United Kingdom

On the morning of the accident flight, G-HYZA was flown for approximately 16 minutes on test flight 85. The flight test team debriefed the results and prepared the aircraft for flight 86. The plan for this flight was for the HV battery to be switched off at the end of the downwind leg then, if able, to fly three or more circuits at 1,000 ft aal using the HFC only to provide electrical power. The flight test team discussed experimenting with combinations of higher airspeeds and propeller rpm that would reduce the aircraft angle of attack and improve the mass flow of air through the radiator which provided cooling for the HFC. This was considered as a potential strategy to manage a slow rise in temperature in the HFC which they had observed in previous flights when flying on that power source alone. The test card for flight 86 was not amended to reflect this intention. At 1406 hrs, following a normal start using both the HV battery and HFC to provide electrical power, the HV was switched off to preserve its electrical capacity. The aircraft taxied to the holding point and was cleared to line up on Runway 03. The weather was fair with good visibility and light winds from 010°. The aircraft entered the runway and backtracked to the threshold where the pilot commenced a run-up of the propulsion system to ensure the HFC could achieve thermal stability within the flight test parameters. Once the temperatures in the HFC were stable, the pilot switched on the HV battery to bring both power sources online and commenced the takeoff run. As the aircraft accelerated and the power lever was advanced, the observer operated the high temperature override switch to maintain the temperature of the HFC within the operating limits. After takeoff, the pilot turned onto the crosswind leg and climbed to the circuit height of 1,000 ft agl. During the downwind leg of the right-hand circuit, the pilot stated the power was set to 95 kW, the propeller to 2,500 rpm and the airspeed to 100 kt. Once stabilized at these parameters, which were at variance with the flight test card conditions, the observer confirmed that the HFC operating temperatures were within limits. He then instructed the pilot to reduce power to 90 kW to assess the effect on the airspeed, which reduced to approximately 95 kt. The pilot increased the power to 95 kW to regain the target speed. The pilot set the power by reference to his display unit which was located below the throttle quadrant. When he looked up from this task, he recognized that the aircraft was in a late downwind position. He turned onto base leg and commented that they were losing speed in the turn. The observer suggested that they could increase power to 120 kW to regain the lost airspeed, then reduce power before turning off the HV battery to re-establish the test conditions. He also suggested a reduction in propeller rpm. The pilot increased power to 120 kW but did not reduce the propeller rpm. As he started to turn onto final, the pilot briefed that once he had established straight and level flight he would reduce the power slightly and turn off the HV battery leaving the electrical motors powered by the HFC. He called final on the radio and was cleared by ATC to fly through at circuit height. Approaching the runway threshold at approximately 940 ft agl, the pilot reduced power to 90 kW, set the airspeed to 90 kt then selected the HV battery to off. Immediately, all electrical drive to the propeller was lost. The pilot and observer made several unsuccessful attempts to reset the system to restore power from the HFC with the observer stating the action to be taken and the pilot making the switch selection. The observer instructed the pilot to select the HV battery to on to reconnect the alternative power source. HV power was not restored so the observer instructed the pilot to attempt a system reset with the HFC in the off position. Electrical power was still not restored and at 440 ft agl the observer declared “the voltage is too high”, to which the pilot replied, “we’ve got to do something quick”. The observer called for a further reset attempt and adjusted the power lever. The aircraft had now travelled the length of the runway and was at approximately 320 ft aal when the observer reported that power could not be restored. The pilot transmitted a MAYDAY call and initiated a turn to the left to position for a landing on Runway 21. Almost immediately he recognized that he did not have sufficient height to complete the manoeuvre so lowered the landing gear and selected full flap for a forced landing in a field that was now directly ahead on a north-westerly heading. The aircraft touched down at approximately 87 kt ground speed on a level grass field. The pilot applied the brakes, and the aircraft continued its movement until it 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 came to an abrupt stop. The pilot and observer were uninjured and exited the aircraft through the upper half of the cabin door. The airport fire service arrived quickly at the scene. The observer returned to the aircraft and vented the hydrogen tank to atmosphere and disconnected the HV battery to make the aircraft safe.

