Aviation Investigation Final Report

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Location:

Jolon, California

Accident Number:

DCA22FA082

Date & Time:

February 16, 2022, 09:58 Local

Registration:

N542AJ

Aircraft:

JOBY AERO INC JAS4-2

Aircraft Damage:

Substantial

Defining Event:

Part(s) separation from AC

Injuries:

N/A

Flight Conducted Under:

Part 91: General aviation - Flight test

Analysis

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

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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.

Probable Cause and Findings

The National Transportation Safety Board determines the probable cause(s) of this accident to be:

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.

Findings

Aircraft

Propeller blade section - Failure

Aircraft

Propeller controlling system - Malfunction

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Factual Information

History of Flight

Maneuvering

Part(s) separation from AC (Defining event)

Maneuvering

Loss of control in flight

Maneuvering

Powerplant sys/comp malf/fail

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.

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Pilot Information

Certificate:

Commercial

Age:

62,Male

Airplane Rating(s):

Single-engine land; Multi-engine

land

Seat Occupied:

None

Other Aircraft Rating(s):

Glider; Helicopter

Restraint Used:

Instrument Rating(s):

Airplane

Second Pilot Present:

Instructor Rating(s):

None

Toxicology Performed:

Medical Certification:

Class 2 With waivers/limitations

Last FAA Medical Exam:

January 13, 2022

Occupational Pilot:

Yes

Last Flight Review or Equivalent:

October 17, 2021

Flight Time:

2965 hours (Total, all aircraft), 43 hours (Total, this make and model), 2641 hours (Pilot In

Command, all aircraft), 80 hours (Last 90 days, all aircraft), 25 hours (Last 30 days, all aircraft),

2 hours (Last 24 hours, all aircraft)

Aircraft and Owner/Operator Information

Aircraft Make:

JOBY AERO INC

Registration:

N542AJ

Model/Series:

JAS4-2

Aircraft Category:

Airplane

Year of Manufacture:

2019

Amateur Built:

Airworthiness Certificate:

Experimental (Special); Special

flight (Special)

Serial Number:

JAS4-201

Landing Gear Type:

Tricycle

Seats:

Date/Type of Last

Inspection:

Condition

Certified Max Gross Wt.:

Time Since Last Inspection:

Engines:

6 Electric

Airframe Total Time:

95.7 Hrs at time of accident

Engine Manufacturer:

AMA/EXPR

ELT:

Not installed

Engine Model/Series:

UNKNOWN ENG

Registered Owner:

JOBY AERO INC

Rated Power:

Operator:

JOBY AERO INC

Operating Certificate(s)

Held:

Certificate of authorization

or waiver (COA)

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.

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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.

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Meteorological Information and Flight Plan

Conditions at Accident Site:

Visual (VMC)

Condition of Light:

Day

Observation Facility, Elevation:

Distance from Accident Site:

Observation Time:

Direction from Accident Site:

Lowest Cloud Condition:

Clear

Visibility

Lowest Ceiling:

None

Visibility (RVR):

Wind Speed/Gusts:

Turbulence Type

Forecast/Actual:

None /

Wind Direction:

Turbulence Severity

Forecast/Actual:

Altimeter Setting:

28.9 inches Hg

Temperature/Dew Point:

11°C / 3°C

Precipitation and Obscuration:

Departure Point:

Jolon, CA

Type of Flight Plan Filed:

Destination:

Jolon, CA

Type of Clearance:

Unknown

Departure Time:

Type of Airspace:

Wreckage and Impact Information

Crew Injuries:

N/A

Aircraft Damage:

Substantial

Passenger

Injuries:

N/A

Aircraft Fire:

On-ground

Ground Injuries:

Aircraft Explosion:

None

Total Injuries:

N/A

Latitude,

Longitude:

35.97087,-121.17582(est)

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.

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Flight recorders

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.

Fire

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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.

Additional Information

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

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.

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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 corresponded to two closely spaced spar ply drops which

created a stress concentration. The unidirectional fibers at the fracture location had a splayed,

broomstraw appearance consistent with a bending failure. The top and bottom blade skins

remained connected together at the tip, but the leading edge and trailing edge joints were

disbonded. Only the 50 mm of spar stub remained attached to the hub. The fractured root areas

of the initial separation blade skins were recovered separately, with some sections not identified.

The separated leading edge “taco” had a kink consistent with impact with the station 4 propeller

blade. There were abrasion marks on the inside of the blade skins at the leading edge consistent

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with the spar being pulled out during the separation. There were irregularities noted in portions

of the adhesive bond area on the upper skin that were consistent with amine blush. There were

impact marks near the trailing edge of the root rib consistent with contact between the root rib

and the motor resulting from chordwise blade flexure with the blades at a steep pitch. The tip of

the station 3 initial failure blade was not identified.

Examination of the other station 3 propeller blade fragments revealed that all had impact marks

near the trailing edge of the root rib consistent with contact between the root rib and propulsion

motor elements at steep pitch. The second blade to depart fractured from the hub near the

outboard end of the titanium fitting and was fractured near the 2/3 span location. The video

showed this blade impacted the right wingtip after departure. The skins of the third and fourth

blades to depart were separated from their spars, and the spars were fractured near the

outboard end of the titanium fittings. The fifth blade to depart fractured from the hub near the

outboard end of the titanium fitting and was mostly intact.

