Urban Air Mobility: Developing a hybrid-electric aircraft

By Manfred Hader and Stephan Baur

Jean-Christophe Lambert is co-founder and CEO of Ascendance Flight Technologies, a Toulouse, France based company developing the hybrid-electric ATEA aircraft and STERNA, its proprietary hybrid technology stack. It was founded by four members of the former Airbus E-Fan team, which built the first all-electric two-engine aircraft to fly across the English Channel in 2015.

Most of the current vertical take-off and landing (VTOL) designs are solely based on electrical propulsion. Why did you choose a hybrid-electric approach for your ATEA aircraft?

Jean-Christophe Lambert: There are basically two cases to be made for hybrid propulsion. First, the technological maturity of electrical propulsion systems is not yet sufficient for long-distance air mobility due to the limits of battery density. We want to promote regional mobility with up to 400 km range, which is not feasible with only battery-electric VTOL aircrafts. Second, a hybrid system is much more scalable, not only for urban air mobility, but also for commercial aviation. In that sense, we're not focusing on building a full battery-electric propulsion, we're much more interested in developing the technology that enables hybrid propulsion systems for aviation in a more agnostic way. Although at least a decade away, we will then for example be able to integrate hydrogen propulsion onto our platform.

Is this also one of the reasons why your design of the ATEA aircraft looks much more like an aircraft than a helicopter?

Jean-Christophe Lambert: That is less attributed to the propulsion system than to the practical aspects of a lift & cruise configuration that we have chosen. This enables us to optimize cruise flight while still being able to have vertical as well as conventional short take-off and landing capabilities. Also, there are safety advantages, since you can cruise and land safely even if the propulsion system fails.

Congratulations on being selected as one of the vehicle partners for the Urban Air Mobility initiative of the 2024 Olympics in Paris! What do you think were the reasons for choosing you as a partner?

Jean-Christophe Lambert: I think we have a clear point of differentiation with our hybrid configuration. However, we believe that the whole aerial mobility ecosystem must be approached systematically and therefore involve the cooperation of many different partners. That is why we have been working together with Groupe ADP , one of the consortium partners, on infrastructure solutions since 2018.

Given that the Olympics are only three years down the road, how far are you in your vehicle design and how ambitious is the timeline to 2024?

Jean-Christophe Lambert: Right now, we are working on both Scale-1 prototype and experimental test benches. We want to present a large-scale prototype at the Olympic Games in 2024. The chosen architecture was one decision that will help us achieve this ambitious timeline, but we still need to be very fast in developing the technology and testing it. One of the most challenging issues is the certification process due to the disruptive nature of the technology.

Speaking of certification, what are the greatest challenges in this process?

Jean-Christophe Lambert: One of the biggest challenges is planning for the certification process – you cannot enter the certification process expecting it to be short and cost-effective. We try to keep the architecture as simple as possible, but the requirements are very high. Key systems must have a critical failure rate of one in a billion. You also have to ensure continued safe flight and landing in a no single point of failure approach to be able to land at any time, which is very challenging for a VTOL aircraft.

What kind of certification are you pursuing? And what kind of advantages does your design have with a view to achieving this certification?

Jean-Christophe Lambert: We are aiming to achieve EASA SC-VTOL certification. Controversially, one advantage Ascendance has is that we started our work later than our competitors, and we therefore knew the rules for compliance with the EASA Special Condition VTOL, which are still in specific development for VTOL aircraft. Also, based on our experience at Airbus , we knew from the start that certification would be the most important issue to address in the development of our aircraft.

Why do you think certification is the most important issue beyond going through this lengthy process?

Jean-Christophe Lambert: There is a critical tradeoff between innovativeness and compliance with certification. The culture of the company must strive towards the highest safety standards in design and production. This makes it incredibly hard to find innovative and disruptive solutions to technological problems. All existing VTOL designs are very innovative – in the end, however, the industry leader will be the one that can turn a light prototype into an industrialized design. You have to find the best compromise between performance, certification and industrialization.

In finding this ideal compromise, is Ascendance planning its vehicle to fly with a pilot or autonomously?

Jean-Christophe Lambert: ATEA, our aircraft, will enter the market piloted. Full autonomy is used at subscale level and for testing purposes, but I do not expect to integrate it into our certified aircraft in the short term. This is simply because we have a tough timeline and will focus on establishing our technological path first. Certifying an autonomous pilot system is incredibly challenging and is unlikely to happen in the next 5-10 years. This does not mean that we won't have systems assisting the pilot to the highest degree.

You mentioned that Ascendance is partnering up for the establishment of an aerial mobility ecosystem – why are you only focusing on the aircraft design?

Jean-Christophe Lambert: We are putting our maximum effort into developing the technological basis for making sustainable aerial mobility possible with both our aircraft, ATEA, and our hybrid technology stack, STERNA, to scale up to the global aviation market. Once we have reached a certain degree of maturity on these two objectives, we will also be able to cover different steps in the value chain. When it comes to building up the market, we believe in a joint approach and are relying on a set of partners for now.

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