NTSB Identification: FTW99LA085.
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Accident occurred Friday, February 26, 1999 in SPRINGTOWN, TX
Probable Cause Approval Date: 11/02/2000
Aircraft: Eurocopter COLIBRI EC120B, registration: N122TH
Injuries: 1 Minor.
NTSB investigators may not have traveled in support of this investigation and used data provided by various sources to prepare this aircraft accident report.
While at cruise altitude, the helicopter's engine began to surge. The pilot initiated an autorotation to an open field. During the downwind forced landing, the skids dug into the ground, and the helicopter nosed over and came to rest on its left side. An examination of the engine revealed that the P3 air pipe, which supplies air to the fuel control, was broken close to the fitting on the combustion chamber casing. Examination of the P3 pipe revealed that its fracture was 'consecutive to the propagation of a fatigue fissure,' as a result of the pipe being 'subjected to an excessive vibratory excitation.' Engine vibration tests were conducted and two main excitation sources were identified. The two sources were the starter-generator and the breather gear assembly. The proposed solution to correct for this vibration consists of fitting an attachment clamp at mid-length of the P3 pipe, 'which moves the natural frequencies out of engine power ratings.'
The National Transportation Safety Board determines the probable cause(s) of this accident to be: The improper touchdown executed by the pilot. Factors were the tail wind condition, and the manufacturer's inadequate design of the P3 pipe supports, which resulted in a fatigue fracture of the P3 pipe. Full narrative available
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