Station 4 (located on the right wing outboard) was found separate from the aircraft with the

propeller intact except for a single propeller blade, which was not attached. Examination of the

non-attached blade found that it exhibited significant mid-span leading edge damage consistent

with the impact of another blade. The spar was intact in this blade and was broken similar to

other failed blades.

A propeller blade from station 6 separated from the propeller before the station separated from

the aircraft. The initial separation blade has not been identified. Two blades have significant

damage but are not believed to be the initial separation blades based on video evidence. The

three remaining blades have no obvious impact marks that would be consistent with collision

with flying debris, unlike the damage seen on the separated Station 4 blade.

Stations 1 and 5 were found with intact propellers and cowlings near station 4. Stations 1 and 5

had no identified propulsion unit or propeller damage that could be attributed to impact with the

terrain. Station 2 suffered a propeller rupture just prior to impact. The propulsion unit and tilt

mechanism separated from the main aircraft immediately prior to impact and was located near

the main crash site. The tilt mechanism was in nominal condition and was found in the cruise

configuration.

Joby Aviation reported that computed tomography non-destructive testing scans from most of

the propeller blades on the aircraft had been conducted prior to the accident. Ultrasonic testing

(UT) inspections had also been conducted on all propeller blades on the day prior to the accident

flight. The station 3 initial failure blade was found to be a typical blade prior to the accident in

comparison to past inspections. Previous CT scans showed a small anomaly on the inner

surfaces of the spar likely affecting a single ply, but scan comparisons of this anomaly over time

revealed that this feature would not have met the criteria to remove the blade from service and

was not likely a factor in the accident. There was no evidence of a skin-to-spar disbond on the

initial failure blade.

The recovered station 3 propulsion unit – including the propulsion motor, propeller pitching

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system, and propeller hub – were disassembled and examined on February 24, 2022, by Joby

and observed by NTSB and FAA representatives.

The unit sustained significant mechanical damage to its propeller pitch system components and

was fire-damaged. Elements of the blade pitching system were forced into the propeller hub by

the impact; the portion of the system responsible for maintaining uniform blade pitch was

crushed, with elements sheared off, and all components of the system were damaged. Although

all five propeller blades had separated prior to impact, all five stub spars and their associated

fittings remained installed in the hub. All blade retention hardware was intact. The blade pitch

linkage was all present and similarly damaged by the impact. Two of the propeller hub-to-rotor

fasteners were broken consistent with the inverted nose-low impact attitude. Some damage was

found to the station 3 blade pitch stops. Damage to the stops was most observable on blade 1.

Evidence of mechanical interference on Station 3 between blade 2 and 3 linkage prior to post-

crash fire could represent abnormal propeller operation prior to impact. The interference was

between the pitch arm on the blade root and the fastener connecting the pitch link to the pitch

arm on the adjacent blade.

The propeller variable pitch actuator was found damaged and compressed consistent with

impact. The corners of the hexagon-shaped thread locking washer under the retaining nut for

the bearing between the rotating propeller and fixed actuator were mostly bent forward, away

from the bearing and opposite of the direction expected from impact. The outer bearing grease

seal showed signs of wear, heat, and deformation.

Joby indicated that these two observations were consistent with damage that could occur if the

bearing and associated pitching system components were operating at or beyond the designed

operational angular travel.

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Administrative Information

Investigator In Charge (IIC):

Hauf, Michael

Additional Participating

Persons:

Jill Wilson; Joby Aviation; Santa Cruz, CA

David A. Gerlach; Federal Aviation Administration

Original Publish Date:

February 7, 2024

Last Revision Date:

Investigation Class:

Class 3

Note:

Investigation Docket:

https://data.ntsb.gov/Docket?ProjectID=104654

The National Transportation Safety Board (NTSB) is an independent federal agency charged by Congress with

investigating every civil aviation accident in the United States and significant events in other modes of transportation—

railroad, transit, highway, marine, pipeline, and commercial space. We determine the probable causes of the accidents

and events we investigate, and issue safety recommendations aimed at preventing future occurrences. In addition, we

conduct transportation safety research studies and offer information and other assistance to family members and

survivors for each accident or event we investigate. We also serve as the appellate authority for enforcement actions

involving aviation and mariner certificates issued by the Federal Aviation Administration (FAA) and US Coast Guard, and

we adjudicate appeals of civil penalty actions taken by the FAA.

The NTSB does not assign fault or blame for an accident or incident; rather, as specified by NTSB regulation,

“accident/incident investigations are fact-finding proceedings with no formal issues and no adverse parties … and are

not conducted for the purpose of determining the rights or liabilities of any person” (Title 49 Code of Federal Regulations

section 831.4). Assignment of fault or legal liability is not relevant to the NTSB’s statutory mission to improve

transportation safety by investigating accidents and incidents and issuing safety recommendations. In addition,

statutory language prohibits the admission into evidence or use of any part of an NTSB report related to an accident in a

civil action for damages resulting from a matter mentioned in the report (Title 49 United States Code section 1154(b)). A

factual report that may be admissible under 49 United States Code section 1154(b) is available here.