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Final report on the ZeroAvia crash investigation: It was not hydrogen

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ZeroAvia/ AAIB
ZeroAvia/ AAIB

During flying testing in April of 2012, a hydrogen-powered ZeroAvia test aircraft lost power and crashed. The Air Accidents Inquiry Branch (AAIB) of the United Kingdom has concluded its investigation and determined that the hydrogen system was not to blame.

ZeroAvia is at the forefront of a wave of businesses aiming to commercialize clean aviation fueled by green hydrogen. Hydrogen, which is more energy-dense than batteries, may provide greater endurance and range than battery packs, whether it is used in combustion engines or used to power an electric powertrain via a fuel cell stack, as was the case with ZeroAvia’s aircraft.

Since the Hindenburg disaster in 1937, there has been a great deal of anxiety around hydrogen, particularly in the aviation industry. According to Eli Dourado, a senior research scholar at Utah State University, much of this worry is unwarranted and primarily the consequence of opportunistic lobbying by the young US helium business. Regardless, today’s fuel cell aviation pioneers are conscious that hydrogen needs a bit of a public relations makeover as we enter the age of clean aviation, so any problems that occur during flight testing are of great interest to the whole industry.

The test aircraft for ZeroAvia was a modified Piper Malibu Mirage two-seater with a single variable-pitch front propeller and two YASA electric motors. The energy source was a hybrid system consisting of a 16.25 kWh Li-NMC battery pack and a hydrogen fuel cell. The aircraft could operate in three modes: battery only (100 kW maximum output), fuel cell solely (100 kW maximum output), or both sources active (100 kW maximum output) (200 kW max output).

On the afternoon of April 29, 2021, the aircraft took off from Cranfield Airport in Bedfordshire with a pilot and a flight test observer on board. The aim was to reach a height of 1,000 feet (305 meters) with both the battery and fuel cell functioning, then switch to fuel cell power and complete at least three loops.

However, as soon as the battery was disconnected, the plane lost all power to the propeller, and after many unsuccessful efforts to reset the system, the pilot was forced to make an emergency landing in a field adjacent to the runway, touching down at around 100 mph (160 km/h). The jet then crashed into a ditch after ripping off its left wing while flying through a hedge. The two unhurt crew members exited the aircraft, waited for the airport fire department, and then carefully evacuated the hydrogen tank into the atmosphere and unplugged the batteries. There was no fire, and there were no detectable hydrogen leakage.

As it turned out, the problem was that when the battery system was turned off and power to the propeller was interrupted, the airflow through the propeller began to drive it, forcing the electric motors to serve as generators. Several other aircraft, including one earlier that day, have accomplished this feat, albeit at lesser airspeeds. As ZeroAvia enlarged the flying envelope to test greater speeds, the voltage created by the free-wheeling propeller rose to the point where the inverter’s overvoltage protection shut down the system.

In fact, this had occurred twice before without necessitating a forced landing; the first occurrence prompted ZeroAvia to install a fault clear button, which was successfully utilized the second time power was lost. Assuming that the issue had been resolved, the team continued on, only to be surprised by the increased velocity.

According to the AAIB investigation, ZeroAvia’s culture as a tech startup as opposed to an established aviation corporation was partially to blame.

“The flight test crew was extremely driven to accomplish the project’s objectives because they believed in the technology’s promise and the necessity to improve aviation’s sustainability. They were motivated to make a quick judgment by the operator’s culture and the competitive atmosphere in which they were operating. No member of the flight test crew had official flight test training or experience in a professional flight test programme, therefore they lacked the knowledge to adopt a more careful and methodical approach.”

According to the report, the team’s errors included insufficient ground testing, ineffective emergency procedures, insufficient investigation of previous power outages, a lack of risk assessment review following the previous power outages, and ad hoc changes to the flight test plan without following proper procedure.

The company issued a statement welcoming the report’s findings, stating that its own internal investigation had reached similar conclusions, and that it has since strengthened its corporate culture, added “extensively qualified” team members to its design, airworthiness, and flight test teams, and implemented an aviation-grade safety management system.

In August of last year, when we last met with ZeroAvia CEO Val Miftakhov, he addressed the incident. “Everyone’s OK,” he assured us. “No leaks whatsoever. The reaction team said that if this had been a fossil fuel aircraft, the wing separation would have certainly caused a fire, as the fuel lines would have been severed near the hot engine.”

When asked what would happen if a collision compromised the hydrogen system, he explained the following: “It is difficult to have a genuinely catastrophic scenario there unless the tank is breached. It’s an extremely durable piece of pressurized equipment with thick carbon fiber walls; it’s the last thing on an airplane that’s likely to fail. The most risky components are located downstream of the tank; you may rupture a gasoline line. The tank, however, is equipped with integrated valves, overpressure protection, overflow prevention, and overtemperature protection. If a gasoline line is ruptured, the flow rate will increase, triggering the overflow protection and stopping the flow.”

Therefore, the whole clean aviation industry may breathe a sigh of relief; the hydrogen fuel cell powertrain did not contribute in any way to this tragedy. Even though nobody was injured, ZeroAvia has received a stern wake-up call from this incident. It’s also a caution to the numerous digital companies rushing into the aviation industry to make claims in the decarbonization gold rush: in this game, you must be meticulous.

Source: AAIB Incident Report

Nedim Husomanovic

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