Accident Overview
The aircraft, serving as the company's engineering test bed, was conducting evaluations of a fly-by-wire rudder actuator and ground minimum control speed (Vmcg). During the final high-speed ground test run, the aircraft abruptly veered left and became airborne. It entered a left turn, climbed to approximately 250 feet, departed controlled flight, and impacted the ground. All seven occupants were killed.
Investigation Findings
Investigation revealed a design feature in the rudder actuator that removes hydraulic pressure within the actuator if the rudder position commanded by the pilot exceeds the actual rudder actuator position for a specified time. When this occurs, the rudder aerodynamically trails. The actuator had previously disengaged in flight. The company did not conduct a system safety review of the rudder bypass feature and its consequences to all flight regimes, nor of the Vmcg test. The flight test plan specified that engine power be retarded if the rudder became ineffective. Neither pilot had received training as an experimental test pilot. The company allowed experimental flight tests at a confined, metropolitan airport.
Probable Cause
The official probable cause is: Disengagement of the rudder fly-by-wire flight control system resulting in a total loss of rudder control capability while conducting ground minimum control speed tests. The disengagement was a result of the inadequate design of the rudder's integrated actuator package by its manufacturer; the operator's insufficient system safety review failed to consider the consequences of the inadequate design to all operating regimes. A factor which contributed to the accident was the flight crew's lack of engineering flight test training.