# NTSB SEA98LA160: McDonnell Douglas 369D, N1096L, Oso, WA, August 10, 1998 - Type: NTSB accident investigation - Classification: Accident · Minor injuries · Limited investigation · Final report - Date: August 10, 1998, 10:05 local (17:05 UTC) - Place: Oso, WA - Coordinates: 48.180358, -121.669464 (estimated) - Nearby events: https://openairdocs.org/map.md?lat=48.1804&lon=-121.6695&mi=10 - Phase of flight: [Enroute](https://openairdocs.org/ntsb-phases/400) (OpenAirDocs' grouping) - Highest injury: Minor injuries - Injuries: 1 minor - Status: Completed, report January 11, 2000 - Report: Approved by Office Director - NTSB case key (mKey): 43715 - NTSB case page: https://web.ntsb.gov/investigations/?ntsbnumber=SEA98LA160 - NTSB report (PDF, downloads): https://data.ntsb.gov/carol-repgen/api/Aviation/ReportMain/GenerateNewestReport/43715/pdf - NTSB docket: https://data.ntsb.gov/Docket?ProjectID=43715 - Page: https://openairdocs.org/ntsb/SEA98LA160 ## Probable cause The separation of the 90 degree gearbox and tail rotor assembly for undetermined reasons followed by the inability of the pilot-in-command to maintain control of the rotorcraft. ## Sequence of events In NTSB's coding before 2008 (https://www.ntsb.gov/GILS/Documents/codman.pdf). - Rotor failure/malfunction, cruise - rotor drive system,tail rotor gearbox(90 deg) - separation (cause) - reason for occurrence undetermined (cause) - Loss of control - in flight, cruise - In flight collision with terrain/water, descent - uncontrolled - terrain condition - ground - aircraft control - not possible, pilot in command (cause) ## Analysis At about 250 feet above the ground, the pilot-in-command heard a bang and saw the tail rotor gear box fly past the rotorcraft on the left side. The rotorcraft then 'nosed over and started turning to the right at a fast rate of speed.' The pilot-in-command then entered an autorotation and the rotorcraft impacted the ground. The tail rotor gearbox, tail rotor output shaft, fork assembly, swash-plate with pitch links, tail-boom aft fitting, and tail rotor driveshaft flex-coupling were recovered. Those items were inspected and tested at Boeing Helicopters, Mesa. The fracture mode for all of the components evaluated was ductile overload. The output shaft exhibited severe bending displacement of approximately 0.925 inches relative to a theoretical line along the centerline, characteristic of a brief, unbalanced load. The conical bearing surfaces of the tail rotor fork assembly were examined and determined to be void of heavy or unusual wear patterns. The tail rotor blades were not found during the course of the investigation. ## Factual narrative On August 10, 1998, approximately 1005 Pacific daylight time, a McDonnell Douglas 369D helicopter, N1096L, registered to and being operated by Eagle Air Helicopters, Inc., and being flown by a commercial pilot, was substantially damaged during a forced landing after the tail rotor gear box and tail rotor departed the aircraft. The pilot sustained minor injuries. Visual meteorological conditions prevailed and no flight plan had been filed. The flight, which had just departed a staging site, was en route downslope to the work site where the pilot would be lifting and transporting cedar shake blocks. The flight was to have been operated under 14CFR133. The pilot stated that, while en route to pick up another sling load of cedar blocks, "at about 250 feet above the ground, I heard a bang and seen [sic] the T/R [tail rotor] gear box fly past the A/C [aircraft] on the left side." The helicopter then "nosed over and started turning to the right at a fast rate of speed. I then entered an autorotation and rolled off the engine. I hit the ground with the tail downhill. The bear paws [pads on the skids] hung up on a stump that kept the A/C from rolling down the hill." The tail rotor gearbox, tail rotor output shaft, fork assembly, swash-plate with