What happened
The pilot was conducting an approach to a specific location in a helicopter to inspect items of interest on the ground. The site was characterized by large rocks, high brush, and rising terrain. As the pilot made a shallow approach and slowed the aircraft into a hover, the rotor blades became loaded.
Upon reaching a hover, the pilot observed that the manifold pressure was at 25 inches. Shortly thereafter, the rotor RPM decayed toward the bottom of the green arc, causing the helicopter to begin settling toward the ground. In an attempt to recover, the pilot reduced the collective slightly and applied full throttle; however, this action caused the aircraft to settle even faster.
To avoid a more violent impact with the terrain ahead, the pilot used the collective to cushion the landing. The skids struck a large rock upon touchdown, which resulted in substantial damage to the skids, the fuselage structure, and the tail boom. The pilot was not injured. After the aircraft came to rest, the rotor RPM recovered once the collective was lowered completely, allowing the pilot to perform a standard engine cooldown and shutdown.
The investigation
An analysis of the performance charts in the Helicopter Flight Manual indicated that the density altitude at the accident site—located at 2,200 feet MSL—was approximately 5,000 feet. Because the aircraft was equipped with a normally aspirated engine with a critical altitude of 4,000 feet, the maximum available manifold pressure at that altitude was limited to 24.8 inches. The investigation found that the operating conditions were very near the performance limits for an out-of-ground-effect hover for the estimated weight of the aircraft.