An accident last year in which Kitty Hawk’s Heaviside eVTOL prototype aircraft was badly damaged was caused by a software timing error, according to an investigation by the U.S. National Transportation Safety Board. A report published on March 27 concluded that an operator error resulted in a battery charging script not being correctly terminated before the unmanned electric aircraft took off for a test flight on Oct. 17, 2019.
About 10 minutes into the test flight in Tres Pinos, California, a ground-station operator informed the pilot in command that multiple flight computer limits had been exceeded. The pilot took manual control of the aircraft, which was then in conventional wing-borne horizontal flight, in order to land it.
As the pilot lined up the aircraft to land into the wind, he reported that the controls felt degraded. As the aircraft approached the landing site, he transitioned the Heaviside to its hover configuration but then experienced even more pronounced loss of control. The prototype incurred “substantial” damage after landing in long grass with around 37 knots of forward airspeed and with 20 degrees of nose down pitch and 20 degrees left roll.
Investigators found that the incorrect termination of the battery charging script resulted in it continuing to run in error mode. The script used significant processing resources at a time when it should not have been running at all.
NTSB’s report said that Kitty Hawk’s 30-year-old pilot holds an FAA commercial license and had 237 flight hours at the time of the accident. The test flight was supported by a visual observer near the pilot and a team member in the ground-control station.
In response to the NTSB report, Kitty Hawk said that it is making changes to ground-station and support-equipment procedures. The company added that it will introduce software changes to address processor priority issues to prevent similar timing errors from occurring. It also stated that it will have these changes validated by a third-party review process.