SEA98LA160

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.

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.