On July 12, 1990, a Bell 47D-1 (registration N8117) was involved in an aviation accident near Fair Oaks, AR. Investigators recorded the probable cause as: The tail rotor cable became disengaged from the tail rotor pitch drum and wrapped around the flange of the pitch change drum, locking the tail rotor blades in the right pedal position. This summary draws on records from the U.S. National Transportation Safety Board (NTSB) historical archive.
A commercial pilot conducting an aerial application operation experienced vibration and loss of pedal response, followed by an unrecoverable right turn and descent into terrain. Investigation revealed the tail rotor cable had disengaged and wrapped around the pitch change drum, locking the blades.
Incident Overview
A commercial pilot was conducting an aerial application operation when a vibration was felt in the aircraft. Immediately thereafter, the pilot reported no response from the left or right pedals. The pilot initiated a right turn to return to the airstrip, but the helicopter continued turning right and descended until colliding with terrain.
Investigation Findings
Post-accident examination of the helicopter revealed that the tail rotor cable had become disengaged from the tail rotor pitch drum. The cable had subsequently wrapped around the flange of the pitch change drum. As a result, the tail rotor blades were locked in the right pedal position.
Technical Details
The disengagement of the tail rotor cable from the pitch drum prevented normal control of the tail rotor pitch. With the blades locked in the right pedal position, the helicopter would have produced a continuous right yawing moment. The pilot's attempt to return by initiating a right turn likely exacerbated the situation, as the locked blades prevented any corrective input, leading to the uncontrolled descent and ground impact.
Probable cause
The tail rotor cable became disengaged from the tail rotor pitch drum and wrapped around the flange of the pitch change drum, locking the tail rotor blades in the right pedal position.