Leading electric airplane developer ZeroAvia says it has “addressed robustly” many issues contributing to an accident of a modified Piper Malibu Mirage (PA-46-350P) propelled by two electric motors.
In the April 2021 event, the airplane lost power during a flight test, causing a forced landing just outside Cranfield Airport (EGTC), in Bedfordshire, England. Neither member of the two-person flight crew was injured in the accident.
The event serves as a reminder that—although there have been many successes—the electric aviation industry still faces significant challenges in its journey toward making electric flight viable and reliable as an environmentally friendly alternative to fossil fuels.
According to the accident report, released July 7 by the U.K.’s Air Accidents Investigation Branch (AAIB), the airplane was reconfigured so its motors could be powered by either a high-voltage lithium battery or a hydrogen fuel cell. A key part of the powertrain includes devices called inverters that transform direct current (DC) electricity from the energy sources to the motors. In the moments before the accident, “electrical power was lost to both motors as the power source was changed, and the inverters locked out, at a position in the circuit where the aircraft could not safely glide to the runway,” the AAIB report said.
The inverters locked out, the report said, because during the change from battery to hydrogen fuel cell while in flight—a normal process in the electric aircraft’s operation—the propellers windmilled, which turned the motors. The motors acted as generators, producing energy that was fed back to the inverters, which triggered an overvoltage protection protocol that caused the inverters to lock out.
According to the report, the pilot in command identified the power loss from the aircraft response, rather than the instrument display. Nine seconds after the power loss at an altitude of 880 feet above the airfield, the crew said the inverters had been lost.
The pilot issued a mayday call and began a left turn to line up for landing on Runway 21, but “almost immediately he recognized that he did not have sufficient height to complete the maneuver,” the report said. The pilot then lowered the gear and selected full flaps for a forced landing at about 87 knots ground speed on a level grass field. The Piper hit a hedge and plowed through it, breaking off the left wing.
It stopped when its nosewheel and left main wheel rolled into a ditch, the report said. The flight crew exited the airplane unhurt through the “upper half of the cabin door.”
What Went Wrong
The accident report criticized the “location of the system status display and the absence of aural warnings.” As a result, “critical information regarding the motor operation was not readily available to the pilot. The only indication of a loss of power was a change in color of the small symbols M1 and M2 on the cluttered system status display, which was obscured when the pilot’s hand was on the power lever.”
ZeroAvia also did not carry out sufficient ground testing “to determine the effect of the back voltage from a windmilling propeller on the inverter protection system,” the report said.
Shortly after the accident, ZeroAvia began its own internal investigation into the accident. “Many of the issues identified in the report were similarly noted in our internal investigation and have subsequently been addressed robustly,” ZeroAvia said in its July 7 statement. “We have embedded key learnings into both our organizational culture and structure, as well as our future technical designs.” The company said it has created a safety and security review board, as well as added “extensively qualified members in several critical positions, including within our design, airworthiness, and flight test teams.” The company also said it has established a “safety management system based on a ‘just’ aviation culture, including occurrence reporting, investigation, and corrective actions functions.”
ZeroAvia said future projects would incorporate lessons learned in terms of handling back voltage due to windmilling.
According to the report, other factors that contributed to the accident included:
- “The emergency procedure to clear an inverter lock out after the protection system operated was ineffective.
- An investigation had not been carried out into a previous loss of power resulting from an inverter lock out, which occurred three flights prior to the accident flight.
- The risk assessment had not been reviewed 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.”
The airplane “met all the requirements to be flown under” the U.K.’s Civil Aviation Authority regulations for electric airplanes (CAP1220), and “a comprehensive dossier was produced by the competent person. However, this was a complex project, and the competent person was unable to completely fulfill his responsibilities as detailed in CAP1220.” The report said the “competent person’s involvement was restricted in a number of areas due to issues within the organizational relationships, the fast tempo of the project, other work commitments, and restrictions from the COVID-19 pandemic.”
It’s important to note that ZeroAvia is one of the world’s leading developers of electric airplanes. Since 2017, the California-based company has been experimenting with existing small airplane platforms by converting them from traditional fossil-fuel burning powerplants to electric.
In 2020, the company flew what it called the world’s first hydrogen-fuel-cell-powered flight of a commercial-grade aircraft.
In 2021, ZeroAvia partnered with Alaska Air Group to develop a 76-seat electric airliner from a de Havilland Q400 and last May, it announced plans with Mitsubishi’s MHI RJ Aviation Group to electrify its CRJ series of regional jets.
The AAIB report recommended that the CAA develop additional guidance “on the design and positioning of controls and displays used in the operation of the aircraft.”
It also recommended clarifying “the scope of projects considered suitable to be carried out under CAP1220.”
In addition, the report called on CAA to ensure that the “individual nominated as a competent person under CAP1220, Operation of Aircraft Under E Conditions, has the knowledge, skills, experience, and capacity to manage and oversee the experimental test program.”