DCA22FA082

The separation of a propeller blade during expansion flight testing that resulted in cascading effects to include the separation of multiple propulsion motor/propeller assemblies and the loss of remote pilot control of the aircraft. Contributing to the accident was the tilt rotor actuator linkage for propulsion station 3 that allowed some propeller blades to be at a steeper angle than commanded.

On February 16, 2022, Joby Aero Inc. was conducting planned, remotely piloted, airspeed and altitude envelope expansion flight tests on aircraft JAS4-2, the first of their two second-generation, pre-production prototype flight test aircraft. The envelope expansion flight test conditions were beyond the expected operating conditions of the aircraft. During the second test flight, and after reaching a maximum dive speed of 181 knots indicated airspeed (KIAS) at an altitude of approximately 8,900 feet, a propeller blade on propulsion station 3 (located on the right wing inboard) experienced a bending failure near the root of the blade which culminated in the release of the propeller blade. The released blade impacted the propeller on propulsion station 4 (located on the right wing outboard), which subsequently resulted in a release of the impacted blade. Cascading effects resulted from the initial inflight blade failures including the separation of multiple propulsion motor/propeller assemblies and loss of remote pilot control of the aircraft. The aircraft departed controlled flight after the initial inflight blade failure and impacted the ground about 0.5 nautical miles (nm) south-southeast away. Examination of the High-Resolution Recorder data for the accident time period revealed that the variable pitch actuator for station 3 was commanding a typical cruise pitch when the blade release occurred, whereas video evidence indicated a steeper pitch on some blades immediately before the initial blade release. Accelerometer data for station 3 showed a rapid growth in vibration after reaching the accident flights test condition before the initial blade release. Tilt actuator position values for station 3 also showed an oscillation at this time. Examination of prior flight test data by Joby revealed consistent asymmetric behavior between station 2 and station 3, despite identical mirrored designs. In cruise mode, the tilt actuators on station 3 showed increased activity in all flight conditions compared to station 2. Tilt actuator linkage loads were also higher in station 3, which can be an indication of anomalous behavior in the tilt mechanism. The resonant response to this propeller mode crossing in station 3 was also consistently stronger than in station 2, indicative of 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 dive speed of 181 KIAS reached during the speed and altitude envelope expansion flight test in conjunction with an anomalous propeller tilt system condition at propulsion station 3, likely resulted in unanticipated aerodynamic interactions that excited a propeller mode, leading to a non-uniform blade pitch increase beyond its design limitations. This likely caused a load exceedance which resulted in the initial blade failure. Aircraft control was lost as a result of cascading effects following the initial propeller blade separation.

