly, increase the energy efficiency of planes to decrease fuel consumption, and then shift towards biofuels and synthetic fuels, though guaranteed sustainability is still an issue.

Improving airplane energy efficiency

According to the International Energy Agency (IEA) reports [1], the energy efficiency in aviation has improved by around 3% per year between 2000 and 2016 but has been slowing down over the last few years. The IEA’s Sustainable Development Scenario (SDS) expects the aviation sector to achieve those the 3% annual decrease in specific energy consumption by 2040, but regrettably, the ICAO (International Civil Aviation Organisation) objectives are set at 2% by 2050.

The driving forces of energy efficiency in airplanes are diverse: an increase in passenger occupancy and load factor, fleet renewal for more efficient airplanes, improved flight maneuvers and route optimisations.

In addition to the sector’s growth, the occupancy rate (also called load factor) has been increasing over the last years, driven by the competitivity of low-cost carriers that tend to maximise the number of passengers on board to increase profits. The result is substantial: while in the 1990s the load factor was around 65%, it increased each year to reach 82% in 2018. The amount of fuel burnt is primarily determined by the weight of the plane and the amount of fuel carried, therefore the occupancy rate increases the overall efficiency by decreasing the fuel burnt per passenger [2].

Nevertheless, there is a common practice in the airline industry called “tankering”: the aircraft carries more fuel than required in order to avoid refueling at the airport where the cost of fuel is higher. This practice can help save money on a roundtrip flight but increases fuel consumption, and thus the amount of CO2 emitted. EUROCONTROL, Europe’s air traffic management organisation, has assessed the impacts of this practice [3]. On a 1000 km roundtrip flight, this practice can save up to 500€ for the airline and increases CO2 emissions by 3% due to additional fuel consumption. If the sector’s goal is to encourage efficiency measures at every step is the goal of the sector, setting economic incentives to avoid these kinds of operations seems like it should be a priority.

In terms of innovation in new aircrafts, there is much room for improvement. Substantial gains can be achieved by bringing advanced fuel-saving technologies to the market. The list is long, and actions can be taken on materials and processes with advanced composite materials and alloys for airplane structures. Just some examples on the aircraft surfaces are low friction coatings, wingtip devices, smart wings, stability control along with advanced engines that are equipped with higher pressure compressors, geared turbofans, variable nozzles, etc. Today, the new generation A320neo and Boeing 737 MAX planes both advertise a 20% and a 14% fuel consumption reduction compared to the versions they are to replace. As a result of the re-engining of the existing airframes with higher performance propulsion systems, they offer good performance in regards to fuel burn reductions. The International Council on Clean Transportation (ICCT), a non-profit independent organisation that commissioned the research at the start of the diesel gate scandal, emphasises the potential efficiency gains that could be achieved through complete aircraft “clean sheet” designs of the wing and tube. They estimate that a 40% reduction of fuel burn can be achieved through technological improvements of aircraft. However, due to the long lead times that “clean sheet” designs need, most of the expected measures won’t be commercialised until 2030.

Further innovations could be foreseen in the field of electric aviation. This subject, and its prospects to decarbonise or not the sector will be assessed in more detail in the next white paper.

Improving airplane energy efficiency

Beyond technology, improvements in the airlines’ operations can result in fuel savings.

Today, flight routing is inefficient, and the IEA considers a 10% cut in fuel consumption could be achieved if aircraft were to be routed more efficiently. In 2015, The European Space Agency developed the IRIS program that uses a satellite to improve conventional radio traffic management. This technology can reduce the distance flown by aircraft by 10% on average and is currently being implemented in Europe. Improvements can also occur at the airport level operations: reducing the congestion of landing runways can decrease the time spent in the air by travelling airplanes.

Flight operations should also consider the formation of contrails. In addition to GHG emissions, the water condensation of plane engines at high altitudes create a phenomenon known as Aviation Induced Cloudiness (AIC). Unlike CO2 emissions that influence long-term global warming, AIC increases the radiative forcing of the planet in the short term (the atmosphere keeps more energy than it releases due to the presence of the high-altitude clouds). An effective air traffic management system could improve plane routing by flying higher or lower to avoid the formation of contrails. This easy to implement system would significantly reduce contrail warming but would result in a 0.5% increase in fuel burn [4]. Finding the balance between AIC and already optimised routes is complex, and researchers at the Imperial College London, have shown that room for improvement still exist. They have suggested selectively diverting a small portion of flights which could lead to reducing the contrails creation by 60% while only increasing fuel consumption by 0.014% [5].

