Mcdonnell Douglas 369D accident near Oso, Washington, August 10, 1998
On August 10, 1998 at about 5:05 pm local time, a Mcdonnell Douglas 369D (helicopter), registered N1096L, was substantially damaged in an accident near Oso, Washington. It was an other work-use flight under external-load helicopter rules (Part 133). 1 person had minor injuries. The weather was visual conditions (good weather).
The NTSB's probable cause their words, unchanged
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.
Source: NTSB aviation accident database, copy made October 5, 2026. Docket and reports at the NTSB.
What the record shows
- Date
- August 10, 1998 · about 5:05 pm local time
- Place
- Oso, Washington · map
- Type
- Accident
- Injuries
- 1 person had minor injuries.
- Weather
- visual conditions (good weather)
- Aircraft
- Mcdonnell Douglas 369D
- Registration
- N1096L · no longer on the register · serial 800789D
- Damage
- Substantial damage
- Flight
- Other work-use flight · external-load helicopter rules (Part 133)
The NTSB's narrative final · quoted from the NTSB record
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.
The complete narrative as the NTSB published it. The NTSB's docket holds the report as a PDF and any photographs, statements and other documents from the investigation.
The factual record from the NTSB's investigation tables, in plain English
Pilot
- Certificate: commercial pilot, private
- Ratings: single-engine land; rotorcraft: helicopter
- Flight time: 22,982 hours in all; 13,450 in this make and model; 22,913 as pilot in command
- Medical certificate: Class 2 (valid medical--no waivers/lim.)
- Seat: left
The aircraft
- Airframe total time: 5,499 hours
- Last inspection: continuous airworthiness programme, August 28, 1998; 24 hours since
- Maximum gross weight: 3,000 lb
- Seats: 4
- Landing gear: fixed
- Engine: Allison 250C20B (turboshaft); 0 hours total
The flight
- Departed from: AWO Arlington WA at 2:45 pm
- Flight plan: none
- Runway 0
Weather at the time
- Light: daylight
- Wind: from 180° at 3 knots
- Visibility: 10 statute miles
- Sky: not recorded; clear
- Temperature: 66°F (19°C), dew point 55°F (13°C)
- Altimeter: 30.00 inHg
- Observation at 4:55 pm from AWO, 8 miles away
Injuries
| Fatal | Serious | Minor | None | |
|---|---|---|---|---|
| Crew | 1 |
Documents from the investigation the NTSB's docket: the evidence folder behind the report
The docket, the folder of records gathered during an investigation (maintenance records, photographs, witness statements, examinations), has not been read from the NTSB for this case yet; the list appears here once it has. Open the docket at the NTSB.
Other NTSB records under N1096L the same tail number, which may have belonged to a different aircraft at the time
Everything on this page comes from the NTSB's public records. The narrative, probable cause and findings are the NTSB's own words; the coded tables behind the report are written out in plain English, with pilots' ages, home towns and medical details left out. The documents and photographs are the NTSB's docket, shown as the NTSB released them. This site's own text never names anyone involved. A preliminary report can change; the final report usually follows one to two years later, and the page is refreshed when it does.