Hiller UH 12E N107HA accident near Wadesboro, North Carolina

Probable cause

A partial loss of engine power due to the failure of the No. 2 engine accessory gearbox bearing for reasons that could not be determined based on the available information.

NTSB analysis

The airline transport pilot was conducting an aerial application flight in the helicopter. About 75 ft above ground level, the pilot heard three engine compressor stalls, which were followed by a partial loss of engine power. During the ensuing forced landing to a nearby road, the helicopter rolled over into a drainage ditch and came to rest on its right side.

Examination of the engine revealed that the accessory gearbox No. 2 bearing had degraded and fractured. The bearing displayed evidence of heat damage associated with its degradation. The cage of the bearing revealed fatigue fractures with multiple origins, indicative of relatively high-stress concentrations to the bearing. The cage fractures were likely secondary failures following the initial degradation of the bearing. The bearing balls were black in appearance. In addition, they were undersized when compared to engineering drawing requirements and displayed signatures consistent with material loss, likely from wear.

Considering that the engine had accumulated only about 92 hours since the gearbox was installed following an overhaul, it was possible that the bearings experienced an abnormal load. The cause of the abnormal load could have been due to installation error, an alignment issue from a previous hard landing that was noted in the gearbox maintenance records, or from insufficient lubrication within the gearbox assembly. However, the investigation could not determine which of these factors, if any, caused the failure of the No. 2 bearing and resulted in the partial loss of engine power.

Text from the NTSB. Open the NTSB docket for ERA14LA361 for the full record.