Biofuels and alternative fuels

In the 2050 objectives for sustainable development, IATA – through the work of the Air Transport Action Group (ATAG) – expects half of the CO2 reductions to come from biofuels and synthetic fuels which the sector refers to as “sustainable aviation fuels. In 2018, aviation biofuel production was about 15 million liters or 0.1% of total aviation fuel consumption. In order to satisfy this effort in carbon reductions, biofuels will need to annually supply 400 billion liters by 2050.

Most of the biofuels used today come from 1st generation biofuels produced from hydrotreated esters and fatty acids (HEFA): food-based products such as corn, sugar cane or palm oil. Numerous studies and articles, including one from Greenfish, have shown that these biofuels have a limited GHG abatement potential and can be even worse considering the impact of Indirect Land Use Change (ILUC) [6]. The ILUC is responsible for additional indirect GHG emissions due to the destruction of forests to convert them into arable land for the production of 1st generation biofuels. The revision of the European Commission’s Renewable Energy Directive in 2016 (RED II) has considered this issue and now limits the use of food-based biofuels and increases the objectives for advanced biofuels that have proven sustainability in their production. Advanced jet biofuels are Fermented Sugars-to-Synthetic Isoparaffin (HFS-SIP), Alcohol-to-Jet (ATJ) and Fischer-Tropsch obtained from cellulose, biomass, algae, etc. Synthetic fuel (also known as e-fuel) is an emerging type made from the synthesis of hydrogen and CO2 that is captured from air. These new fuels that are not yet commercially available and still need major development, but many pilot projects have proven the technical interest of the concept.

Today, most commercial aviation biofuels come from HEFA biofuels. On top of the absence of proven sustainability of these biofuels, there is also an issue with availability. Today, most of the arable land is used for agriculture. Providing the 400 billion liters of biofuels needed by the aviation sector by 2050 would require a surface of 82.3 million more hectares of arable land (roughly the size of Italy) [7]. Or half of the amazon forest. And that is just to provide for the aviation sector when road transport (passenger and freight) and international maritime transport also count on biofuels to decarbonise their sectors. Synthetic biofuels are especially interesting because they are not land-based, and therefore show a greater potential of capacity.

The future of biofuels will then be greatly determined by the cost-effectiveness of biofuels that have proven sustainability and use technologies that are less land-intensive. Today, conventional jet fuel’s price is between 0,4€L and 0,6€/L. The cost of biofuels is hard to determine since the technologies and the markets are variable. From the literature, the price of the cheapest bio jet fuel is at least 60% more expensive than petrol-based fuels; HEFA is between 0,8€/L and 1,5€/L and the price of advanced biofuels is significantly higher at 1€/L to 3€/L for HFS-SIP biofuels and even around 4–5€/L for e-fuels [8]. With technological advancements and improvements, it is expected that e-fuel prices could fall around the price of conventional biofuels by 2050.

Figure 1 – Price per liter of different fuel-type for aircraft [9]

The jet fuel represents 20% of the carrier’s expenses, a 60% increase in jet fuel price translates to a 20% increase in the price of a ticket, which is not acceptable in a very competitive environment. An increase of the jet fuel price, governmental subsidies towards bio jet fuels and carbon pricing could be ways that would make biofuels competitive. However, that would most certainly happen at the expense of consumers who would pay more for their plane tickets.


A considerable amount of solutions exists today to begin the decarbonisation of the air transport sector. It is very likely that efficiency measures will be progressively implemented as they also reduce the cost of fuel consumption. The digitalisation of the sector shows great potential for the improvement of operations and routes. Biofuels and e-fuels are an extensive field of research today. While some projects have demonstrated their effectiveness, mass-produced biofuels are not an environmentally friendly solution nowadays, and much investment will be required to produce ecological jet fuels on a global scale. Attention should be paid to non-CO2 effects as well, such as AIC, which are also major contributors to climate change in aviation but are harder to quantify and measure.

All in all, and despite the numerous options that exist, there remains a high degree of uncertainty regarding whether those will suffice in order for the sector to reach its crucial climate objectives. In a working paper for its 40th assembly in 2019, ICAO itself has casted doubt on these options for offsetting the GHG emissions resulting from projected traffic growth. The best displayed scenario in this paper predicts a 1.37% annual energy efficiency improvement (both technology and operation), falling short of the 2% intended by 2050. Those computed results, show that the GHG level of the sector would be the same in 2050 as in 2005, exceeding the objective of a 50% reduction target [10]. Can the industry reach those goals without addressing the reduction air traffic ?

Our next white papers in this series of 4, will take a deeper look at the technological improvements that electricity could bring to the aviation sector (part 3) and will focus on a potential solution for air traffic demand reduction (part 4). Stay tuned !