A Piper M Series aircraft being used by ZeroAvia as a flying testbed for its hydrogen propulsion system crashed on April 29 last year after electrical power was lost to both of its motors, according to a report by the UK’s Air Accident Investigation Branch (AAIB). The report found that an emergency procedure to clear an inverter lockout proved ineffective and that the company had failed to investigate previous loss-of-power incidents also caused by inverter lockouts that had happened three flights prior to the accident near Cranfield Airport in England.

In their summary of factors contributing to the non-fatal accident, in which the aircraft was badly damaged after a crash landing in a field, AAIB investigators said that ZeroAvia’s flight test team did not review the risk assessment of the testing following the loss of propulsion on two previous flights. “Ad hoc changes were made to the flight test plan, including the position where the electrical power source was switched, without the knowledge of the competent person,” they concluded.

The report was published as ZeroAvia prepares to make the first test flight of its fuel cell-based powertrain on a larger Dornier 228 aircraft from its UK base at Kemble. It expects that to happen on July 15, subject to the UK Civil Aviation Authority (CAA) issuing a permit to fly. The Piper flight tests had been conducted under the agency’s experimental rules.

ZeroAvia said it welcomed the conclusion of the investigation and that “many of the issues identified in the report were similarly noted in our internal investigation and have been addressed robustly.” The flight testing last year was conducted as part of the HyFlyer 1 project, which received UK government funding.

“We have embedded key learnings into both our organizational culture and structure, as well as our future technical designs,” the California-based start-up said in a written statement. “This includes the establishment of a Safety and Security Review Board, adding to our team with extensively qualified members in several critical positions, including our design, airworthiness, and flight test teams, and establishing a safety management system based on a ‘just’ aviation culture, including occurrence reporting, investigation, and corrective actions functions.”

The AAIB report stated that the Piper lost power when, as part of the testing process, the pilots switched off the battery 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,” it stated. “The pilot and observer were unable to reset the system.”

As part of the accident report, the AAIB has made five safety recommendations for changes to the CAA’s rules on the operation of experimental aircraft. They include changes to the design and positioning of aircraft controls and displays, as well as calling for closer attention to0 the scope of flight testing and the ability of the appointed competent person to safely manage the operations.

Under its HyFlyer 2 project, ZeroAvia is working to secure a supplemental type certificate to convert a regional airliner to hydrogen propulsion and have it ready to enter commercial service by the end of 2024. The company, which has yet to confirm which type it will seek to bring to market first, is considering plans to convert aircraft such as the Cessna Grand Caravan and the Twin Otter. It also aims to provide propulsion for new aircraft including Otto Aviation's planned Celera 750L.

In the longer term, ZeroAvia believes, it could also convert larger 40-seat aircraft and it has received support from operators including British Airways, United Airlines, and Alaska Airlines. It is also working with Mitsubishi on plans to convert regional jetliners such as the CRJ family.

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