Robinson Helicopter R44 N141TM accident near Chatham, Massachusetts

Probable cause

The inadequate maintenance of the lower forward clutch actuator bearing for a prolonged period of time, which resulted in a bearing failure. Contributing to the accident was an erroneous maintenance entry, which resulted in the helicopter being operated beyond its mandatory airframe overhaul time.

NTSB analysis

The commercial pilot reported that, during an aerial photography flight, the helicopter was about 300 ft above a pond when it experienced an engine overspeed, followed by a loss of engine power. The pilot subsequently entered an autorotation to shallow water.

Examination of the engine revealed that the No. 2 cylinder head rocker shaft bosses, the No. 2 intake valve upper spring, and the No. 3 intake valve spring seat were fractured, consistent with an engine overspeed. Examination of the airframe revealed that the engine cooling fan shaft had separated. Without the resistance of the fan shaft, the engine oversped, which resulted in sufficient engine damage for the engine to lose total power. The aft face of the fan shaft's lower sheave exhibited caked-on grease, consistent with grease leaking beyond the lower forward clutch actuator bearing seal for a prolonged period of time. Metallurgical examination of the bearing revealed that its rollers were seized, and no grease was recovered, consistent with a lack of lubrication.

The helicopter manufacturer's maintenance manual required that the entire airframe be overhauled every 2,200 hours, which would include an overhaul of the engine cooling fan driveshaft lower bearing and seal. Review of maintenance records revealed that, due to a maintenance logbook entry error (time since overhaul) that occurred about 9 years before the accident and was carried forward, the helicopter had been operated about 52 hours beyond the mandatory airframe overhaul time limit. The manual also required that the lower clutch actuator bearings be lubricated every 300 hours or 3 years, whichever occurred first. Although the failed bearing had been lubricated with grease both about 1 year and 2 years preceding the accident, there was no record indicating that the bearing had been lubricated with grease during the preceding 4 years 11 months and 685.1 hours of operation, which likely damaged or degraded the bearing and led to its failure during the accident flight.

Text from the NTSB. Open the NTSB docket for ERA16LA216 for the full record.