EVTOL Industry Makes Case For Performance-Based Energy Reserves

Industry has made its case for performance-based energy reserves for electric aircraft as an alternative to the conventional prescriptive time-based reserves proposed by the FAA in its draft operating rules for powered-lift-electric-vertical-takeoff-and-landing (eVTOL) vehicles.

The industry’s concept for managing the range and endurance of battery-electric aircraft is outlined in a report produced by the General Aviation Manufacturers Association’s (GAMA) influential Electric Propulsion and Innovation Committee.

“For aircraft that use these [high-voltage energy storage] systems, traditional fuel management may not always result in safety equivalence,” the report says. GAMA stops short of offering a definitive alternative to the FAA’s proposal, and says the report aims to inform the broader industry about battery systems and provide a foundation for further discussion about a performance-based reserve strategy.

Citing a lack of operational data to support a less-restrictive fuel reserve, the FAA’s proposed Special Federal Aviation Regulation for pilot certification and operations would require powered-lift aircraft to meet the same reserve requirements as fixed-wing airplanes: 30 min. for daytime visual flight rules (VFR) operations and 45 min. for nighttime VFR and day or night instrument flight rules operation.

The European Union Aviation Safety Agency (EASA), in eVTOL operational rules submitted in August for EU adoption, proposes a performance-based reserve requirement, equivalent to the energy required to execute a go-around maneuver. The representative time to perform a go-around with degraded vehicle performance is to be established during initial certification and can be defined for a go-around to a vertical and a conventional landing.

Existing time-based reserve requirements were developed to add considerable fuel margin to cover various contingencies, such as delays, higher-than-anticipated energy consumption, winds, navigation errors, route changes and performance degradation. “A simple time-based energy planning procedure to account for complex variables was a pragmatic necessity. Fuel monitoring was crude, and weather forecasting capabilities were limited,” GAMA says.

In conventionally powered aircraft there is a direct relationship between remaining fuel and endurance and it is relatively easy for the flight crew to determine from an indication of fuel quantity how much longer the aircraft can fly. For electric aircraft, an accurate assessment of a battery’s remaining performance requires more than a single measurement, GAMA says.

“Fuel-based systems can deliver constant power down to essentially the last drop of usable fuel,” the report says. “A battery’s ability to deliver power, by contrast, diminishes as energy is removed from the system.” An electric aircraft will lose some performance capability as battery state-of-charge reduces.

The maximum electric potential energy a battery can produce—its open circuit voltage— decreases as its state of charge declines. The equivalent of fuel exhaustion for a fully electric aircraft is when the open circuit voltage falls below the minimum voltage required to sustain flight, also known as the minimum bus voltage. 

For an eVTOL, minimum bus voltage may vary with phase of flight due to the significantly higher power demand in thrust-borne vertical versus wing-borne horizontal flight, GAMA says, and the aircraft may progressively lose operational capabilities as the battery is depleted.

Also, the portion of total potential energy that is usable is a dynamic because energy loss increases with power demand due to higher electrical resistance and electrochemical dynamics. As batteries age, their energy capacity and ability to deliver power decreases with time and usage. Temperature is another factor that causes battery performance to vary as cold batteries have decreased power capability.

The performance-based reserve concept has the same safety objective as the time-based reserve. But rather than being a general rule incorporated into flight planning, the performance-based concept is based on a mission-specific energy hazard assessment, GAMA says. 

The proposed mission energy hazard assessment would analyze contingencies by planning for alternate landing sites, monitoring inflight energy and evaluating aircraft-specific nominal and off-nominal performance throughout the mission to ensure it can reach the destination with the desired reserve intact. It also identifies contingency maneuvers, which the time-based reserve approach does not.