- Date
- May 27, 1994, 13:00 local (19:00 UTC)
- Place
- Libby, MT
- Injuries
- 1 serious
- Damage
- Substantial damage
- Phase of flight
- Maneuvering
- Aircraft
- N911SWBell 204B
- Weather
- Visual (VMC) · Day
- Status
- Completed · report January 12, 1995
- Report
- Approved by Office Director
- FAA report
- 19940527012969A
- Case key
- 41931
From the NTSB’s public API. By law, NTSB reports can’t be used as evidence in lawsuits over the accident.
On this page
Probable cause
The fatigue fracture of the 42 degree gear box input pinion. Factors include unsuitable, mountainous/hilly terrain.
Sequence of events
NTSB’s coding before 2008: each occurrence and phase of flight, and the causes and factors under it. The codes are defined in NTSB’s coding manual.
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
- Reported as
- Bell 204B
- Category
- Helicopter
- Serial number
- 2025
- Airframe hours
- 12,884
- Engine
- Lycoming T53-13B, 1,400 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.
- Role
- Pilot
- Age
- 44
- Certificates
- 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.