The accident aircraft was the first of two second-generation, pre-production prototype flight test aircraft produced by Joby Aviation. The aircraft was all-electric, fly-by-wire, and capable of vertical takeoff and landing. Provisions for five occupants (a pilot and four passengers), were provided, though it could be piloted remotely. The design's maximum gross takeoff weight was 4,200 pounds. The aircraft was operated with a civil Optionally Piloted UAS Experimental Airworthiness Certificate. The Certificate of Authorization (COA) assigned to the aircraft dated May 5, 2021, was not applicable for the accident flight which occurred in special use airspace. Figure 1. Depiction of a JAS4-2 aircraft (Source: Joby Aviation) The aircraft was configured with six tilting propellers directly driven by six dual-powered electric motors supplied by power from four battery packs. The six electric propulsion unit (EPU) stations are identified numerically based on location as station 1-outboard left wing, station 2-inboard left wing, station 3-inboard right wing, station 4- outboard right wing, station 5-left tail, and station 6-right tail. Each of the six variable pitch propeller assemblies were equipped with five blades and actuated by a single variable pitch actuator driving a mechanical pitch change mechanism. On February 16, 2022, a Joby Aero Inc. JAS4-2 experimental aircraft, N542AJ, was engaged in a planned speed and altitude envelope expansion flight test, beyond expected operating conditions. The aircraft was remotely piloted from the ground and observed from a chase aircraft. The aircraft was performing a developmental flight test, operating under the provisions of 14 Code of Federal Regulations part 91, utilizing an experimental category special airworthiness certificate. The flight began about 09:42 PST with a normal vertical takeoff, transition to wing-borne flight, and climb up to 11,000 feet mean sea level (MSL). After successfully completing one test condition, the remote pilot-in-command (PIC) began descending and increasing the speed of the aircraft in preparation for the next test condition. After reaching a maximum dive speed of 181 knots indicated airspeed (KIAS) at an altitude of approximately 8,900 feet, the propeller on propulsion station 3 (located on the right wing inboard) experienced oscillations in rpm and motor vibrations. Based on a review of video evidence and recorded flight data, about 09:58, the station 3 propeller stabilized at a resonant condition with previously unidentified destructive effects which quickly culminated in a propeller blade release from propulsion station 3. The blade spar failed near the root outside the hub and the blade traveled outboard and impacted the propeller on propulsion station 4, resulting in the separation of a blade from this propeller and the separation of this station from the aircraft. The station 3 propeller continued to rotate with significant imbalance. Shortly thereafter station 6 (right tail propeller) experienced a single blade separation and the separation from the aircraft. Cascading effects (loss of other propeller stations from the aircraft) resulted in the aircraft subsequently breaking up in flight. The aircraft departed controlled flight, rapidly rolling to the left, entered an inverted dive, and crashed in an uninhabited area near Jolon, California. There were no injuries, and the aircraft was destroyed. The aircraft’s main wreckage consisted of the main body of the aircraft, wings, station 3 motor, and most of the tail. The main fuselage of the aircraft impacted the ground about 0.5 nautical miles (nm) south-southeast of the initial in-flight failure event. The separated stations 1, 4, and 5 EPUs (including attached propellers), separated propeller blades and fragments from stations 2, 3, and 6, various nacelle cowlings, skin sections from the right tail, and other lightweight debris were scattered in a debris field up to 4 nm south-southeast of the main wreckage. The station 6 EPU separated from the aircraft with no propeller blades attached and impacted the ground about 0.8 nm southeast of the main wreckage. The station 2 EPU and separated propeller blades were located between the main wreckage and an area about 0.1 nm northeast. Debris at the main wreckage site was largely located in an area about 100 feet in diameter. Battery pack structures failed during the impact and battery cells were found spread in the wreckage. Two main fires occurred at the primary impact site and were largely confined to battery cells that were damaged during the impact. Both fires affected an area about 10 feet in diameter and were contained with handheld fire extinguishers. Several devices onboard the aircraft were capable of recording data or video. The following items were recovered from the wreckage with recoverable information. o 3x High-Resolution Recorders (HRRs) data. o Only one of these included a full data set for the accident. o GoPro 360 video from the onboard pilot eyepoint. o Right door-mounted GoPro video looking out the right wing and slightly aft. Information was unable to be recovered from the data acquisition system (DAQ) and GoPro forward-pointing nose camera. Additionally, the following data and video were captured off aircraft: o Ground control station recordings of the nose and tail First Person Video (FPV) cameras o Ground control station antenna video o Chase aircraft video o Ground control station video The majority of the recorded parametric data came from the Joby Aero Inc. high-resolution recorder (HRR), a custom unit that captured various on-vehicle information, including control system messages. There were multiple HRRs on the aircraft to ensure consistent recording of data between redundant aircraft systems. One HRR, which recorded the data relevant to this incident, was the source for all data used in this report. Joby also employed a commercial off-the-shelf data acquisition system (DAQ) to record additional flight test data. This dataset would have included vibration and strain gauge information. This unit was not designed to be crash-resistant and did not survive the accident. No data was recovered from this device. Valuable information was recorded by a cockpit-mounted GoPro 360-degree video camera. This unit was not crash-resistant. The unit’s files were corrupted in the accident but were recovered by the NTSB flight recorders laboratory. Much of the analysis of the accident sequence – specifically, the behavior of the blades and subsequent blade pitch angles on station 3 – was constructed from this recording. Additional video sources on the aircraft include a camera looking outboard in the right door and a forward looking "stinger" camera mounted on a shaft extending behind the tail and looking forward. Flight Test Video Review Joby performed a detailed frame-by-frame analysis of the cockpit-mounted GoPro 360 view camera to document the station 3 propeller condition prior to and during the accident sequence. The blade pitch angles were estimated by comparison to the station 2 blade pitch angles and a CAD model of the blades at various pitch angles. Joby indicated that, seconds before the first propeller blade separated from station 3, it appeared normal and was operating with blade pitch angles within their normal range. The aircraft was established in a descent, then accelerated to a commanded 181 KIAS test point prior to the accident condition. About 0.07 seconds prior to the first propeller blade separating from propulsion station 3, the failure blade (blade 4) was located near the 2 o’clock position (as viewed from behind looking forward, rotation direction counter-clockwise). Joby indicated that the blade’s pitch angle appeared greater than the other blades. When the blade was near the 10 o’clock position, its pitch angle was estimated to have exceeded the commanded pitch angle a