On December 3, 1999, a Robinson R22 MARINER II (registration N7183Y) was involved in an aviation accident near Minden, NV. Investigators recorded the probable cause as: 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. This summary draws on records from the U.S. National Transportation Safety Board (NTSB) historical archive.
While hover-taxiing at a five-foot hover, a helicopter experienced a sudden uncommanded right yaw, spinning 360 degrees. The pilot's full left pedal input did not stop the rotation. After the lower horizontal stabilizer struck the ground, the aircraft continued spinning another 180 degrees before the pilot reduced power and applied controls, resulting in an abrupt touchdown. Post-incident examination revealed separation of portions of both tail rotor blades.
Incident Description
During a hover-taxi maneuver, the pilot experienced a sudden uncommanded right yaw. The aircraft was at a five-foot hover when it began to spin 360 degrees to the right about the mast. The pilot applied full left pedal in an attempt to stop the rotation, but the aircraft did not respond. After completing one full rotation, the lower horizontal stabilizer struck the ground. The aircraft continued to spin an additional 180 degrees, at which point the pilot rolled off the throttle, applied full up collective, and pressed the right pedal. The helicopter then touched down abruptly next to the taxiway.
Damage Examination
Following the incident, it was found that the outboard 11 inches of one tail rotor blade had separated, along with the outboard 6 inches of the second blade. Both separated blade sections were recovered and showed similar leading edge damage near the blade tips. The geometric design of the vertical and horizontal stabilizers acts as a guard for the tail rotor. Examination of the lower horizontal stabilizer showed evidence of ground contact, consistent with lateral contact during a right yaw. The vertical stabilizer exhibited damage from mechanical loading originating from movement of the lower horizontal stabilizer, resulting in left-to-right buckling just below the tail boom. This damage was also consistent with a right yaw. There was no evidence of contact between either stabilizer and the tail rotor. The extent of damage to the stabilizers and the geometric position of the tail rotor did not support witness reports of contact between the tail rotor and the ground. The differential loading from the unequal tail rotor blade separation created an unbalanced system that fed back to the tail rotor gearbox.