The crew were scheduled to operate two cargo flights from Exeter Airport (EXT), Devon, to East Midlands Airport (EMA), Leicestershire, and return. The co-pilot was the PF for both sectors, and it was night. The sector from EXT to EMA was uneventful with the crew electing to landed with FLAP 40. The subsequent takeoff and climb from EMA to EXT proceeded without event. During the cruise the crew independently calculated the landing performance, using the aircraft manufacturer’s software, on their portable electronic devices. Runway 26 was forecast to be wet, so they planned to use FLAP 40 for the landing on Runway 26, with AUTOBRAKE 3. With both pilots being familiar with EXT the PF conducted a short brief of the pertinent points for the approach. However, while they did mention that the ILS had a 3.5° glideslope (GS), they did not mention that the stabilized approach criteria differed from that on a 3° GS. From the ATIS they noted that the weather seemed to be better than forecast and the surface wind was from 230° at 11 kt. The ATC provided the flight crew with radar vectors from ATC to the ILS on Runway 26 at EXT. The landing gear was lowered and FLAP 25 selected before the aircraft intercepted the GS. FLAP 40 (the landing flap) was selected on the GS just below 2,000 ft amsl. With a calculated VREF of 134 kt and a surface wind of 10 kt the PF planned to fly the approach with a VAPP of 140 kt. At about 10 nm finals, upon looking at the flight management computer, the PM noticed there was a 30 kt headwind, so a VAPP of 144 kt was selected on the Mode Control Panel (MCP). The crew became visual with the runway at about 1,000 ft aal. The PF then disconnected the Auto Pilot and Auto Throttle; the Flight Directors remained on. As the wind was now starting to decrease, the VAPP was then reduced from 142 to 140 kt at about 600 ft aal. As the wind reduced, towards the 10 kt surface wind, the PF made small adjustments to the power to maintain the IAS at or close to VAPP. At 500 ft radio altimeter (RA) the approach was declared stable by the crew, as per their standard operating procedures. At this point the aircraft had a pitch attitude of 2.5° nose down, the IAS was 143 kt, the rate of descent (ROD) was about 860 ft/min, the engines were operating at about 68% N1 and the aircraft was 0.4 dots above the GS. However, the ROD was increasing and soon thereafter was in excess of 1,150 ft/min. This was reduced to about 300 ft/min but soon increased again. At 320 ft RA, the aircraft went below the GS for about 8 seconds and, with a ROD of 1,700 ft/min, a “SINK RATE” GPWS alert was enunciated. The PF acknowledged this and corrected the flightpath to bring the aircraft back to the GS before stabilizing slightly above the GS; the PM called this deviation too. As the PF was correcting back to the GS the PM did not feel there was a need to take control. During this period the maximum recorded deviation was ¾ of a dot below the GS. At about 150 ft RA, with a ROD of 1,300 ft/min, there was a further “SINK RATE” GPWS alert, to which the PM said, “WATCH THAT SINK RATE”, followed by another “SINK RATE” alert, which the PF responded by saying “AND BACK…”. The commander recalled that as the aircraft crossed the threshold, at about 100 ft, the PF retarded the throttles, pitched the aircraft nose down, from about 5° nose up to 4° nose down, and then applied some power in the last few feet. During these final moments before the landing, there was another “SINK RATE” alert. The result was a hard landing. A “PULL UP” warning was also triggered by the GPWS, but it was not audible on the CVR. The last surface wind transmitted by ATC, just before the landing, was from 230° at 10 kt. During the rollout the commander took control, selected the thrust reversers and slowed down to taxi speed. After the aircraft had vacated the runway at Taxiway Bravo it became apparent the aircraft was listing to the left. During the After Landing checks the co-pilot tried to select FLAPS UP, but they would not move. There was then a HYDRAULIC LP caution. As there was still brake accumulator pressure the crew were content to taxi the aircraft slowly the short distance onto Stand 10. Once on stand the listing became more obvious. It was then that the crew realized there was something “seriously wrong” with the aircraft. After they had shut the aircraft down, the flight crew requested that the wheels were chocked, and the aircraft be connected to ground power before going outside to inspect the aircraft. Once outside a hydraulic leak was found and the airport RFFS, who were present to unload the aircraft, were informed.