22 June 2023
Authors : Nicolas Meunier

What if low-carbon aircrafts already existed?

For some years now, the carbon-zero aircraft has been on everyone's mind. From artist's renderings of hydrogen-powered aircraft to futuristic designs, aeronautical research is hard at work for the medium term (we're talking about a new aircraft by 2035-2040), and it's going to cost a lot! In France, the government has deployed a massive support plan to ensure the emergence of the "ultra-sober" aircraft, with 2.4 billion euros invested since 2020 through the Conseil pour la Recherche Aéronautique Civile (CORAC) [1]. Across the Atlantic, NASA and Boeing are jointly investing $1 billion to develop a commercial aircraft demonstrator offering a 30% reduction in fuel consumption [2].

However, the technology to significantly reduce aircraft fuel consumption is already available, using turboprops rather than the usual turbofans seen on most tarmacs. These are actually two technological variants of a turbine engine: a turboprop has a propeller driven by the engine, while a turbofan has a shrouded fan instead of a propeller (see image below). Turboprops are used on short-haul regional jets, such as the ATR42 and ATR72 or the Dash 8-Q400, as well as on many business jets, but not much more than that, since they are not used for distances over 1,000 km. Their cruising speed is lower (~600 km/h vs. ~830 km/h for an A320NEO), and the difference in flight time with a turbojet is deemed prohibitive beyond a certain distance (e.g. a difference of around 40 minutes on a Paris-Madrid flight)They are also a little noisier (both in the cabin and outside), and subject to more turbulence because they fly at a lower altitude. Not to mention the fact that the propellers give an (erroneous) impression of an aircraft from another age, which doesn't correspond to the image of modernity supposedly sought by passengers. They do, however, consume much less fuel - in the order of 20% to 40% less than a regional turbofan - even if this reduction diminishes with distance flown, the fuel consumption savings being greatest during the take-off and climb phases.

Furthermore, the benefits of today's turboprops also come from the oversizing of their competitors on a given route, with regional turbofans (ERJ, CRJ) or medium-haul turbofans (A320, 737) having a greater range and therefore being correspondingly heavier. This is a replication of the phenomenon observed in road transport: having a multipurpose vehicle for greater operational flexibility, which is consequently sub-optimized for certain routes.

A turbofan aircraft, commonly known as a "jet" (top) and a turboprop aircraft (bottom)

Another significant advantage for the climate is that turboprop aircraft fly at a lower altitude than turbofan aircraft, and therefore produce no contrails. They therefore have no non-CO2 radiative effects, which multiply the purely CO2 impact of aviation by around 2, according to the ADEME (see our previous article). The reduction in the climate impact of a turboprop flight can therefore reach 50% to 60%, which is very significant!

This example illustrates the different possible paths to decarbonization. To obtain a significantly less carbon-intensive aircraft, it is possible to opt for a technological gamble, by investing massively in aeronautical research to develop a medium-term aircraft with roughly the same performance and greater energy efficiency, able to avoid areas where contrails form (and therefore non-CO2 effects) using predictive technologies. Or to follow a path based on existing turboprop technology, which implies a change in user behavior by accepting slightly degraded on-board comfort (noise and turbulence) and longer flight times (~3h in turboprop vs ~2h in A320 for a 1500 km Paris-Tunis flight).

In the age of speed and relatively cheap energy (even since the Russian-Ukrainian conflict), quality of service still takes precedence over carbon, and the first path over the second one. However, there is a risk in any technological gamble, and the urgency of climate change means that we need to rapidly find solutions to achieve carbon neutrality by 2050, rather than wait for breakthrough solutions promised for 2035-2040 at best. Moreover, these two socio-technological paths do not contradict each other, and it would be possible to develop longer-range turboprops today, while investing in parallel in research on subjects such as contrails avoidance, and technologies halfway between the turboprop and the turbojet, such as the open rotor [3].

Achieving carbon neutrality within the next 27 years is a major and highly ambitious challenge and will necessarily involve technological innovations and changes in the way aviation is used. Is flying slower a big sacrifice, if it means flying at all?