There is definitely advantages of both. Direct combustion gets you a much greater specific power of propulsion, and the technology for turbine engines is so optimised that it sounds like a really attractive option. We expect we can get higher efficiency with fuel cells and the engine maintenance costs will be lower as fuel cells and electric motors operate at low temperature. Both of these cost drivers lead to a lower CASM.

I have seen claimed that the system with battery powered electric engine would become practically maintenance free. Can you comment on this based on your experience? How would fuel cell based system compare to this?

I think I would be hard pressed to say maintenance free but definitely significantly lower maintenance costs. Turbine blades are intrinsically expensive, made of titanium and operate at high temperatures under high loads. Electric motors have much simpler operation and when made at scale, can really benefit from a replace after 20,000 hours model, instead of a overhaul every 1800 hours, with 200hrs inspection intervals, which we see in some turboprop engines. Fuel cells and batteries are much the same, replace after a certain life, and with built in tolerance to failures. Unfortunately, with fuel cells, as opposed to batteries, we still have some balance of plant that will need maintained, compressors for example, however unlike a turbine engine, they don't operate at high temperatures and are a smaller part of the system, so we can design for much longer lifetimes that you can get away with in a turboprop. All this leads us to estimating 65% saving on engine maintenance. Battery electric companies are going after more like a 90% saving.