The project suffered a setback on 12 March 1969, when the rotor on prototype #3 (s/n 66-8828) struck the fuselage and caused the aircraft to crash, killing the pilot, David A. Beil. The accident occurred on a test flight where the pilot was to manipulate the controls to excite 0.5P oscillations (or half-P hop) in the rotor; 0.5P is a vibration that happens once per two main rotor revolutions, where P is the rotor's rotational speed. The accident investigation noted that safety mechanisms on the controls had apparently been disabled for the flight. The investigation concluded that the pilot-induced oscillations had set up a resonant vibration that exceeded the rotor system's ability to compensate. After the investigation, the rotor and control systems were modified to prevent the same problem from occurring again.