Flight Test Plan
On the morning of the accident flight, G-HYZA was flown for approximately 16 minutes on test flight 85. The flight test team debriefed and prepared the aircraft for flight 86. The plan was to switch off the HV battery at the end of the downwind leg and, if possible, fly three or more circuits at 1,000 ft aal using only the hydrogen fuel cell (HFC) for electrical power. The team discussed experimenting with higher airspeeds and propeller rpm to reduce angle of attack and improve airflow through the HFC radiator, as they had observed a slow temperature rise when flying on HFC alone. The test card for flight 86 was not amended to reflect this intention.
Pre-takeoff and Takeoff
At 1406 hrs, after a normal start using both the HV battery and HFC, the HV battery was switched off to preserve capacity. The aircraft taxied to the holding point and was cleared to line up on Runway 03. Weather was fair with good visibility and light winds from 010°. After backtracking to the threshold, the pilot ran up the propulsion system to ensure HFC thermal stability within test parameters. Once stable, he switched on the HV battery, brought both power sources online, and commenced takeoff. During acceleration, the observer operated the high temperature override switch to maintain HFC temperature within limits.
Circuit and Power Loss
After takeoff, the pilot turned onto crosswind and climbed to 1,000 ft agl. On downwind in a right-hand circuit, he set power to 95 kW, propeller to 2,500 rpm, and airspeed to 100 kt — parameters at variance with the test card. The observer confirmed HFC temperatures were within limits, then instructed a power reduction to 90 kW, which reduced airspeed to about 95 kt. The pilot increased power back to 95 kW. When he looked up from setting power, he recognized a late downwind position and turned base leg, noting a loss of speed. The observer suggested increasing power to 120 kW to regain speed, then reducing before switching off the HV battery, and also suggested reducing propeller rpm. The pilot increased power to 120 kW but did not reduce propeller rpm. While turning final, he briefed that he would reduce power slightly and switch off the HV battery once straight and level. He called final and was cleared by ATC to fly through at circuit height. Approaching the threshold at about 940 ft agl, he reduced power to 90 kW, set airspeed to 90 kt, then selected the HV battery off. Immediately, all electrical drive to the propeller was lost. Several unsuccessful attempts were made to restore power from the HFC, with the observer instructing actions and the pilot making switch selections. The observer then instructed selecting the HV battery on, but power was not restored. A system reset with the HFC off was attempted but failed. At 440 ft agl, the observer stated “the voltage is too high”; the pilot replied “we’ve got to do something quick”. Another reset attempt and power lever adjustment failed. At about 320 ft aal, the observer reported power could not be restored.
Forced Landing
The pilot transmitted a MAYDAY and initiated a left turn to land on Runway 21. He quickly realized insufficient height and lowered the landing gear and selected full flap for a forced landing in a field directly ahead on a north-westerly heading. The aircraft touched down at about 87 kt ground speed on a level grass field. The pilot applied brakes; the aircraft struck and passed through a hedge, during which the left wing broke away. The nosewheel and left main wheel entered a ditch, and the aircraft stopped abruptly. The pilot and observer exited through the upper half of the cabin door. The airport fire service arrived. The observer returned, vented the hydrogen tank to atmosphere, and disconnected the HV battery to make the aircraft safe.