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By Elan Head

An award-winning journalist, Elan is also a commercial helicopter pilot and an FAA Gold Seal flight instructor with helicopter and instrument ratings. Follow her on Twitter @elanhead

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NTSB: Software timing error led to Heaviside eVTOL crash

Just two weeks after Kitty Hawk unveiled its single-seat Heaviside eVTOL last year, a prototype was substantially damaged after a software timing error affected the controllability of the aircraft, accident investigators say.

Heaviside eVTOL in crash
Kitty Hawk’s Heaviside has a total of eight VTOL electric motors with pusher-type propellers: six on the main wing, and two on the canard. The accident aircraft was serial number H2.2-003. Kitty Hawk Photo

According to a newly released report from the U.S. National Transportation Safety Board (NTSB), the all-electric Heaviside 2 aircraft registered N221HV — the same one that appeared in Kitty Hawk’s Oct. 3 reveal video — was performing an unmanned developmental test flight on a privately owned ranch near Tres Pinos, California, at the time of the accident on Oct. 17. Kitty Hawk has been authorized by the Federal Aviation Administration to conduct flight testing up to 5,000 feet mean sea level (MSL) at two locations on the ranch.

The report describes how, approximately 10 minutes into the test flight, the aircraft encountered an anomaly while in conventional wing-borne flight at 2,000 feet MSL (approximately 700 feet above ground level). The ground station operator notified the remote pilot-in-command (PIC) that multiple flight computer limits were exceeded. The PIC took manual control of the aircraft and began descending it in preparation for an immediate landing.

“The PIC stated that he felt the aircraft had degraded control, and because the location on the test site was a large open field that allowed for a land out maneuver in any direction, he made the decision to perform a conventional landing into the wind,” the report states.

The prototype touched down in a field of tall grass with approximately 37 knots forward airspeed, 20 degrees nose-down pitch, and 20 degrees left roll. Heaviside was not designed to land with forward velocity on unimproved surfaces, and the prototype suffered substantial damage.

According to the report, the canopy and nose section separated from the fuselage, and the left canard separated from the nose section. The fuselage showed cracking on its lower left side, and the landing gear and tail skeg were damaged and separated from the aircraft. While the primary wing remained mostly intact, damage was noted to the outboard motor fairings, and the left outboard motor propeller was detached. There was no evidence of fire.

A review of the recorded data by Kitty Hawk revealed that the aircraft experienced a software timing error “caused by a battery charging script that, due to operator error, was not properly terminated at the ground station prior to the test flight. The improper termination resulted in the battery charging script running in an error state when it was not intended to be running, and the priority associated with the script allowed for the script to utilize significant processing resources,” the report states. “The manufacturer confirmed both the cause and effects of this timing error by recreating the sequence of events using a hardware in the loop simulator.”

Kitty Hawk told the NTSB it is making improvements to ground station and support equipment procedures as a result of the accident. Future software versions will address processor priority issues to prevent similar timing errors from occurring, and the company intends to have its corrective actions validated by an external party review, the report states.

A Kitty Hawk spokesperson told eVTOL.com, “The event happened during routine testing of Kitty Hawk’s Heaviside prototype vehicle. No one was on board the aircraft as we do not fly people. We are in a new world with electric VTOL vehicles and we want to push our aircraft to the limits to find any issues before we fly people. This was one of the tests we were doing when the aircraft was damaged. We plan to conduct multiple extensive tests on our unmanned vehicle to understand every failure mode to ensure utmost safety before we make them available to people.”

This is not the first time the NTSB has been involved in an eVTOL accident investigation. Last year, the agency published a report on the June 4 crash of the Passenger Air Vehicle developed by Boeing subsidiary Aurora Flight Sciences, which was attributed to resonant aircraft vibrations incorrectly activating the vehicle’s ground mode, commanding its motors to shut down.

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