ZeroAvia and the hydrogen fuel cell propulsion system they are developing have become a regular feature on aviation news websites. Most often, this is due to its many high-profile supporters such as Alaska and United Airlines that have been eager to sign agreements that could help propel them toward net-zero carbon operations.

However, a little over a year ago, the company hit the headlines due to the crash of its test aircraft, a modified Piper PA-46-350P, near Cranfield Airport in Bedfordshire, UK. A report now says the incident, which took place on April 29, 2021, occurred during a battery-off test, which led the electrical engines to lose power.

In this case, the battery-off test entailed switching off the electric battery that was powering the aircraft to have it operate solely on the hydrogen fuel cell installed in the plane. The Air Accidents Investigations Branch (AAIB) released a report late last week that said,

"During this interruption, the windmilling propeller on the aircraft generated voltage that was high enough to operate the inverter protection system. This then locked out the power to the motors and the pilot and observer were unable to reset the system and restore electrical power."

Damaged ZeroAvia test aircraft
The aircraft was severely damaged in the crash. Photo: AAIB

Plane lost a wing

Onboard the plane were the pilot and one flight observer. The observer stated that the voltage was too high, to which the pilot responded, "we have got to do something quick." They carried out an emergency landing in a field, upon which the plane went through a hedge, and the left wing broke off. Fortunately, the pilot and observer were both uninjured as a result of the incident. Meanwhile, the AAIB concluded that several factors contributed to the accident.

Insufficient review of prior incidents

Among the issues the AAIB raised was a lack of sufficient ground testing to determine the effect of the back voltage from a windmilling propellor on the inverter protection system. This was coupled with an ineffective emergency procedure to clear an inverter lockout after the protection system operated.

Furthermore, the AAIB said that ZeroAvia had not sufficiently investigated a similar incident occurring three flights prior and failed to review risk assessment following the loss of propulsion on two previous flights.

ZeroAvia Dornier 228
ZeroAvia is working hard to produce a hydrogen-powered version of the Dornier 228. Photo: ZeroAvia

Commercial pressure major culprit

The AAIB also found that the "fast tempo" of the project meant that the "competent person's involvement was restricted in a number of areas," including not being aware of ad hoc changes to the test flight plan, such as where the electrical power source was switched.

ZeroAvia has set itself an incredibly ambitious timeline of having a 19-seater passenger aircraft operate a commercial flight between the UK and the Netherlands by 2024. The report stated that the test team was experiencing a "high workload and there was pressure on them to achieve a long duration demonstration flight by the end of May 2021".

The report resulted in five safety recommendations from the AAIB, and ZeroAvia has taken several safety precautions to address the findings. These specifically include managing what is termed "commercial pressure" so it does not compromise safety.