Introduction
G-HYZA, a modified Piper PA-46-350P (Malibu Mirage) originally built in 1997, was part of a development programme for zero-emission aircraft propulsion. The piston engine had been replaced with two YASA electric motors on a common shaft, powered by a high-voltage lithium battery and a hydrogen fuel cell. The aircraft was operated under E Conditions (CAP 1220) at Cranfield Airport.
History of the Flight
On 29 April 2021, the aircraft performed two experimental flights. During the second flight (flight 86), the test plan called for the high-voltage battery to be switched off on the downwind leg, leaving the hydrogen fuel cell as the sole power source. After takeoff from Runway 03, the crew flew a right-hand circuit at 1,000 ft. While on the downwind leg, the pilot adjusted power and propeller settings, deviating from the test card. The observer instructed the pilot to reduce power and later increase it to 120 kW to regain airspeed. As the aircraft turned final, the pilot reduced power to 90 kW and selected the high-voltage battery off. Immediately, all electrical drive to the propeller ceased. The crew attempted multiple resets, including reconnecting the battery and cycling the fuel cell, but power could not be restored. At approximately 320 ft, a MAYDAY was declared. The pilot initiated a forced landing in a grass field. The aircraft touched down at about 87 kt, struck a hedge, losing the left wing, and came to a stop in a ditch. Both crew members exited unharmed.
Aircraft Information
The electric powertrain could operate in three modes: combined (battery and fuel cell), battery only, or fuel cell only. The propeller was a variable-pitch type without feathering capability. The aircraft retained standard flight controls and landing gear. Cockpit displays showed power plant parameters. A 24 V battery and dual alternators powered avionics.
Conclusion
The loss of power occurred during the interruption of the power supply when the battery was selected off. The windmilling propeller generated a voltage that triggered the inverter protection system, locking out power to the motors. The crew could not reset the system. Five Safety Recommendations were made regarding CAP 1220. The operator took Safety Action to address findings.