Primary finding
Probable cause
The failure and subsequent disintegration of the oil cooler fan forward hanger bearing.
Investigator assessment
Analysis narrative
About 5 minutes after takeoff, while in cruise flight, the pilot felt a few short 'clunks' and then heard grinding noise and felt violent shaking. The pilot elected to perform a precautionary, autorotative, off-airport landing. During the landing flare, the tail rotor and tail stinger contacted the ground. The tail rotor blades were damaged, the stinger broken, and the tail rotor gearbox was fractured. The vibration that the pilot felt was traced to a failed bearing at the oil cooler fan forward hanger. The bearing was sent to the helicopter manufacturer for examination; however, a fracture mode could not be identified due to damage to the fracture surfaces. An airworthiness directive applied to the bearing and was complied with 11 flight hours prior to the accident.
Source record
Factual narrative
On April 16, 2000, at 1208 hours mountain standard time, a Bell 407, N175PA, was substantially damaged during a precautionary, autorotative, off-airport landing near Grand Canyon, Arizona. The commercial certificated pilot and five passengers were not injured. The local area sightseeing flight was operated under 14 CFR Part 135 by Papillon Airways, Inc. A company VFR flight plan was filed. Visual meteorological conditions prevailed for the flight that departed from the Grand Canyon National Park airport at 1205. The pilot reported he felt a few short "clunks" while in cruise flight about 5 minutes after takeoff. Next there was "a very loud grinding noise and the aircraft began to SHAKE VIOLENTLY." The pilot decided to autorotate to a nearby clearing. During the landing flare, the tail rotor and tail stinger contacted the ground. The tail rotor blades were damaged, the stinger broken, and the tail rotor gearbox was fractured. The vibration that the pilot felt was traced to a failed bearing at the oil cooler fan forward hanger. The bearing was sent to Bell Helicopter/Textron for examination; however, the laboratory report states that a fracture mode could not be identified due to damage to the fracture surfaces. Airworthiness Directive 2000-02-12 applied to the bearing and was complied with 11 flight hours prior to the accident.