The Future of Advanced Air Mobility

Bonding Issue with Propeller Caused Vertical's VX4 eVTOL Prototype To Crash

Vertical Aerospace says a bonding issue in a propeller was the root cause of an August 9 accident that badly damaged its VX4 eVTOL prototype aircraft during flight testing at Cotswold Airport in the UK. On Thursday, the company said it had submitted a report to the country’s Air Accident Investigation Branch and is preparing to start flying a second, more advanced, prototype of the four-passenger all-electric vehicle in early 2024.

“The early-generation propeller had already been redesigned prior to the incident, with the issue fully resolved ahead of the next phase of testing,” the UK start-up said in a press statement. “Further recommendations by the investigation [team] are being implemented by Vertical.”

According to Vertical, the fault that triggered the release of a single propeller started when another of the VX4’s electric propulsion units was intentionally disabled for a test. This resulted in an excessive balance load, which caused the failure of one of the supporting pylon structures.

Vertical’s engineering team reported that proprietary technology, including the high voltage and battery systems, performed well during the incident. “Voltage, current, and power all stayed within acceptable limits, and cell temperatures were considered normal during and after the incident,” the report concluded.

The company said it will issue a more comprehensive update on the accident once the AAIB’s investigation is complete. Its initial statement indicated that the unexpected fault had caused “the aircraft to enter a stable descent before being damaged on impact with the ground.”

Vertical's first VX4 eVTOL prototype
The first prototype for Vertical's VX4 eVTOL aircraft crashed during flight testing on August 9 due to a failure in a single propeller.

Despite the setback of the accident, Vertical said it managed to complete what it called its remote thrustborne flight test campaign with the VX4 during July, with the aircraft achieving its target speed of 40 knots (70 mph) and “demonstrating exceptional overall stability and control.” The company also reported that performance targets for the remotely piloted, untethered flights were generally exceeded by 10 to 30 percent during hover and low-speed flights.

“The prototype performed especially well in sustained hover, typically the most challenging regime for a VTOL aircraft, where it maintained level flight for longer than anticipated,” said Vertical’s initial report. “The aim of these thrustborne flight tests was to verify acceptable stability, battery efficiency, and control characteristics, aerodynamics, structural loads, performance, and vibration throughout this speed range—all of which were achieved.”

Type Certification Timeline Unchanged

Stressing that type certification timelines for the program are unchanged, Vertical said it is also working to build another full-scale prototype, which would be ready to fly in the second half of next year. Honeywell, GKN Aerospace, Hanwha, Solvay, Leonardo, and Molicel are contributing as key program partners.

The two aircraft being prepared will have improved structures and subsystems that will get them closer to a production specification to prepare for type certification, which is anticipated in 2026. Both will require a permit to fly from the UK’s Civil Aviation Authority (CAA) to be released to start piloted flights.

“We are pleased with our flight test progress to date and the data, insights, and invaluable learnings we have collected,” commented Vertical’s founder and CEO Stephen Fitzpatrick. “While a fault of any sort is disappointing, it is not wholly unexpected at this stage of testing a novel aircraft. I am pleased that as a result our expert team has isolated the cause of the fault and has been able to provide the AAIB with our report within 14 days of the incident.”

The aircraft that crashed on August 9 will now be used only for ground testing. The type of propeller blade on this first prototype will not be used on later models.

"Test flights are a key part of advancing aviation safety and technology," commented the CAA's joint interim chief executive Rob Bishton. "As the regulator it's important to create conditions in which companies can safely innovate. That environment is one that is safe, robust and sustainable, one which enables companies to develop and test their solutions in the knowledge that we are supporting them in an open and clear way."