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Photo credit to JobyThe NTSB found that a propeller blade separation led to the 2022 Joby flight testing crash. The separation of the propeller blade and the resulting separation of multiple propulsion motor/propeller assemblies led to the loss of remote pilot control of the eVTOL and caused the crash during the expansion of flight test testing.
On Feb. 16, 2022 Joby Aero was conducting planned, remotely piloted, airspeed and altitude envelope expansion flight testing on its eVTOL JAS4-2, the first of Joby's second-generation, pre-production prototype flight test aircraft. The prototype was all-electric, fly-by-wire and capable of vertical takeoff and landing. The final report said the envelope expansion flight test conditions were beyond the expected operating conditions of the aircraft. In the second test, after the prototype reached a maximum dive speed of 181 knots indicated airspeed at an altitude of about 8,900 feet, a prop blade on propulsion station 3, found on the right wing inboard, experienced a bending failure near the root of the blade. This resulted in a release of the impacted blade, which then impacted the propeller on propulsion station 4, found on the right wing outboard, which then released the impacted blade. Subsequently, there was a separation of multiple propulsion motor/propeller assemblies and the loss of the remote pilot of the aircraft. The report notes that the aircraft left controlled flight after the initial blade failure, impacting the ground about 0.5 nautical miles south-southeast away.
Based on the examination of High-Resolution Recorder data for the accident time period, the variable pitch actuator for station 3 was commanding a normal cruise pitch when the blade release occurred. Video evidence revealed a steeper pitch on some of the blades right before the initial blade release. Accelerometer data for station 3 also showed growth in vibration after it reached the accident flights test condition prior to the initial blade release. The tilt actuator position values for station 3 were also showing oscillation at that time.
The NTSB said the aircraft was operated with a civil Optionally Piloted UAS Experimental Airworthiness Certificate, which was assigned May 5, 2021 and was not applicable for the accident flight that occurred in a special use airspace. The main wreckage consisted of a main body, wings, station 3 motor and most of the tail. Some of the separated pieces were scattered around a debris field about 4 nm south-southeast of the main wreckage. There were two main fired at the primary impact site, largely confined to the battery cells damaged during the impact.
Joby had several data or video recording devices on board and most was recovered. While the unit's GoPro 360-degree video camera was damaged in the crash. The NTSB flight recorders laboratory recovered the corrupted files.
Prior flight test data from Joby showed consistent asymmetric behavior between station 2 and station 3, despite the identical mirrored designs. While in cruise mode, the tilt actuators on station 3 were showing an increase in activity during all flight conditions compared to station 2. The tilt actuator linkage loads were also found to be higher in station 3, which can indicate anomalous behavior in the tilt mechanism. The resonant response to the station 3 prop mode crossing was also seen as stronger than that shown in station 2. This showed a coupled interaction with the anomalous tilt mechanism.
"While prior flights excited the propeller mode in transition flight, the strong excitation in cruise was not predicted; post-accident analysis revealed this strong excitation was due to aerodynamic interactions that only became significant when the airspeeds were beyond the expected operating conditions of the aircraft," the NTSB final report said.
The 181 KIAS dive speed achieved during the speed and altitude envelope expansion flight test along with the anomalous prop tilt system at propulsion station 3 had likely resulted in unanticipated aerodynamic interactions which excited the prop mode, leading to a non-uniform blade pitch increase beyond the design limitations of the prototype. The report said this likely caused a load exceedance and resulted in the initial blade failure. The remote aircraft control was lost as a result of the cascading effects of the initial prop separation.
The probable cause was attributed to the prop blade separation during flight testing and the resulting effects, including the separation of multiple propulsion motor/prop assemblies and the loss of remote pilot control of the aircraft. The report also noted that the tilt rotator actuator linkage for the propulsion station 3 which allowed some of the prop blades to be at a steeper angle than commanded contributed to the crash.