Hughes OH-6A N6639R accident near Horseshoe Bend, Idaho

Probable cause

A partial loss of engine power due to a leak in the fuel cell outlet valve.

NTSB analysis

While conducting wildlife damage management operations at low altitude over hilly terrain, the pilot of the helicopter noted a loss of main rotor rpm. To regain main rotor rpm, he reduced the engine power demand by descending into a draw; however, the helicopter impacted terrain, rolled over, and came to rest on its left side. Two days before the accident flight, the pilot experienced a similar loss of main rotor rpm; however, he was able to safely land the helicopter. The pilot was unable to replicate the loss of main rotor rpm and continued the flight. The pilot believed that the event was an isolated anomaly and did not notify maintenance personnel.

Postaccident examination of the helicopter revealed an air leak located in the fuel cell outlet valve near the firewall. Subsequent examination of the fuel cell outlet valve revealed circumferential scratch marks around the barrel of the pipe. Damage due to cross-threading was observed on the first two threads. When the O-ring was removed, metal chips were located embedded throughout the O-ring. A review of the maintenance records indicated that the fuel system components were inspected during the 100-hour inspection, along with correspondence from the maintenance facility that a vacuum check of the fuel system was conducted during that inspection, with no anomalies noted. No entries were found in the maintenance records of any work done to the fuel outlet valve. The approved maintenance manual for the helicopter contained multiple warnings that air in the fuel system will cause a power reduction or flame out. It could not be determined when, or how, the fuel cell outlet valve developed a leak.

Text from the NTSB. Open the NTSB docket for WPR21LA107 for the full record.