NTSB Identification: FTW01LA087.
The docket is stored in the Docket Management System (DMS). Please contact Records Management Division
Accident occurred Wednesday, March 21, 2001 in Ardmore, OK
Probable Cause Approval Date: 07/30/2001
Aircraft: Bell 47G2, registration: N1575
Injuries: 2 Uninjured.
NTSB investigators may have traveled in support of this investigation and used data provided by various sources to prepare this aircraft accident report.
During a night flight, the helicopter was in level flight, about 1,400 feet MSL over a lake as the student pilot commenced a turn to the left with a forward airspeed of about 45 to 50 knots. The CFI then "came onto the controls" to assist with the coordinated turn, and noticed that the cockpit turn indicator showed that the helicopter was "slipping." Subsequently, the CFI applied left pedal to correct, and as soon as the left pedal was applied, the airframe shuddered and started to yaw to the right. The airspeed started to decrease and the helicopter began to "spin to the right rapidly." The CFI then applied full left pedal and the helicopter continued to spin to the right. After lowering the collective and reducing the throttle, the helicopter stopped spinning. The CFI then lowered the nose and maneuvered the helicopter toward shallow water near the heavily vegetated shoreline, flaring and pulling up on the collective to cushion the landing. The helicopter came to rest on its left side in the shallow water. Examination of the helicopter revealed that the splined coupling at the aft end of the forward tail rotor drive shaft segment was damaged. Removal of the dust cover for the coupling revealed that the screw and washer, part numbers AN501A10-4 and AN960-10, were loose with several of the threads stripped, thus allowing grease to weep during operation. The forward tail rotor drive shaft assembly, part number 47-644-18-9, was replaced on January 29, 2001. The helicopter had accumulated 70 hours of flight time since the last 100- hour inspection and 23 hours since the last 50- hour lubrication.
The National Transportation Safety Board determines the probable cause(s) of this accident to be: the loss of tail rotor control during maneuvering resultant from the failure of the tail rotor drive shaft coupling due to a stripped thread which allowed the grease lubricating the shaft to leak out. A factor was the lack of suitable terrain for the forced landing (autorotation). Full narrative available
Index for Mar2001 | Index of months