The crash of a hydrogen-powered test plane in the English countryside last year was not caused by the H2 or fuel cell on board, an official investigation on behalf of the UK government has concluded.

The incident was in fact caused when the battery was switched off mid-flight for test purposes, and the wind flow against the plane caused the propeller to rotate — turning it into a windmill — which generated voltage in the battery and caused the electrical system to lock and fail, the Air Accident Investigation Branch (AAIB) found.

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The pilot and a passenger escaped the wreckage unharmed after crash landing near Cranfield airfield in southern England in April 2021.

The pair were carrying out a one of a series of test flights of the two-seater Piper PA-46-350P propeller aircraft, which had been modified to fly with two electric motors in place of a piston engine, powered by a high-voltage lithium battery and a hydrogen fuel cell.

As planned, the flight team switched off the battery during the flight to power the plane on the hydrogen fuel cell alone, however, at this point the aircraft suffered the electrical system failure and crashed into a field close to the runway, shearing off the left wing and damaging the landing gear and nose of the aircraft.

There was no fire, neither the fuel cell nor the battery leaked, and the flight team were able to vent the hydrogen from the fuel cell safely after the airport fire service had arrived.

But the AAIB, a branch of the UK Department for Transport, was critical of the communication within the flight team, and between the team and designated “competent person” responsible for the safe management of the experiment — a key condition of the tests being allowed to go ahead.

Specifically, it noted that previous in-flight power failures had not been investigated, nor had the risk assessment been reviewed, and that the competent person had not been informed about changes to the flight plan.

In addition to the power failures, the team had been experiencing some problems with the hydrogen fuel cell. On one of the previous test flights, it had shut down in-flight, so the pilot landed using the battery alone. On another planned test, the fuel cell had shut down during last-minute checks before take-off.

During the last test, the flight team were experimenting with maximising the air flow through the cooling radiator for the hydrogen fuel cell, in order to manage the raised temperature they had observed on previous tests.