Incident Overview
A helicopter experienced a loss of rotor rpm and descended into a rough, rock-strewn field while the pilot attempted to hover. The accident site was at an elevation of 2,200 feet mean sea level (msl), with a density altitude of approximately 5,000 feet. The helicopter's normally aspirated engine has a critical altitude of 4,000 feet, and the Rotorcraft Flight Manual indicates that the maximum available manifold pressure at that altitude is 24.8 inches of mercury.
Pilot Account
The pilot stated that he selected a spot suitable for hovering to obtain a closer view of items of interest on the ground. He performed a shallow approach to the spot and loaded the rotor blades as the helicopter slowed. Upon reaching a hover, the manifold pressure was 25 inches of mercury. The pilot then realized that the rotor rpm was near the bottom of the green arc and the helicopter was settling toward the ground.
In response, the pilot reduced the collective slightly and rolled the throttle fully open. However, the helicopter began to settle faster. The pilot applied collective to cushion the landing, but the skids contacted a large rock hard, resulting in bending of the skids and fuselage structure, as well as damage to the tail boom.
After the helicopter came to rest, the pilot lowered the collective completely, and the rotor rpm recovered. He then performed a normal engine cool-down and shutdown.
Performance Considerations
The pilot noted that when the rpm first decayed and the helicopter began settling, he initially considered flying forward out of the descent. However, large rocks, high brush, and rising terrain directly ahead made that option seem likely to result in worse damage and potential injury.
Review of the performance charts in the Helicopter Flight Manual revealed that, given the reported temperature and pressure altitude at the accident site and the estimated weight of the helicopter, the conditions were very near the limit for an out-of-ground-effect hover.