Vertical Aerospace details failure sequence that preceded VX4 crash

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Three weeks after Vertical Aerospace’s first full-scale VX4 prototype crashed during flight testing, the company has shared preliminary results from its internal investigation into the accident, revealing that the event sequence was initiated by the failure of a propeller blade.

The U.K.-based electric vertical take-off and landing developer said it has submitted a report to its country’s Air Accidents Investigation Branch (AAIB), which will complete its own investigation before publishing a final report on the occurrence in the months to come. While Vertical intends to provide a further update to industry once that process is complete, CEO Stephen Fitzpatrick and Chief Engineer David King provided extensive details about the accident in an exclusive interview with The Air Current.

It is a striking display of transparency in an industry where trade secrets are closely guarded and where, in the U.S., the National Transportation Safety Board maintains extremely tight control of information surrounding all investigations in progress. California-based Joby Aviation has been unable to share any details about the February 2022 crash of its own uncrewed eVTOL prototype as the NTSB continues to plod through that investigation, 18 months later.

“There’s some proprietary information that I think we have picked up, and you know, we’re not going to share every single data point that we got,” Vertical’s Fitzpatrick said. “But I think in terms of … all the lessons that we’ve got that will affect passenger safety and aircraft design that we think the whole industry can benefit from, we’re very keen to share.”

The accident occurred on August 9 at Vertical’s Flight Test Centre at Cotswold Airport near Kemble, about 85 miles west of London. The prototype, called “Aircraft One”, had been conducting extensive uncrewed flight testing there since June 16, reaching forward speeds of up to 40 knots in thrust-borne flight.

Related: Vertical discloses first VX4 eVTOL flight tests

At the time of the accident, the aircraft was conducting one-engine-out tests, which are necessary for sizing the battery packs on the VX4. They were also a critical safety check before proceeding to untethered flights with a pilot on board, which Vertical had hoped to accomplish later this year, although it had already completed an initial round of planned flight testing with the prototype.

The VX4 features four tilting propellers along the front of its wing and four pairs of lifting props in the rear, each powered by its own electric motor. According to King, the aircraft had just completed a hover-out-of-ground-effect test point, with the motor for the left outboard tilting propeller disabled, at a height of 30 feet above the runway. As it was beginning to translate to a 10-knot test point, a blade departed the right inboard tilting propeller, creating an out-of-balance load that caused the failure of the supporting pylon.

That structural failure was actually consistent with Vertical’s prior modeling of the scenario, King said. The flight control system automatically shut down the right inboard motor and spooled up the motor that had been intentionally disabled for the test. Normally, that would have permitted a safe descent.

However, the structural failure in the pylon had an unexpected effect on a Controller Area Network (CAN) bus — the digital data bus through which the flight control computers communicate with the motors — that led to degraded power in two additional motors on the right wing. “Then it was essentially three of the motors on the right wing had lost power at a hover 30 feet above the ground. So the aircraft began to descend quickly,” King said.

The asymmetrical power loss meant that the right wing experienced a torsional moment on impact, which caused it to fail outboard of the inner pylons, as it is designed to do for crashworthiness. Although the landing gear was also damaged, the cockpit and cabin remained intact and the onboard battery packs withstood the impact.

King said it was quickly apparent from videos and physical evidence that the sheathing had departed from the spar of the failed propeller blade. Vertical determined that as the sheathing was coming off the spar, it created a bending moment that snapped the spar off at the root. Ongoing destructive inspections aim to shed more light on the factors that led to degradation of the blade’s adhesive bonding.

Whatever conclusions are drawn from that testing, the blade failure in and of itself is unlikely to have much impact on Vertical’s development program, since the company is already following very different design and manufacturing processes for its second full-scale VX4 prototype, now under construction. Vertical said that Aircraft Two remains on track for a first flight early next year, while a third, identical prototype is expected to start flying in the second half of 2024.

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“Aircraft One was designed as a full-scale prototype to be 100% representative for aerodynamics and control laws, so the intent was really to understand the performance and the aerodynamics and get as much of the critical data as quickly as possible,” King said. By contrast, Aircraft Two, while not fully representative of the production VX4, will have “the structures and the critical parts and controls that are the same as what we would do for a type design in a certification program.”

In addition to propellers that follow “standard best practices for rotor blades”, King said, Aircraft Two will feature a second-generation powertrain with lithium-ion battery cells from Molicel and battery packs designed to meet thermal runaway safety requirements. It will also include more refined flight control software covering the full transition flight envelope and an ejection seat for additional safety during piloted flight testing.

While the propeller quality problem may be readily solved, the cascading failure that resulted in an unanticipated power loss could be more consequential. King said that Vertical is still analyzing the intermittent effect on the CAN data bus, which is currently believed to be related to high-voltage arcing. The secondary effects of a wiring failure that causes a high-voltage short circuit are “a very difficult thing to analyze,” he said. “It’s kind of a new requirement for electric aircraft to be able to analyze what are the particular risks, zonal common cause effects of it. So this data point is extremely useful to really look at what those effects are.”

Fitzpatrick said the accident is “a great reminder that these are complex aircraft — there’s a lot of novel technologies. In and of themselves, electric motors, batteries, not entirely new, but … when you put them all together, you have a very complex system that’s hard to model. … I think when you’re designing something that’s as novel as an electric VTOL aircraft, you need to learn by doing.”

Vertical’s challenge now is to persuade investors to continue supporting its vision. The company said it is funded into the second half of 2024, and it does not expect the accident to delay its progress toward certification. Even before its hard landing, however, Vertical was in need of more cash, with just £90 million (around $114 million) remaining in its coffers as of June 30, 2023.

“The kind of investors that we are talking to, I have to say that they have a lot of experience in aerospace; they understand the process of developing type-certified aircraft and the risks and the process of flying a prototype aircraft,” Fitzpatrick said. “[The accident] hasn’t had any impact on the conversations that we’re having, and we expect to complete a fundraise in the second half of this year.”

Write to Elan Head at elan@theaircurrent.com