Air Accidents Investigation Branch
Summary:
The electrically powered aircraft was undertaking experimental flight tests, under E Conditions , when power to the electrical motors was lost. A forced landing was carried out close to Cranfield airfield during which the aircraft was severely damaged.
The loss of power occurred during an interruption of the power supply when, as part of the test procedure, the battery was selected OFF with the intention of leaving the electrical motors solely powered by the hydrogen fuel cell. During this interruption the windmilling propeller generated a voltage high enough to operate the inverter protection system, which locked out the power to the motors. The pilot and observer were unable to reset the system and restore electrical power.
Five Safety Recommendations were made regarding Civil Aviation Publication (CAP) 1220, Operation of experimental aircraft under E Conditions. The operator has also taken Safety Action to address a number of findings from this accident.