# NTSB SEA94LA128: Bell 204B, N911SW, Libby, MT, May 27, 1994

- Type: NTSB accident investigation
- Classification: Accident · Serious injuries · Limited investigation · Final report
- Date: May 27, 1994, 13:00 local (19:00 UTC)
- Place: Libby, MT
- Coordinates: 48.320678, -115.360084 (estimated)
- Nearby events: https://openairdocs.org/map.md?lat=48.3207&lon=-115.3601&mi=10
- Phase of flight: [Maneuvering](https://openairdocs.org/ntsb-phases/450) (OpenAirDocs' grouping)
- Highest injury: Serious injuries
- Injuries: 1 serious
- Status: Completed, report January 12, 1995
- Report: Approved by Office Director
- NTSB case key (mKey): 41931
- NTSB case page: https://web.ntsb.gov/investigations/?ntsbnumber=SEA94LA128
- NTSB report (PDF, downloads): https://data.ntsb.gov/carol-repgen/api/Aviation/ReportMain/GenerateNewestReport/41931/pdf
- Page: https://openairdocs.org/ntsb/SEA94LA128

## Probable cause

THE FATIGUE FRACTURE OF THE 42 DEGREE GEAR BOX INPUT PINION. FACTORS INCLUDE UNSUITABLE, MOUNTAINOUS/HILLY 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,intermediate gearbox(42 deg) - fractured (cause)
  - rotor drive system,intermediate gearbox(42 deg) - fatigue (cause)
- Forced landing, descent - emergency
- Hard landing, descent - emergency
  - terrain condition - mountainous/hilly
  - terrain condition - none suitable (contributing factor)

## Analysis

JUST AFTER LIFTING A SINGLE-LOG EXTERNAL LOAD, THE HELICOPTER EXPERIENCED A FAILURE OF THE 42 DEGREE GEAR BOX INPUT PINION GEAR. THE PILOT RELEASED THE LOAD, AND BEGAN AN EMERGENCY DESCENT IN ORDER TO ATTEMPT A FORCED LANDING IN THE MOUNTAINOUS/HILLY TERRAIN. WHILE ATTEMPTING TO INITIATE THE LANDING FLARE, THE AIRCRAFT IMPACTED THE TERRAIN.

## Factual narrative

On May 27, 1994, approximately 1300 mountain daylight time (MDT), a Bell 204B, N911SW, impacted the terrain during Part 133 operations 38 miles north of Libby, Montana. The commercial pilot received serious injuries, and the aircraft sustained substantial damage. The long-line logging flight, which was moving felled trees to a loading area, was being operated in visual meteorological conditions at the time of the accident. No flight plan had been filed, and the ELT, which was activated by the impact, was turned off at the scene.

According to witnesses, just as the aircraft was lifting  a "light load," a loud bang or pop came from the aircraft. Witnesses then saw the helicopter begin spinning to the right, and heard the engine begin to wind down. The aircraft then descended almost straight down and impacted the forested  terrain.

An NTSB supervised teardown analysis of the 42 degree gear box revealed that a fatigue crack originated in the root radius of one of the pinion teeth. This crack had extended through the pinion gear itself, and then far enough around the pinion gear shaft fillet radius to cause the pinion assembly to fail.

## Aircraft

- Registration: N911SW (not matched to a registry record)
- Reported make and model: BELL 204B
- Category: Helicopter
- Serial number: 2025
- Airframe hours: 12884
- Engines: LYCOMING T53-13B 1400 horsepower turbo shaft
- Damage: Substantial damage
- Flight: Other work use, Part 133
- Registered owner: Westerberg, Dennis & Sloane
- Airworthiness certificate: Standard, Transport
- Last inspection: 100 hour, May 25, 1994, 12 hours before

## Flight crew

As NTSB recorded them. NTSB leaves out names.

- Pilot; age 44; commercial, private; airplane ratings: single-engine land; instrument ratings: helicopter; medical: class 2, valid medical--no waivers/lim.; seat: left; flight hours: 8,780 total, 2,150 in this make and model, 8,780 as pilot in command, 150 in the last 90 days, 58 in the last 30 days, 8 in the last 24 hours

## Weather

- Conditions: Visual (VMC), Day
- Wind: 310° at 5 knots
- Visibility: 50 statute miles
- Lowest ceiling: none
- Temperature: 70 °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 19940527012969A](https://openairdocs.org/incident/19940527012969A), May 27, 1994: Bell 204B, Libby, MT, Substantial damage. Lifting a log 42 degree gear box pinion gear failed. Dropped log. Autorotated into hilly terrain.

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.

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