Bell 206B N24FS accident near Rockville, Idaho

Probable cause

The in-flight failure of the engine-to-transmission drive shaft due to a fatigue fracture of one of the KAflex flex frames caused by a loose bolt, which resulted in a total loss of engine power and a subsequent hard landing.

NTSB analysis

The commercial pilot reported that, while conducting an aerial application flight, he performed a 180° turn at an altitude of about 40 to 50 ft above ground level and heard a growling noise, which was followed by the sound of a loud bang and a subsequent loss of power. The pilot lowered the collective, likely to initiate an autorotation to a nearby open field, and the helicopter landed hard.

Postaccident examination of the helicopter airframe and engine revealed that the middle portion of the engine-to-transmission drive shaft was separated between the engine and transmission KAflex couplings. No additional evidence of any preexisting mechanical malfunction was observed.

Metallurgical examination of the shaft and couplings revealed that the shaft had failed from a pair of fatigue cracks at a bolt hole in one element of the flexure coupling (flex frame). This bolt hole exhibited rotational wear severe enough to cause material loss on one face of the bolt hole wall. This circular-wear depression was consistent with repeated rotational contact with an adjacent washer. It is likely that, once the fatigue cracks at the bolt hole propagated deeply enough, the remaining flexure element cross-section fractured from overstress. The failure of this flexure element led to multiple and subsequent overstress failures in other flexure elements, eventually compromising the entire drive shaft assembly.

The circular-wear depression and the longitudinal wear inside the flexure element bolt hole were both consistent with a loose bolt assembly. Because the bolt assembly was loose, the washer would have been able to rub on the flexure element. In addition, the bolt shaft would have been able to rub or gouge the inner wall surfaces of the bolt hole. The bolt and associated washer were not located in the wreckage. None of the maintenance performed on the helicopter about 1 month before the accident involved the flex frame bolt assemblies, so the investigation could not determine how one of the bolts became loose.

Text from the NTSB. Open the NTSB docket for WPR17LA113 for the full record.