Md Helicopter 369E/500E N633JH accident near Kekaha, Hawaii

Probable cause

Maintenance personnel’s noncompliance with maintenance manual procedures that required the removal of the overrunning clutch output bearing during the regreasing procedure, which resulted in inadequate regreasing of the bearing and the subsequent failure of the overrunning clutch assembly.

NTSB analysis

The pilot reported that, while conducting a doors-off helicopter sightseeing flight near the northern shoreline on the island of Kauai, he smelled smoke and immediately began flying toward a beach for a precautionary landing. Very shortly thereafter he heard a loud “pop,” followed by the engine out aural warning tone. The pilot immediately lowered the collective control and entered an autorotation to land at a nearby smaller, remote sand-covered beach. As the helicopter touched down, it rocked forward and nosed down into the sand. The helicopter then rolled to the right and came to rest on its side, resulting in substantial damage to the tail boom, fuselage, and main rotor system.

An initial examination revealed that the overrunning clutch and engine-to-transmission driveshaft were fractured. The overrunning clutch was found to have failed at its output (forward) end, with significant thermal damage to the clutch inner race and output bearing. The overrunning clutch assembly, Kaflex driveshaft, and the grease sample from the engine power output pad were collected and sent to the NTSB Materials Laboratory. Further examination of the overrunning clutch found the thermal damage was consistent with frictional heating of the output bearing, leading to the output bearing’s failure, causing the clutch inner race to become very hot and degrade its material strength.

The mechanics who maintained and inspected the accident helicopter during its last 100-hour and 300-hour inspections had not removed the output bearing from the overrunning clutch when repacking the bearing with new grease during the inspections. The removal of the output bearing from the overrunning clutch during the regreasing procedure was required by the maintenance manual to allow visual confirmation that the old grease had been pushed out of the bearing during the repacking process.

By neglecting to remove the output bearing from the overrunning clutch during the regreasing procedure, the mechanics could not confirm that the old grease had been completely displaced from the output bearing. A significant quantity of old grease likely remained within the output bearing due to the bearing not being removed during the regreasing procedure. Over time, and with continued use, this residual grease likely degraded further, ultimately leading to the failure of the output bearing.

Text from the NTSB. Open the NTSB docket for ANC24LA017 for the full record.