# NTSB CHI99LA012: Hiller UH-12C, N9739C, Clinton, IA, October 24, 1998 - Type: NTSB accident investigation - Classification: Accident · No injuries · Limited investigation · Final report - Date: October 24, 1998, 11:30 local (16:30 UTC) - Place: Clinton, IA - Coordinates: 41.839511, -90.229331 (estimated) - Nearby events: https://openairdocs.org/map.md?lat=41.8395&lon=-90.2293&mi=10 - Phase of flight: [Enroute](https://openairdocs.org/ntsb-phases/400) (OpenAirDocs' grouping) - Highest injury: No injuries - Injuries: 2 uninjured - Status: Completed, report February 16, 2001 - Report: Approved by Office Director - NTSB case key (mKey): 45140 - NTSB case page: https://web.ntsb.gov/investigations/?ntsbnumber=CHI99LA012 - NTSB report (PDF, downloads): https://data.ntsb.gov/carol-repgen/api/Aviation/ReportMain/GenerateNewestReport/45140/pdf - Page: https://openairdocs.org/ntsb/CHI99LA012 ## Probable cause the failure of the 90 degree gearbox for reasons undetermined. Factors were the failure of the gearbox, and it not being possible for the pilot to maintain control of the aircraft. ## Sequence of events In NTSB's coding before 2008 (https://www.ntsb.gov/GILS/Documents/codman.pdf). - Airframe/component/system failure/malfunction, cruise - normal - rotor drive system,tail rotor gearbox(90 deg) - failure,total (contributing factor) - reason for occurrence undetermined (cause) - Forced landing, descent - emergency - autorotation - initiated, pilot in command - aircraft control - not possible, pilot in command (contributing factor) - In flight collision with terrain/water, emergency descent/landing ## Analysis During cruise flight the helicopter suffered a loss of tail rotor authority. The pilot initiated an autorotational landing; however, during the approach he lost all control of the aircraft. An inspection of the tail rotor gearbox after the accident revealed a failure of the 90 degree gearbox. The log books indicated an overhaul 195 hours prior to the accident with no other maintenance on the gearbox noted. An inspection of the helicopter was conducted one month prior to the accident and the helicopter had accumulated about 33 hours since that inspection. ## Factual narrative On October 24, 1998, at 1130 central daylight time, a Hiller UH-12C, N9739C, owned by a corporation and operated by a commercial pilot, sustained substantial damage during a forced landing four miles east of the Clinton Municipal Airport, near Clinton, Iowa. The pilot said that he lost tail rotor control and initiated an autorotational landing into a field. He and the passenger reported no injuries. The 14 CFR Part 91 sightseeing flight was operating in visual meteorological conditions. No flight plan was on file. The local flight departed Clinton about 1120. An examination of the helicopter after the accident revealed that the "90 degree" gearbox had suffered a failure of the internal gears. When disassembled it was determined that the drive gear and the pinion gear in the transmission were both missing teeth. A check of the maintenance log books for the helicopter indicated that the last overhaul and maintenance on the "90 degree" gearbox occurred 195 hours prior to the failure. The helicopter had received an inspection the month prior to the accident with no note of maintenance on the gearbox or gears. The helicopter had accumulated about 33 hours since the most recent inspection. ## Aircraft - Registration: [N9739C](https://openairdocs.org/aircraft/N9739C), registered now, S/N 847, built 1956 (OpenAirDocs) - Reported make and model: Hiller UH-12C - Registry model: [Hiller UH-12C](https://openairdocs.org/model/hiller-uh-12c) (registry code 4360116) (OpenAirDocs) - Registry engine: [Franklin 6V4 series](https://openairdocs.org/engine/franklin-6v4-series) (registry code 27033, reciprocating) (OpenAirDocs) - Category: Helicopter - Serial number: 847 - Airframe hours: 3300 - Engines: Franklin 6V335-50 210 horsepower reciprocating - Damage: Substantial damage - Flight: Business, Part 91 - Registered owner: Aerocraft And Aviation, Inc - Airworthiness certificate: Standard, Normal - Last inspection: Annual, September 30, 1998, 33 hours before - Departed: CWI, CLINTON, IA ## Flight crew As NTSB recorded them. NTSB leaves out names. - Pilot; age 26; flight instructor, commercial; airplane ratings: single-engine land; instrument ratings: helicopter; medical: class 2, valid medical--no waivers/lim.; seat: left; flight hours: 475 total, 173 in this make and model, 362 as pilot in command, 8 in the last 90 days, 8 in the last 30 days ## Weather - Conditions: Visual (VMC), Day - Wind: 210° at 6 knots - Visibility: 10 statute miles - Lowest ceiling: none - Temperature: 64 °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 19981024040399A](https://openairdocs.org/incident/19981024040399A), Oct 24, 1998: Hiller UH12C, Clinton, IA, Substantial damage. (-23) While in cruise flight, the pilot was unable to maintain altitude and heading. An emergency landing was initiated which resulted in loss of control… 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.