Continental Copters Inc. 47G2 N9021R accident near Lohn, Texas

Probable cause

The improper installation of the tail rotor gearbox by maintenance personnel, which led to accelerated wear of the ring and pinion gears, resulting in the loss of tail rotor effectiveness and subsequent forced landing.

NTSB analysis

The pilot performed a preflight inspection of the helicopter to include checking the tail rotor oil level through its sight glass. He had flown the helicopter for more than an hour when the tail yawed to the left, slowly at first then rapidly. The pilot “cut” the throttle, flew the helicopter “to the ground as fast as possible,”and impacted terrain. A postaccident examination of the wreckage revealed that the tail rotor gearbox did not operate when it was rotated. A teardown revealed that the interior surfaces of the gearbox were covered with a black oily substance consistent with residual lubricating oil contaminated with liberated metal particles. The oil level sight glass exhibited no discoloration or other hindrance to normal oil level detection in service.

Examination of the ring and pinion gears revealed that the teeth on the pinion gear were deformed and worn almost down to the tooth root in some locations. Wear was observed on the top land and drive faces of the ring gear. Wear patterns were noted on the coast faces of the ring and pinion gear teeth. The observation of wear on the coast faces of the gear teeth is most probably an indication that the spacing between the gears was too tight as a result of improper alignment during installation, causing accelerated wear. A logbook entry showed that the helicopter’s last annual inspection was completed about 6 months before the accident during which the tail rotor gearbox was overhauled and installed.

Text from the NTSB. Open the NTSB docket for CEN12LA120 for the full record.