Forced landing of the first hydrogen fuel cell powered test aircraft

A hydrogen and batteries driven aircraft was forced to land after a flight incident.

According to the aircraft company the aircraft was forced to an emergency landing, but landed normally on its wheels in a flat grass field and almost came to a stop, but was damaged as it caught the left main gear and wing in the uneven terrain at the end of the field at low speed.

According to the sources, the flight conformed to the approved test route over the airport, and the aircraft structural integrity was maintained throughout the incident sequence.
The plan was to reach an altitude of 1,000 ft. (305 m) with both the battery and fuel cell active, then switch the plane to fuel cell only, and fly three or more circuits. But as soon as the battery was switched off, the plane lost all drive to the propeller, and after several unsuccessful system reset attempts, the pilot made a forced landing.
There were no unintended hydrogen or electrical releases and no fire.
After the landing, the crew were able to safeguard the battery and safely release hydrogen from the on-board tanks.
No fluid leaks were observed at the time and full data logs were preserved for use in an investigation.

The INITIATING CAUSE was due to an expected sequence of events when performing airborne tests.
When the battery system was switched off and power to the propeller was cut, the airflow through the propeller begun to drive it, causing the electric motors to begin acting as generators.


The same situation had already occurred on two previous occasions, without the need of a forced landing; the first time prompted ZeroAvia to install a fault clear button, which was used successfully the second time power was lost. Thinking the problem was solved, the team continued the tests, but were caught out at the higher airspeed.

Fuel cell piper, on-board batteries, hydrogen storage

The aircraft flew its first successful test flight in September 2020, completing taxi, take-off, a full pattern circuit, and landing.
The test which caused the incident, had already been performed in several previous flights, including one earlier that day, but at slower airspeeds. As ZeroAvia expanded the flight envelope to test higher speeds, the voltage generated by the free-wheeling propeller increased to a point where it triggered the overvoltage protection on the inverter, shutting the system down.

During previous flight, in addition to the power failures, the team had been experiencing some problems with the hydrogen fuel cell. In one flight, 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.

After the landing, the crew were able to safeguard the battery and safely release hydrogen from the on-board tanks. No fluid leaks were observed at the time and full data logs were preserved for use in an investigation..
The company stated that the incident will disrupt its planned six-seat demonstration programme.

The result of the investigation is summarised in the reference.
This event was related to a airplane prototype, and occurred when testing it. It served the manufacturer to improve the design and develop better operative and emergency procedures.