NTSB Identification: LAX03LA084.
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Accident occurred Wednesday, February 05, 2003 in Casa Grande, AZ
Probable Cause Approval Date: 12/28/2004
Aircraft: Cobb International Rotorway Exec 90, registration: N9876D
Injuries: 2 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.
The helicopter made a hard landing following a failure of the antitorque system. While in the landing pattern, the student and CFI heard a "bang." The CFI took the flight controls, initiated an autorotation, and turned to an open area for an emergency landing. During the autorotation the CFI noticed a loss of his ability to maintain directional control. He lowered the collective and continued with the autorotation. As he pulled collective to cushion the landing the helicopter yawed to the left, landed firmly on the ground, and rolled over on to its right side. The CFI's execution of an autorotative landing failed to adjust for the engagement of the engine when he pulled in collective to cushion the landing. Engagement of engine power in the autorotative flare caused the helicopter to yaw left, which was not compensated for by the failed tail rotor. The helicopter encountered ground resistance when it landed, and the force of the uncontrolled left yaw rolled it over onto its right side. The FAA Rotorcraft Flying Handbook describes the appropriate response to an antitorque failure as an entry into an autorotation profile by reducing the collective and rolling off the throttle. Rolling off the throttle will prevent the engine from engaging during the autorotative landing. The manufacturer's flight manual does not address the issue of performing an autorotative landing with a tail rotor failure. Examination of the helicopter revealed that the secondary drive shaft appeared to have fractured between the lower bearing and the lock collar. Examination of the shaft revealed fretting and corrosion in the area contacted by the lock collar. The fracture was a result of fatigue cracking that initiated in the area of fretting and corrosion. This secondary drive shaft had been involved in one helicopter rollover and two tail rotor strikes. It is possible that excessive loads generated during these previous events was sufficient to loosen the lower bearing on the shaft and initiate the fretting and corrosion process that eventually led to the initiation of a fatigue crack.
The National Transportation Safety Board determines the probable cause(s) of this accident to be: the flight instructor's failure to maintain directional control of the helicopter during the flare and landing. Factors in the accident were the failure of the secondary drive shaft and inadequate tail rotor failure procedures specified by the manufacturer. Full narrative available
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