N3979C

On September 23, 1993, approximately 0815 Pacific daylight time (PDT), a Bell UH-1B, N3979C, landed hard during an attempted forced landing about 60 miles north of Pierce, Idaho. The commercial pilot, who was the sole occupant of the aircraft, received minor injuries, and the aircraft sustained substantial damage. The pilot, who had just dropped off a work crew to assist in the FAR Part 133 logging operation, was operating in visual meteorological conditions at the time of the accident. No flight plan had been filed, and there was no report of an ELT activation.

The operator of the aircraft said that the pilot was returning to a fueling site when, at about 300 feet above the ground, the 90 degree gear box separated from the aircraft. The pilot, who was operating in hilly/mountainous terrain, then attempted a run-on forced landing on a nearby logging road. During the flare/touchdown, the aircraft landed hard, which resulted in substantial damage to the airframe. Damage included collapse of the skids and separation of the remaining portion of the tail boom.

In a post-accident conversation, the pilot said that on the night before the accident, he heard two very loud bangs or pops after shutting down the aircraft. The bangs, which occurred about one- tenth of a second apart, were heard just as the rotors were coasting to a stop.

A metallurgical examination conducted by the NTSB Office of Research and Engineering revealed that all six stud-bolts that attach the 90 degree gearbox to the airframe had experienced fatigue failure. All but one of the bolt sections remaining in the gearbox housing showed clear evidence of reverse bending fatigue, with one of the bending directions being more prominent than the other. The three bolt sections that were found sheared from the housing all contained nuts, and all showed wear marks on their surfaces about midway along their length on the side corresponding to the origin of the prominent fatigue zone.

The historical service record obtained from the operator indicated that this gearbox (serial number ABC-50294) was a new part, and had not been used or overhauled prior to being installed on the accident aircraft on 8/18/93. According to the operator's inspection records, the gearbox had been on this aircraft approximately 136 hours prior to the accident. Discussions with Bell Helicopter and the parts broker who supplied the gearbox to HELI-TECH Logging revealed that Bell had shipped this unit to Manila, Philippines as a new spare part in September of 1981. Records indicated that the gearbox also spent time in Australia prior to returning to the United States.