Availability of en-route alternate aerodromes as potential limitation in flight planning for hybrid-electric regional aircraft

To reduce carbon emissions, hybrid-electric aircraft could replace conventional turboprop aircraft on regional operations. Because of the limited energy density provided by batteries and requirements concerning reserves for operations under instrument flight rules (IFR), a range extender will be required to achieve commercially relevant ranges. In a serial hybrid concept with electric motors driving propellers, a gas turbine in combination with a generator would provide sufficient electricity for longer ranges, on top of the energy stored in the batteries. It is advantageous that the initial flight stages of missions beyond the battery-electric range are operated with the range extender, with battery power reserved for the final part of the mission. If the gas turbine fails, full battery power will remain available to return to the origin airport, to reach the planned destination or to divert to a suitable en-route alternate airport. Therefore, in cases of longer missions, flight planning must consider the availability of alternate en-route airports within the remaining battery-electric range of the aircraft. This paper examines whether the requirement to reach an alternate airport within the remaining battery electric range in case of a failure of the gas turbine / generator is a major constraint on hybrid-electric aircraft operations. For this purpose, actual airport pairs operated by larger regional turboprop aircraft (ATR42/72 and Dash 8-400) as listed in global flight schedules for the year 2019 are analyzed. The authors find that only a small fraction of routes is constrained by the availability of en-route alternate airports, but this result depends on the battery-electric range, for which a sensitivity analysis has been conducted. Minimizing operational constraints can be an important factor for the acceptance and market uptake of new aircraft concepts.