The June 4 crash of Boeing’s urban air mobility prototype occurred after resonant aircraft vibrations incorrectly activated the vehicle’s ground mode, commanding the motors to shut down, according to new details released by the National Transportation Safety Board (NTSB).
In an updated preliminary report, the NTSB describes how the Passenger Air Vehicle (PAV) developed by Boeing’s independent subsidiary Aurora Flight Sciences “descended vertically at a high rate and impacted the runway” during flight testing at the Manassas Regional Airport in Virginia.
No one was injured in the crash of the unoccupied eVTOL aircraft, which was being remotely piloted at the time of the accident. However, the impact broke the aft booms and horizontal stabilizer, and caused other significant structural damage. Additionally, the aircraft’s No. 3 motor “exhibited a brief flash of fire or sparking on impact,” the report states.
According to the report, the aircraft took off normally for its vertical takeoff and landing and low-speed maneuvering test. The remote pilot entered pre-planned flight test stability check maneuvers, including brief lateral maneuvers followed by forward translations along the runway centerline.
After the flight test engineers noted some “brief data dropouts and abnormal motor speeds,” the team decided to terminate the flight. The pilot entered the autoland command, which is the normal landing maneuver for the PAV.
As the NTSB’s report explains, the autoland function “establishes a vertical descent and transitions to on-ground mode using a combination of squat switches and a time derivative of acceleration known as ‘jerk’ logic.” Either the closing of the squat switches or an acceleration spike caused by ground contact will switch the logic to ground mode and command the aircraft’s eight electric VTOL motors to idle. (The aircraft also has a rear-mounted electric motor and pusher propeller for cruise flight, but these were disabled for this particular test.)
On the accident flight, the aircraft initially responded to the autoland command, but the motors went to idle after a small descent. A review of the recorded data “revealed that airframe vibration occurred in a resonant mode and was transmitted through the structure into the flight controller. The accelerations resulting from the vibrations briefly exceeded the jerk logic threshold and the aircraft entered the ground mode, subsequently commanding the motors to shut down.”
Although the aircraft was equipped with a radar altimeter, it was not used for ground detection in the autoland sequence in this test configuration.
An Aurora Flight Sciences spokesperson did not immediately respond to a request for comment from eVTOL.com. Aurora recently told AIN that the company “learned some lessons from that incident” and expects to resume test flying the second PAV prototype early in the New Year.