Hughes 369D N500LW accident near Kooskia, Idaho

Probable cause

A complete loss of engine power due to the failure of the No. 2 bearing, which was precipitated by electrical arcing that occurred at an unknown time prior to the accident flight.

NTSB analysis

The pilot was maneuvering the helicopter for a wolf-capturing mission above rugged terrain densely populated by trees. As the pilot maneuvered the helicopter in a hover about 300 to 400 feet above the ground, a loud "bang" emitted from the engine compartment, followed by a loss of engine power and slight vibration. With the low rpm light illuminated, the pilot lowered the collective and entered an autorotation; the helicopter landed hard.

A postaccident examination of the engine revealed metal particles adhering to both engine chip detector plugs. Disassembly of the engine revealed that the No. 2 bearing failed. The damage identified on the bearing’s outer race was consistent with electrical arcing. The extensive wear and mechanical damage to the bearing raceways and the bearing balls precluded the identification of arcing damage on the raceways and ball surfaces. However, the arcing damage found was sufficient to precipitate a bearing failure such as was found on accident engine.

The bearings are a common source of electrical grounding between the case of a turbine engine and the rotating components. Hence, bearings are a probable location of electrical arcing if conditions are sufficient. Although the exact source of electrical arc could not be determined for the helicopter, it is possible that a lightning strike or stray electrical current could have precipitated such damage. The metal splatter deposits present on the oil slinger surfaces were approximately aligned in the same direction suggesting that the arcing associated with the oil slinger occurred when the engine compressor was not rotating.

Although the chip detector caution indicator was found to be inoperative, it is unknown whether it would have alerted the pilot soon enough to the impending engine failure for him to execute a precautionary landing.

Text from the NTSB. Open the NTSB docket for WPR10GA102 for the full record.