Accident Overview
On February 5, 2021, about 1400 mountain standard time, a Hughes OH-6A helicopter, registration N6639R, was substantially damaged when it impacted sloped terrain and rolled over near Horseshoe Bend, Idaho. The pilot sustained serious injuries, and a passenger sustained minor injuries. The helicopter was being operated as a public use flight for wildlife damage management operations.
Accident Sequence
While maneuvering at low altitude over hilly terrain, the pilot noted a loss of main rotor rpm. He maneuvered the helicopter into a draw to reduce engine power demand and regain rotor rpm. The helicopter then impacted sloped terrain, rolled over, and came to rest on its left side.
Previous Incident
Two days before the accident flight, the pilot experienced a similar loss of rotor rpm while over flat terrain and was able to land safely. On the ground, he used the "BEEP" switch (N2 governor control switch) to increase rotor rpm, but it had no effect. He then moved the throttle from fly to idle and observed no abnormalities. After returning the throttle to fly, main rotor rpm was regained. He repeated the process several times without duplicating the loss, believed it was an isolated anomaly, and did not notify maintenance personnel.
Postaccident Examination
Examination revealed substantial damage to the fuselage and tail boom. An air leak was found at the fuel cell outlet valve connecting the fuel supply line from the fuel shutoff valve to the frangible in-line fuel valve on the self-sealing engine fuel inlet hose at the firewall. Flight control continuity was confirmed. The engine was removed and tested, running for about an hour with no anomalies that would have caused a sudden loss of power, though two decelerations and a brief engine stall occurred, considered normal for a mid-life engine.
Fuel System Findings
The accident helicopter had a non-pressure-boosted fuel system. The engine fuel pump generated a vacuum at its inlet to draw fuel from the tank. According to the engine manufacturer, a leak in the fuel delivery system before the engine can produce air entrainment, interrupting homogeneous fuel flow. The Rolls Royce maintenance manual warns that air leaks in the fuel system can cause flameouts, power loss, or overspeed.
Examination of the fuel cell outlet valve by the NTSB Materials Laboratory using a pressure test with soapy water revealed bubbles at the pipe barrel and valve, indicating an air leak. Visual inspection showed circumferential scratch marks on the pipe barrel and damaged threads due to cross-threading. The O-ring was compressed with flattened surfaces and embedded metal chips. Maintenance records did not indicate the age of the valve or any prior maintenance on it.