McDonnell Douglas 369D N1096L

Summary

On 10 August 1998 at about 17:05 local time, a McDonnell Douglas 369D registered N1096L was involved in an accident near Oso, Washington, United States. One person was on board and one had minor injuries. The aircraft was substantially damaged. The NTSB has published a probable cause for this accident; it is quoted in full below.

Date

10 August 1998 at 17:05

Classification

Accident

Location

Oso, Washington, United States

Coordinates

48.1804, -121.6695

Aircraft

McDonnell Douglas 369D

Registration

N1096L

Category

Helicopter

Engines

1

Operator

Not recorded

Operating rule

Part 133: Rotorcraft Ext. Load

Purpose of flight

Other Work Use

Phase of flight

Not recorded

Route

Arlington → Unknown

Aircraft damage

Substantial

Weather

VMC

Light

Day

NTSB number

SEA98LA160

1 person had minor injuries.

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.

Quoted verbatim from the NTSB record. This site does not paraphrase or interpret it.

Read the full NTSB narrative

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.

Quoted verbatim from the NTSB record.