LAX00LA167 · Eurocopter AS-350 BA

The pilot attempted a run on landing after a loss of engine power over rough terrain. The helicopter bounced back into the air after initial touchdown and came to a sudden stop after a skid caught in a ditch. The pilot saw the engine oil pressure light illuminate followed by an illuminated generator light and the sound of the low rotor rpm audio. He informed his passengers that he was making a precautionary landing and briefed them on emergency procedures. After entering autorotation, he unsuccessfully attempted to restart the engine. He continued to the preselected landing zone, and after clearing obstacles, landed in an open area. One passenger seated in the back recalled smelling fuel about 15 seconds prior to the low rpm audio warning. During the on-scene inspection of the fuel system, the T-fitting was partially attached to the ignition solenoid housing. It was further noted that the safety wire was not a field installation. Maintenance personnel indicated that they had not done fieldwork in this area, and the last overhaul was conducted at the manufacturer's facilities in June 1999. The engine was later examined at the manufacturer's facility. Prior to removal of the ignition solenoid valve, T-fitting, and start drain components, manufacturer representatives confirmed that the safety wire, a swedged unit, was installed at their facility during the last overhaul. A visual inspection of the T-fitting and ignition solenoid valve revealed that the threads of both units were covered in a black substance with metallic particles sticking to the threads. The O-ring on the T-fitting was flattened with material missing from it. A metallurgical examination of the ignition solenoid valve and T-fitting was conducted. The T-fitting is made from steel, and the ignition solenoid valve is made from an aluminum alloy. The O-ring is located in a recessed groove that provides sealing between the threaded portion of the ignition solenoid valve and T-fitting. An energy dispersive x-ray spectroscopy (EDS) test indicated that the black substance found on the threads of both components contained peaks of aluminum. It also revealed that the inside threaded portion of the ignition solenoid valve housing exhibited deterioration consistent with fretting. An examination of the chamfer area of the ignition solenoid housing was outside of the manufacturer's design specifications. The chamfer area is the recessed groove in the ignition solenoid housing and is designed to guide the O-ring into position and prevent it from being pinched between the T-fitting and ignition solenoid valve once installed. The O-ring would not have seated properly during installation due to the chamfer area being outside of design specification limits. This would have prevented adequate clamping forces to be applied and may have allowed the lock nut to loosen. The engine was the placed in a test cell for an acceptance run. The engine started and ran for approximately 20 minutes while a range of acceptance tests were conducted. No discrepancies were noted with the engine run-up.

Verbatim NTSB analysis and probable cause from the NTSB dataset