# NTSB SEA93LA207: Bell UH-1B, N3979C, Pierce, ID, September 23, 1993 - Type: NTSB accident investigation - Classification: Accident · No injuries · Limited investigation · Final report - Date: September 23, 1993, 08:15 local (15:15 UTC) - Place: Pierce, ID - Coordinates: 46.160423, -116.920089 (estimated) - Nearby events: https://openairdocs.org/map.md?lat=46.1604&lon=-116.9201&mi=10 - Phase of flight: [Maneuvering](https://openairdocs.org/ntsb-phases/450) (OpenAirDocs' grouping) - Highest injury: No injuries - Injuries: 1 uninjured - Status: Completed, report October 20, 1994 - Report: Approved by Office Director - NTSB case key (mKey): 41773 - NTSB case page: https://web.ntsb.gov/investigations/?ntsbnumber=SEA93LA207 - NTSB report (PDF, downloads): https://data.ntsb.gov/carol-repgen/api/Aviation/ReportMain/GenerateNewestReport/41773/pdf - Page: https://openairdocs.org/ntsb/SEA93LA207 ## Probable cause SEPARATION OF THE 90 DEGREE GEARBOX AFTER FATIGUE FAILURE OF THE GEARBOX ATTACH BOLTS. FACTORS INCLUDE HILLY/MOUNTAINOUS TERRAIN. ## Sequence of events In NTSB's coding before 2008 (https://www.ntsb.gov/GILS/Documents/codman.pdf). - Airframe/component/system failure/malfunction, maneuvering - rotor drive system,tail rotor gearbox(90 deg) - separation (cause) - Forced landing, descent - emergency - Hard landing, landing - flare/touchdown - terrain condition - mountainous/hilly (contributing factor) - terrain condition - roadway/highway ## Analysis 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. ## Factual narrative 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. ## Aircraft - Registration: [N3979C](https://openairdocs.org/deregistered/N3979C-2014-12-02), registration cancelled 2014-12-02, S/N 62-2048, built 1962 (OpenAirDocs) - Reported make and model: BELL UH-1B - Registry model: [Bell UH-1B](https://openairdocs.org/model/bell-uh-1b) (registry code 1181410) (OpenAirDocs) - Registry engine: [Lycoming T-53 series](https://openairdocs.org/engine/lycoming-t-53-series) (registry code 41549, reciprocating) (OpenAirDocs) - Category: Helicopter - Serial number: 62-2048 - Airframe hours: 7922 - Engines: LYCOMING T53-L11-D 1150 horsepower turbo shaft - Damage: Substantial damage - Flight: Other work use, Part 133 - Operator: Clearwater Management Co. Inc., doing business as Heli-Tech Logging, Inc. - Registered owner: Clearwater Management Co. Inc. - Airworthiness certificate: Special, Restricted (Special) - Last inspection: Annual, August 31, 1993, 90 hours before ## Flight crew As NTSB recorded them. NTSB leaves out names. - Pilot; age 49; commercial; airplane ratings: single-engine land; medical: class 2, valid medical--w/ waivers/lim; seat: left; flight hours: 22,000 total, 18,000 as pilot in command, 200 in the last 90 days, 40 in the last 30 days, 6 in the last 24 hours ## Weather - Conditions: Visual (VMC), Day - Wind: calm - Lowest ceiling: none - Temperature: 55 °F ## FAA reports on this event (1) The FAA's own accident and incident reports, matched to the aircraft by N-number and date (OpenAirDocs). - [Accident 19930923036609A](https://openairdocs.org/incident/19930923036609A), Sep 23, 1993: Bell UH1B, Pierce, ID, Substantial damage. 90 Degree gear box separated from helicopter. Landed hard during run on landing. Chopped tail. Attach bolts broke. From the NTSB's public API. Aircraft are matched to registry aircraft by N-number and event date, checked against the reported serial number, make and model; the registry's history has gaps, so older matches can be wrong. Compare the reported make and model. --- OpenAirDocs.org is an independent research project of [Contrail Labs](https://contraillabs.com/). It is not affiliated with the FAA or the NTSB. Source documents are U.S. government works in the public domain.