Accident Bell UH-1B N3979C, Thursday 23 September 1993

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Date:	Thursday 23 September 1993
Time:	08:15 LT
Type:	Bell UH-1B
Owner/operator:	Clearwater Management Co. Inc.
Registration:	N3979C
MSN:	62-2048
Year of manufacture:	1962
Total airframe hrs:	7922 hours
Engine model:	Lycoming T53-L11-D
Fatalities:	Fatalities: 0 / Occupants: 1
Other fatalities:	0
Aircraft damage:	Substantial
Category:	Accident
Location:	Pierce, ID - United States of America
Phase:	Manoeuvring (airshow, firefighting, ag.ops.)
Nature:	Unknown
Departure airport:
Destination airport:
Investigating agency:	NTSB
Confidence Rating:	Accident investigation report completed and information captured

Narrative:
WHILE MANEUVERING NEAR A REFUELING SITE IN HILLY/MOUNTAINOUS TERRAIN, THE 90 DEGREE GEARBOX SEPARATED FROM THE AIRCRAFT. THE PILOT THEN ATTEMPTED A RUN-ON FORCED LANDING ON A NEARBY LOGGING ROAD. DURING THE FLARE, THE AIRCRAFT LANDED HARD, RESULTING IN A COLLAPSE OF THE SKIDS, AND SEPARATION OF THE REMAINING PORTION OF THE TAIL BOOM. METALLURGICAL ANALYSIS REVEALED THAT ALL SIX GEARBOX ATTACH BOLTS HAD EXPERIENCED REVERSE-BENDING FATIGUE FAILURE. THE OPERATOR RECORDS INDICATED THAT THIS WAS A NEW SURPLUS PART WHICH HAD 136 HOURS OF OPERATION SINCE NEW. HISTORICAL RECORDS CONFIRMING THE UNIT'S TOTAL TIME SINCE BEING SHIPPED TO THE PHILIPPINES AS A NEW SURPLUS PART IN 1981 COULD NOT BE LOCATED.

Probable Cause: SEPARATION OF THE 90 DEGREE GEARBOX AFTER FATIGUE FAILURE OF THE GEARBOX ATTACH BOLTS. FACTORS INCLUDE HILLY/MOUNTAINOUS TERRAIN.

Accident investigation: