Accident Overview
A revenue sightseeing helicopter operating under 14 CFR Part 91 experienced a partial loss of power and attempted a forced landing on the summit of Mauna Loa volcano. During the landing, the aircraft collided with lava rock.
Pilot Report
The pilot stated that while in a shallow downwind turn at approximately 200 feet above ground level (AGL) and an airspeed of about 40 mph, the helicopter began to settle. He continued to add power, but the aircraft was too low to increase airspeed. After striking the lava rock, the helicopter skidded 12 feet, turned to the right, and rolled over onto its left side.
System Function and Testing
Both the flight controls and the engine functioned normally until touchdown. Subsequent tests disclosed no discrepancies in the fuel system.
Height-Velocity Diagram Considerations
Investigators noted that the height-velocity (H-V) diagram, which defines safe combinations of altitude and airspeed, had only been tested up to a density altitude of 10,000 feet. The accident occurred at a density altitude of 15,500 feet. Moreover, the H-V diagram does not apply above a density altitude of 5,400 feet at a gross weight of 3,600 pounds, which was the estimated weight of the helicopter at the accident site.