NTSB Identification: LAX00LA050.
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Accident occurred Friday, December 03, 1999 in MINDEN, NV
Probable Cause Approval Date: 07/01/2002
Aircraft: Robinson R22 MARINER II, registration: N7183Y
Injuries: 2 Uninjured.

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 hover-taxiing, the pilot experienced a sudden uncommanded right yaw. The aircraft was at a 5-foot hover when it spun 360 degrees (about the mast) to the right. The pilot attempted to stop the turn with full left pedal but the aircraft did not respond. After a full rotation, the lower horizontal stabilizer struck the ground. The aircraft had continued to spin an additional 180 degrees when the pilot rolled off the throttle, applied full up collective, and pressed the right pedal. The aircraft touched down abruptly next to the taxiway. The outboard 11 inches of one tail rotor blade had separated along with the outboard 6 inches from the second blade. Both separated blade sections were recovered with both showing similar leading edge damage near the blade tips. The geometric design of the vertical and horizontal stabilizer acts as a "stinger" or guard for the tail rotor. While there is evidence of ground contact with the lower horizontal stabilizer, it is indicative of lateral contact consistent with damage incurred during a right yaw. The damage to the vertical stabilizer shows evidence of mechanical loading originating from the movement of the lower horizontal stabilizer. The resultant loading produced left to right buckling of the vertical stabilizer just below the tail boom. This damage was also consistent with a right yaw. There was no evidence of contact between the either stabilizer and the tail rotor. The extent of damage to the stabilizers and geometric position of the tail rotor did not support the witness contention of contact between the tail rotor and the ground. The differential loading created by the unequal tail rotor blade separation resulted in an unbalanced system feeding back to the tail rotor gearbox.

The National Transportation Safety Board determines the probable cause(s) of this accident to be:

the result of a foreign object contacting the tail rotor blade(s) while the aircraft was hover taxiing, which caused blade separation with subsequent loss of anti-torque (lateral) control.

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