What happened
During the second traffic pattern of a training flight, the instructor and student were practicing an autorotation entry. The crew climbed to approximately 1,000 feet AGL with the intention of performing a glide, followed by a power recovery and a go-around at roughly 200-300 feet AGL. During the final approach, both pilots were at the controls. Upon lowering the collective, the engine RPM dropped to zero. The flight instructor took control of the aircraft while the student attempted an engine restart, but no further events were recalled until after the impact.
A witness reported that the helicopter struck the runway with significant force, causing the landing skids to split laterally. The aircraft then rolled onto its left side, which led to a fire.
The investigation
Investigators calculated the density altitude at 7,16 and feet. Prior to the accident, weight and balance calculations performed by the crew indicated the helicopter weighed 1,859.6 pounds. However, according to the Pilot's Flight Manual performance diagrams, at a density altitude of 7,162 feet, the maximum allowable weight to maintain proper autorotation characteristics was 1,600 pounds. An examination of the engine and aircraft systems revealed no anomalies.
Findings
- The helicopter was operating at a weight of 1,859.6 pounds, which exceeded the recommended maximum weight of 1,600 pounds for the calculated density altitude of 7,162 feet.
- The engine RPM dropped to zero immediately following the lowering of the collective.