Incident Overview
During the initial climb of a takeoff, as the helicopter accelerated through 25 to 30 miles per hour, the 90-degree gearbox fractured through the center of its case and separated from the tail boom. The pilot managed to slow the spin rate and executed a hovering autorotation. As the helicopter descended vertically, a portion of the tail boom struck a barbed wire fence.
Examination Findings
A metallurgical examination of the gearbox case fractures revealed overload features. An internal inspection of the gearbox and gears showed that the pinion and gear teeth had disengaged. This disengagement lasted only a short period. The rotation of the disengaged gear and pinion against each other caused the gearbox case to become overloaded and subsequently separate.
The wear patterns on the drive side of the pinion and gear teeth did not conform to the specifications in the maintenance manual. The patterns extended beyond the toe ends and the tips of the teeth.
Maintenance Background
The gearbox is an 'on-condition' component, meaning it is monitored for wear rather than replaced at fixed intervals. It was last overhauled or repaired in 1984. There is no requirement for an internal inspection of the tail rotor gearbox during routine or annual maintenance inspections.