What happened
On 29 April 2021, a modified Piper PA-46-350P, registration G-HYZA, was conducting experimental flight testing near Cranfield Airport. The aircraft, which had been converted from a piston engine to an electric propulsion system powered by hydrogen fuel cells and a high-voltage battery, was performing a test to evaluate flight using only the fuel cell as a power source.
During the downwind leg of the circuit, the pilot switched off the high-voltage battery to transition to fuel-cell-only power. Immediately following this transition, the aircraft lost all electrical drive to the propeller. Despite multiple attempts by the crew to reset the system and restore power, the propulsion remained inactive. Realising that the aircraft lacked sufficient altitude to complete a standard circuit, the pilot initiated a forced landing in a nearby field. During the landing, the aircraft struck a hedge, causing the left wing to detach and the landing gear to collapse. There were no injuries to the two crew members on board.
The investigation
The AAIB investigation focused on the electrical architecture of the modified propulsion system. The aircraft utilised two YASA electric motors and two inverters configured in parallel. The investigation examined the interaction between the hydrogen fuel cell, the high-voltage battery, and the inverter protection software. Investigators also reviewed the flight test procedures and the crew's attempts to manage the transition between power sources.
Findings
- The loss of power was triggered by a voltage spike during the transition between power sources.
- When the battery was disconnected, the windmilling propeller generated a voltage high enough to trigger the inverter protection system's overvoltage lockout.
- The inverter software locked out power to the motors to protect the system, and the crew was unable to reset this fault in flight.
- The flight test parameters at the time of the incident were at variance with the established flight test card.