What happened
During flight, the pilot of a helicopter experienced a loss of engine power while flying over rough terrain. The pilot observed the engine oil pressure light illuminate, followed by the generator light and a low rotor RPM audio warning. One passenger noted the smell of fuel approximately 15 seconds before the low RPM warning occurred.
The pilot informed the passengers that a precautionary landing was being made and briefed them on emergency procedures. After entering autorotation, the pilot attempted to restart the engine but was unsuccessful. The pilot continued to a preselected landing zone, cleared obstacles, and landed in an open area. During the landing attempt, the aircraft bounced back into the air after the initial touchdown and came to a sudden stop when a skid caught in a ditch. All passengers were not injured.
The investigation
An on-scene inspection of the fuel system revealed that the T-fitting was only partially attached to the ignition solenoid housing. Investigators noted that the safety wire was not a field installation; manufacturer representatives confirmed the swedged safety wire had been installed during the last overhaul in June 1999.
A visual inspection of the T-fitting and the ignition solenoid valve showed that the threads on both components were covered in a black substance containing metallic particles. The O-ring on the T-fitting was found to be flattened with material missing.
A metallurgical examination was performed on the steel T-fitting and the aluminum alloy ignition solenoid valve. Energy dispersive x-ray spectroscopy (EDS) confirmed that the black substance on the threads contained aluminum. The investigation also revealed that the internal threaded portion of the ignition solenoid valve housing showed deterioration consistent with fretting.
Furthermore, the chamfer area of the ignition solenoid housing—the recessed groove designed to guide the O-ring and prevent pinching—was found to be outside of the manufacturer's design specifications. This deviation prevented the O-ring from seating properly, which likely prevented adequate clamping forces from being applied and may have allowed the lock nut to loosen.