DEN08LA027 · Agusta A119

The pilot said he was landing on top of an 11,900-foot mountain to allow his passengers to "stretch their legs." The approach was "normal" into a headwind of less than 10 knots. Indicated airspeed was 45 knots and main rotor rpm was 70 percent. The low rotor rpm warning horn sounded and he lowered the collective in an attempt to regain rotor rpm. As the helicopter descended from approximately 50 feet, he increased the collective to cushion the landing. The helicopter hit the mountain, spreading and fracturing both forward and aft skids. The engine was later functionally tested. Minor NF (power turbine speed) and NG (compressor turbine, or gas generator, speed) instability in MEC (mechanical mode) and EEC (engine electronic control) modes were noted. According to the engine manufacturer, this would point to the pressure regulator in the fuel control unit (FCU). Engine behavior and power response (slow and rapid acceleration and deceleration inputs) to CLP (collective pitch) inputs were within acceptable limits. The BOV (bleed off valve) closing point was also within limits. The FCU was tested and revealed indications of wear in the pressure regulator, which was reflected in the NF and NG instability. Analysis indicated that this instability would not have been detected by the pilot and would not have prevented the engine from achieving full rated power. According to the nearest weather reporting facility (elevation 5,102 feet msl) AWOS (Automated Weather Observation Station) 1255 observation, the temperature was 8 degrees Celsius (C.), and the altimeter setting was 30.12 inches of Mercury (Hg). The density altitude at the weather facility was computed to be 5,222 feet msl, and the estimated density altitude at the accident site was approximately 13,000 feet msl.

Verbatim NTSB analysis and probable cause from the NTSB dataset