CONTINENTAL COPTERS INC. 47G2 near Lohn, TX — 2012-01-01

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 narrative

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.

Analysis

Primary failure mode

Mechanical failure

First missed decision gate

Pilot could have conducted more thorough checks on tail rotor gearbox before flight.

NTSB coding

• Flight control sys malf/fail · Maneuvering-low-alt flying
• Maintenance

Evidence available

• Photos
• 10 docket documents

Docket documents10

• Evidence Control Formform
• Other Pertinent Forms and Reports - Scott’s – Bell 47 Inc. Party Reportform
• Pilot/Operator Aircraft Accident Report, NTSB Form 6120.1form
• Retention / Release of Wreckage Formform
• Statement of Party Representatives to NTSB Investigationinterview
• Miscellaneous - Bell 47 Walk-around Inspection Checklist Excerptother
• Miscellaneous - No Records In Reference To Tail Rotor Gearboxother
• Miscellaneous - Shim measurement from the tailrotor gearbox supplied by the FAAother
• View of a gear in the tailrotor gearbox supplied by the FAAother
• Materials Laboratory 15 - Factual Report 13-028 (with 8 embedded images)photos

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