ASN Wikibase Occurrence # 30812
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| Date: | Monday 14 December 1998 |
| Time: | 13:00 LT |
| Type: |  Bell UH-1H |
| Owner/operator: | Northwest Helicopters Inc. |
| Registration: | N94NW |
| MSN: | 66-1101 |
| Year of manufacture: | 1967 |
| Total airframe hrs: | 7240 hours |
| Engine model: | Lycoming T53-L13B |
| Fatalities: | Fatalities: 0 / Occupants: 1 |
| Aircraft damage: | Substantial |
| Category: | Accident |
| Location: | Darrington, Washington -
 United States of America
|
| Phase: | Taxi |
| Nature: | Unknown |
| Departure airport: | |
| Investigating agency: | NTSB |
| Confidence Rating: |  Accident investigation report completed and information captured |
While in a hover during heavy-lift aero-logging operations, the vertical fin separated from the tail boom, resulting in an uncontrolled descent into the terrain. During the investigation, the NTSB Materials Laboratory determined that the separation was due to the propagation of a number of fatigue cracks which had initiated within the fourth rivet hole above where the left side of the spar fin cap protruded from the tail boom. Although the FAA had previously issued Airworthiness Directive (AD) 97-20-09 to address the separation of vertical fins from UH-1 helicopters used in repeated heavy-lift operations, the Rotorcraft Directorate determined that the AD was not adequate. As a result, the Directorate initiated the formulation of a Priority Letter Airworthiness Directive requiring the replacement of the original fin spar with one that '...has been shown to account for the effects of repeated heavy-lift operations.'
Probable Cause: The total separation of the vertical fin (stabilizer), as the result of fatigue cracks which initiated in the fourth rivet hole above where the fin extends from the tailboom.
Accident investigation:
 |
|
Sources:
NTSB SEA99LA020
Location
Revision history:
| Date/time | Contributor | Updates |
|---|---|---|
| 27-Sep-2008 01:00 | ASN archive | Added |
| 29-Jan-2012 19:26 | Dr. John Smith | Updated [Time, Total fatalities, Total occupants, Other fatalities, Location, Country, Phase, Nature, Departure airport, Destination airport, Source, Damage, Narrative] |
| 04-Apr-2024 11:58 | ASN Update Bot | Updated [Time, Cn, Operator, Other fatalities, Phase, Nature, Departure airport, Destination airport, Source, Damage, Narrative, Category, Accident report] |
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