# NTSB SEA99LA020: Garlick UH-1H, N94NW, Darrington, WA, December 14, 1998 - Type: NTSB accident investigation - Classification: Accident · Serious injuries · Limited investigation · Final report - Date: December 14, 1998, 11:00 local (19:00 UTC) - Place: Darrington, WA - Coordinates: 48.189357, -121.210533 (estimated) - Nearby events: https://openairdocs.org/map.md?lat=48.1894&lon=-121.2105&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 11, 2000 - Report: Approved by Office Director - ADs cited in the narratives: [AD 97-20-09](https://openairdocs.org/ad/97-20-09) (OpenAirDocs) - NTSB case key (mKey): 45496 - NTSB case page: https://web.ntsb.gov/investigations/?ntsbnumber=SEA99LA020 - NTSB report (PDF, downloads): https://data.ntsb.gov/carol-repgen/api/Aviation/ReportMain/GenerateNewestReport/45496/pdf - Page: https://openairdocs.org/ntsb/SEA99LA020 ## 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. ## Sequence of events In NTSB's coding before 2008 (https://www.ntsb.gov/GILS/Documents/codman.pdf). - Airframe/component/system failure/malfunction, hover - vertical stabilizer - fatigue (cause) - vertical stabilizer - fractured (cause) - vertical stabilizer - separation (cause) - In flight collision with terrain/water, descent - uncontrolled ## Analysis 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.' ## Factual narrative On December 14, 1998, approximately 1100 Pacific standard time, a Garlick UH-1H helicopter, N94NW, owned by Northwest Helicopters, and operated as a 14 CFR Part 133 aero-logging operation, impacted the terrain after experiencing a separation of the vertical fin in flight. Visual meteorological conditions prevailed, and no flight plan was filed. The aircraft sustained substantial damage, and the commercial pilot received serious injuries. The flight originated at a nearby helicopter service pad. According to the pilot, he was in a hover at the end of a 150 foot long-line that had just been attached to a set of logs. He reported that he had not yet began to lift the logs, but had taken the slack out of the line when he heard a loud bang. The aircraft immediately began spinning to the right and settled into the terrain. The aft portion of the tail boom was sent to the NTSB materials laboratory for examination. That examination revealed the presence of fatigue cracking in all five layers of the left side of the vertical fin spar cap. According to the laboratory, the cracks initiated from the hole for the fourth rivet above where the fin protrudes from the tail boom, and propagated through most of the left side of the spar cap prior to final separation. During the investigation, it was determined that the vertical fins of at least two other UH-1 helicopters had previously separated in flight, although not through the fourth hole. The FAA had previously issued Airworthiness Directive (AD) number 97-20-09 to address the problem of fin separation from UH-1 helicopters used in repeated heavy-lift operation, but the FAA Rotorcraft Directorate subsequently determined that the AD was not adequate. As a result, the Directorate initiated the formulation of a Priority Letter Airworthiness Directive calling for the replacement of the original equipment manufacturer's (OEM) fin spar with one that "...has been proven to account for the effects of repeated heavy-lift operations." ## Aircraft - Registration: [N94NW](https://openairdocs.org/aircraft/N94NW), registered now, S/N 66-01101, built 1996 (OpenAirDocs) - Reported make and model: GARLICK UH-1H - Registry model: [Bell UH-1H](https://openairdocs.org/model/bell-uh-1h) (registry code 1181407) (OpenAirDocs) - Registry engine: [Lycoming T-53 series](https://openairdocs.org/engine/lycoming-t-53-series) (registry code 41549, reciprocating) (OpenAirDocs) - Category: Helicopter - Serial number: 66-1101 - Airframe hours: 7240 - Engines: Lycoming T53-L13B 1400 horsepower turbo shaft - Damage: Substantial damage - Fire: On-Ground - Flight: Other work use, Part 133 - Registered owner: Northwest Helicopters Inc. - Airworthiness certificate: Special, Restricted (Special) - Last inspection: Continuous airworthiness, December 10, 1998, 10 hours before ## Flight crew As NTSB recorded them. NTSB leaves out names. - Pilot; age 50; commercial; medical: class 2, valid medical--w/ waivers/lim; seat: left; flight hours: 6,336 total, 800 in this make and model, 6,336 as pilot in command, 210 in the last 90 days, 66 in the last 30 days, 7 in the last 24 hours ## Weather - Conditions: Visual (VMC), Day - Wind: calm - Visibility: 10 statute miles - Lowest ceiling: none - Temperature: 39 °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 19981214044099A](https://openairdocs.org/incident/19981214044099A), Dec 14, 1998: Bell UH1H, Darrington, WA, Destroyed. On December 14, 1998, approximately 1100 pacific standard time, a garlick UH-1H helicopter, N94NW, which is owned by northwest helicopters, and was being… 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.