How Does The Future of Sustainable Aviation Look?

Whether it’s the roar of Avgas powered pistons or the thunder of Jet A-1 powered turbines, this is the standard that many of us are accustomed to in the aviation industry, and it has been this way for many years. I have distant memories of inhaling the somewhat unpleasant exhaust from a pair of Spey-512s or JT8-Ds during pushbacks, but the health and safety focus was always on the need for ear defenders, which even then to be honest were not always used. Even now, we still tend to look at the contrails of overflying aircraft from an aesthetic point of view, rather than considering the environmental impact; but things are noticeably beginning to change, and a relatively quiet revolution in aircraft powertrain design and alternative fuel sources, which actually started several years ago, is starting to come ever more frequently to the fore of aviation industry discussion forums.

There are in fact many experimental powertrain projects currently underway in a number of countries, with the majority being Europe and North America based. It is fair to say that the attention is focused on hybrid electric, pure electric and hydrogen fuel cell technology, as these are all viewed as providing potential for technically and commercially viable powertrains for commercial aircraft.

The automotive industry can take much of the credit for directing attention and investment resources towards developing battery technology, but whilst there have been a number of research projects experimenting with pure electric powered flight, current technology capability limits its application for the most part to small light aircraft with a modest range such as the Pipistrel Alpha 2-seater and the Pure plane ΦNIX 2-seater. Battery performance will undoubtedly improve in the years to come, but it will be a while before we will have the likes of a pure electric powered Dash 8 or ATR 42/72 equivalent. Current projects such as the Heart Aerospace ES-19 (19-seater), Ampaire Tailwind (9-seater) and Eviation Alice (9-seater), are focused on modest cabin sizes and limited flight range, but are predicted to be ready for service within the current decade.

At the other end of the scale, hydrogen fuel cell powertrains utilising green hydrogen, which is essentially produced by the electrolysis of water using renewable energy sources, would appear to be a desirable long term objective for commercial use. Having said that, the technology has already been proven on a small scale by the likes of ZeroAvia which has successfully test flown a Piper Malibu utilising a hydrogen powered fuel cell.

The attraction of hydrogen based zero emission flight is obvious, but its not without its challenges in terms of both technology and infrastructure development. Storing hydrogen in compressed gaseous form is one thing, but for larger and longer range aircraft, the hydrogen would need to be stored in liquid form which of course is possible with cryogenic tanks, but will present design and certification challenges. Equally, current airport fueling infrastructure is geared around standard aviation fuels, and whilst it is feasible and probably quite likely that airport fuel providers will eventually buy into the concept of hydrogen fuel storage and provision for aircraft use, it will likely be a number of years before this becomes readily and widely available. One further point in relation to hydrogen use for aircraft use is that for a flight to be considered truly zero emission, green hydrogen would need to be utilized, and whilst this is entirely possible in time, hydrogen supply in the shorter term would likely be grey hydrogen which comes from natural gas, but this process produces carbon waste and accordingly would have a negative impact on the overall carbon reduction benefit.

That leaves hybrid electric power, which is seen by many to be the most likely first step on the critical path to what is considered by many to be the long term ultimate goal of green hydrogen fuel cell powertrains.

Much attention has been focused on hybrid electric projects, from the concept of retrofitting hybrid electric powertrains to current airframes, to design and production of totally new aircraft platforms. The benefit of a hybrid electric aircraft is that it combines the benefits of electric flight, such as reduced noise and emissions on take-off, with the benefit of the range and infrastructure security of a traditional aviation fuel powered aircraft. If such aircraft utilize a sustainable aviation fuel (SAF), then the carbon reduction benefit is greater still. Aviation fuel suppliers are responding to the significant level of interest expressed by many airlines around the world in reducing their carbon footprint, and although costs are currently higher for SAF than standard aviation fuels, the difference will likely reduce in time. Some of the largest aviation fuel providers such as Shell, Air BP and Chevron are either currently supplying or working on SAF development projects, either independently or with partners.

