N542AJ

HISTORY OF FLIGHTOn 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. AIRCRAFT INFORMATIONThe 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. AIRPORT INFORMATIONThe 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. WRECKAGE AND IMPACT INFORMATIONThe 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. ADDITIONAL INFORMATIONFlight 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 and appeared to be near or beyond the physical pitch stop. The outer one-third of the trailing edge of the blade appeared to be disbonded. The following propeller blade (blade 5) also exhibited evidence of a core disbond.

Blade 4 failed near the root when it was near the 7 o’clock position; its trailing edge root skins appeared to be ruptured and the blade pitched and deflected in a way consistent with a failure of the blade spar near the root.

At this time, blade 5 was near the 10 o’clock position and its pitch angle was estimated to have exceeded the commanded pitch angle and appeared to be near or beyond its physical pitch stop. Shortly after blade 4 separated, the video showed it approaching the station 4 propeller; subsequently, the station 4 EPU began to separate from the aircraft.

Following the separation of blade 4 from station 3, three additional propeller blades separated from the station 3 propeller within a couple of seconds, and the fifth blade separated during the aircraft’s descent to the ground.

Flight Test Data Review

Examination of the HRR 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. All propellers were at different operating speeds at the time of the incident – an intended behavior of the control system – and the station 3 propeller speed was recorded near a predicted propeller natural frequency (resonance) crossing mode. Accelerometer data for station 3 showed a rapid growth in vibration after reaching the accident flight condition before the initial blade release. Tilt actuator position values for station 3 also showed an oscillation at this time.

Further examination of prior flight test data by Joby revealed that there was 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, which experienced steady loading after achieving zero tilt angle. Tilt linkage loads were also higher in station 3, which analysis by Joby determined could 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 become significant above the design never-exceed airspeed.

Aircraft Component Examination

The recovered station 3 propeller blade segments were examined. Each of the blades was conclusively identified using distinct markings from the video. The initial failure blade spar was separated from the blade skins and was fractured about 50 mm outboard of the end of the titanium root fitting. This location ...