What happened
On November 29, 2006, a Bell 47G helicopter was conducting a training flight at the Rendsburg-Schachtholm airfield. The flight, which included a flight instructor and a student pilot, initially consisted of hovering exercises and approaches with the hydraulics deactivated. During the fourth circuit, the instructor attempted to demonstrate an autorotation from an altitude of 800 ft AGL.
As engine power was reduced for the maneuver, the pilot was unable to lower the collective pitch lever. This resulted in the main rotor RPM dropping below the red line on the tachometer, leading to a rapid descent rate of 2,000 ft/min. Because the engine power and rotor RPM could not be recovered, the helicopter struck the ground with significant forward momentum. The aircraft's tail rotor blades struck the ground first, followed by the skids, causing the helicopter to tilt onto its right side. The student pilot sustained light injuries, while the aircraft was heavily damaged.
The investigation
The BFU examined the mechanical condition of the Bell 47G and the maintenance history of the aircraft. The investigation focused on the failure of the pilot to maintain rotor RPM and the subsequent inability to recover from the descent. Technical inspections of the rotor assembly were conducted to determine if any mechanical interference prevented the collective pitch lever from functioning correctly.
Findings
Technical examination revealed a critical mechanical failure in the rotor head assembly. The mast nut, which secures the rotor head to the rotor mast, had loosened to the point where only a few threads remained engaged. Furthermore, the associated securing plate had not been properly utilized, as none of the securing tabs had been engaged. This instability caused the rotor head to move relatively against the mast, resulting in severe damage to the rotor mast threads. This mechanical loosening of the rotor head assembly was the primary factor that prevented the necessary pitch adjustment during the autorotation attempt.