pitch links, tail-boom aft fitting, and tail rotor driveshaft flex-coupling were recovered. Those items were inspected and tested at Boeing Helicopters, Mesa. The fracture mode for all of the components evaluated was ductile overload. The output shaft exhibited severe bending displacement of approximately 0.925 inches relative to a theoretical line along the centerline. The conical bearing surfaces of the tail rotor fork assembly were examined and determined to be void of heavy or unusual wear patterns (refer to attached engineering laboratory report). The tail rotor blades were not found during the course of the investigation. ## Aircraft - Registration: [N1096L](https://openairdocs.org/deregistered/N1096L-2014-11-19), registration cancelled 2014-11-19, S/N 800789D, built 1980 (OpenAirDocs) - Reported make and model: McDonnell Douglas 369D - Registry model: [Hughes Helicopters HUGHES 369D](https://openairdocs.org/model/hughes-helicopters-hughes-369d) (registry code 4470706) (OpenAirDocs) - Registry engine: [Allison 250 SER 250HP](https://openairdocs.org/engine/allison-250-ser-250hp) (registry code 03002, turbo-shaft) (OpenAirDocs) - Category: Helicopter - Serial number: 800789D - Airframe hours: 5499 - Engines: Allison 250C20B turbo shaft - Damage: Substantial damage - Flight: Other work use, Part 133 - Registered owner: Eagle Air Helicopters, Inc. - Airworthiness certificate: Standard, Normal - Last inspection: Continuous airworthiness, August 28, 1998, 24 hours before - Departed: AWO, ARLINGTON, WA ## Flight crew As NTSB recorded them. NTSB leaves out names. - Pilot; age 50; private, commercial; airplane ratings: single-engine land; medical: class 2, valid medical--no waivers/lim.; seat: left; flight hours: 22,982 total, 13,450 in this make and model, 22,913 as pilot in command ## Weather - Conditions: Visual (VMC), Day - Wind: 180° at 3 knots - Visibility: 10 statute miles - Temperature: 66 °F, dew point 55 °F - Altimeter: 30.00 inHg - Observed at: AWO, 8 nm away ## 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 19980810020269A](https://openairdocs.org/incident/19980810020269A), Aug 10, 1998: Hughes 369D, Darrington, WA, Substantial damage. Helicopter crashed when the tail rotor failed while performing logging operation. (-23) see attached. No narrative attached. (.19) on August 10, 1998,… ## Other investigations on N1096L (3) 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. - [NTSB SEA99FA018](https://openairdocs.org/ntsb/SEA99FA018), Dec 8, 1998: Hughes 369D (N1096L), Taholah, WA, substantial damage. Probable cause: Turbine wheel failure as the result of a low-cycle fatigue crack that initiated in a beyond-limits grinding gouge. Factors include improper maintenance by unknown parties and tree stumps that… - [NTSB SEA85LA017](https://openairdocs.org/ntsb/SEA85LA017), Nov 13, 1984: Hughes 369D (N1096L), Humptulips, WA, destroyed - [NTSB SEA82FYM16](https://openairdocs.org/ntsb/SEA82FYM16), May 19, 1982: Hughes 369D (N1096L), Birkenfeld, OR, substantial damage ## Docket (3 files) The files NTSB released with the investigation on March 8, 1999, on NTSB's docket system: https://data.ntsb.gov/Docket?ProjectID=43715. 1.6 MB in all. - Pilot/Operator Aircraft Accident Report, NTSB Form 6120.1 (item 1, .PDF, 719 KB): https://data.ntsb.gov/Docket/Document/docBLOB?ID=40474559&FileExtension=.PDF&FileName=Pilot%2FOperator%20Aircraft%20Accident%20Report,%20NTSB%20Form%206120.1-Master.PDF - Boeing Helicopters, Mesa - Engineering Laboratory Report (item 2, .PDF, 735 KB): https://data.ntsb.gov/Docket/Document/docBLOB?ID=40474560&FileExtension=.PDF&FileName=Boeing%20Helicopters,%20Mesa%20-%20%20Engineering%20Laboratory%20Report-Master.PDF - Receipt of Parts (item 3, .PDF, 97 KB): https://data.ntsb.gov/Docket/Document/docBLOB?ID=40474561&FileExtension=.PDF&FileName=Receipt%20of%20Parts-Master.PDF 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.