On August 10, 1998, a Mcdonnell Douglas 369D (registration N1096L) was involved in an aviation accident near Oso, WA. Investigators recorded the probable cause as: 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. This summary draws on records from the U.S. National Transportation Safety Board (NTSB) historical archive.
While climbing at about 250 feet, a helicopter's pilot heard a bang and saw the tail rotor gearbox separate. The aircraft nosed over and yawed right. An autorotation was performed but the helicopter impacted the ground.
Incident Summary
At approximately 250 feet above ground level, the pilot-in-command heard a bang and observed the tail rotor gearbox pass the left side of the rotorcraft. The helicopter then nosed over and began turning to the right at a fast rate. The pilot-in-command entered an autorotation, and the aircraft subsequently impacted the ground.
Examination Findings
Investigators recovered several components, including 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. These parts were examined and tested at Boeing Helicopters in Mesa. Fractographic analysis indicated that all evaluated components failed in a ductile overload mode. The output shaft exhibited severe bending displacement of approximately 0.925 inches relative to a theoretical centerline, characteristic of a brief, unbalanced load. The conical bearing surfaces of the tail rotor fork assembly showed no heavy or unusual wear patterns. The tail rotor blades were not located during the investigation.