There are a number of large corporate OEMs and new start up SMEs that are involved in hybrid electric aircraft projects, with much experience having been gained in recent years. Some projects have been abandoned such as the joint Airbus/Rolls Royce/Siemens E-Fan X, but nevertheless, much was learned through the experimental process.

Whilst some projects have focused on retrofitting an electric powertrain to an existing airframe such as the Ampaire EEL (Cessna Skymaster) and the Zunum Aero ZA10 which hit investment problems, others have taken the path of designing a totally new aircraft, specifically designed for an electric powertrain. Such an approach is likely to be an attractive proposition to many airlines, as a new purpose built platform can incorporate a broad spectrum of more recently developed technology driven benefits, reducing operational costs on a greater scale than a retrofitted aircraft, and providing greater passenger appeal. A good example of this is the Faradair Bio Electric Hybrid Aircraft (BEHA), which presents a wide range of technology and functionality design innovations, including an ingenious triple box wing design for STOL performance, carbon composite structure, and of course a hybrid electric powertrain which includes a gas turbine which can utilize SAF. This is a clean sheet design which is intended to be a hybrid electric vehicle initially, but capable of being updated to fully electric or possibly hydrogen fuel cell powered as the technology matures to a sufficient level to be commercially viable. This purpose built new design approach must surely be the best short to medium term solution for a technically and commercially viable route to significantly reducing an aircraft operator’s carbon footprint.

It is perhaps worth mentioning at this stage that there are a number of current electric VTOL projects, some of which are autonomous, with a benefit case based on potentially being able to offer more direct point to point operational capability; the typical operational scenario being based around an air-taxi service. There may well be a future for such platforms, but technical challenges aside, the certification requirements considering the safety implications of such vehicles flying over populated areas, would likely mean that it would probably be some considerable time before we see any such platforms in commercial operation.

The greatest hurdle to maximizing the benefits of research and development into sustainable aviation technologies, may well be securing the confidence of those organisations capable of providing the necessary financial investment for such projects. This is likely to be less of an issue for the large corporates, but what about the many SMEs that are often responsible for some of the most ingenious technology advances? With respect to the UK, think of Sir Barnes Wallis and R.J. Mitchell, they had it tough in their day securing acceptance of their designs, but is it really much different today?

Attracting investment for aviation projects has always been a challenge, and successive UK governments have been slow to nurture or even protect UK design and manufacturing expertise. There is undoubtedly a general move in the right direction of supporting innovation, with the establishment of institutions such as the Aerospace Technology Institute, the Aerospace Growth Partnership and Innovate UK, and public sector funding is becoming available, although on a limited scale. The establishment of the Jet Zero Council with the aim of zero-emission flight by 2050 may well focus greater attention on supporting SMEs actively involved in this field, but investment from the private sector is also crucial if we are to reap the benefits of some of the remarkably innovative engineering design expertise that is to be found in some of our SMEs.

So how does the future of sustainable aviation look? It actually looks quite bright, there is clearly industry, government and public interest and focus on gravitating towards a greener and more sustainable future for all industries. We are not lacking aviation engineering design expertise; perhaps the hardest task is convincing both the public and private sector, that electric powered flight is undoubtedly the way ahead for the global aviation industry in order to ensure its survival and continued growth, and just as the automotive industry has clearly mapped out its future using this technology, the aviation industry must do the same. Hybrid electric aircraft of 19 seats or less, to keep within EASA CS-23 certification requirements rather than the more demanding CS-25 requirement for larger aircraft, offers the most logical starting point on this map, and then as the technology matures, adoption of full electric powertrains and perhaps ultimately green hydrogen powered fuel cells, may be the optimum route to sustainable aviation nirvana.

The future of aviation looks decidedly green, with a growing determination to utilize technologies that will ultimately result in quieter and less polluted airports and skies, and a more sustainable aviation industry.