What happened
On February 5, 2021, at approximately 1400 mountain standard time, a Hughes OH-6A helicopter, registration N6639R, was performing wildlife damage management operations near Horseshoe Bend, Idaho. While flying at a low altitude over hilly terrain, the pilot noticed a loss of main rotor RPM. In an attempt to reduce engine power demand and recover the rotor speed, the pilot maneuvered the aircraft into a draw. During this maneuver, the helicopter struck sloped terrain, rolled over, and settled on its left side. The accident resulted in one serious injury to the pilot and one minor injury to a passenger.
Two days prior to the accident, the pilot had experienced a similar loss of rotor RPM while flying over flat terrain. During that previous event, the pilot successfully landed the aircraft and attempted to use the N2 governor control switch to increase RPM, but the action had no effect. After cycling the throttle from fly to idle and back to fly, the pilot regained RPM and was unable to replicate the issue on the ground. Believing the event was an isolated anomaly, the pilot did not report the occurrence to maintenance personnel.
The investigation
Post-accident inspections of the aircraft revealed substantial damage to the tail boom and fuselage. Investigators identified an air leak at the fuel cell outlet valve, which connects the fuel supply line to the engine fuel inlet hose at the firewall.
Examination of the fuel cell outlet valve by the N/TSB Materials Laboratory revealed the following:
- The first two threads of the fitting showed damage from cross-threading.
- Circumferential scratch marks were present on the barrel of the pipe.
- The O-ring was found to be compressed and flattened, with metal chips embedded in the rubber.
While the engine was tested in a run cell, no mechanical anomalies were found that would cause a sudden loss of power, though a brief engine stall and two decelerations were noted. The engine manufacturer noted that because the fuel system is non-pressure-boosted and relies on a vacuum to draw fuel, any leak prior to the engine can introduce air into the fuel flow, interrupting the supply.
Maintenance records showed a 100-hour inspection had been completed on May 19, 2020, which included a vacuum check of the fuel system that showed no issues. No specific maintenance or repairs were recorded for the fuel cell outlet valve.