CULLMAN, AL, USA
N110AL
SIKORSKY S-76A
THE PILOT REPORTED THAT DURING CRUISE FLIGHT, AT ABOUT 200 FEET AGL, HE HEARD A 'SNAP' SOUND, AND THE HELICOPTER YAWED ABOUT 30 DEGREES TO THE RIGHT. HE OBSERVED THE LEFT ANTITORQUE PEDAL TO BE INOPERATIVE. DIRECTIONAL CONTROL WAS MAINTAINED WITH ENGINE POWER ADJUSTMENTS. A RUN ON LANDING WAS PERFORMED AT THE BIRMINGHAM AIRPORT. THE HELICOPTER WAS THEN TOWED FROM THE RUNWAY. AN INSPECTION OF HELICOPTER REVEALED THAT THE LEFT TAIL ROTOR CABLE ASSEMBLY HAD FAILED IN THE VICINITY OF A PULLEY. METALLURGICAL EXAMINATION INDICATED THAT THE CABLE FAILED FROM SEVERE WEAR OF THE INDIVIDUAL WIRES OF THE CABLE STRANDS. THE CABLE WAS INSPECTED 521 HOURS PRIOR TO THE INCIDENT; MAINTENANCE MANUAL DICTATES CABLE INSPECTION AT EVERY 750 HOUR INSPECTION. CABLE WAS ORIGINAL EQUIPMENT ON THIS HELICOPTER (4110 HRS TOTAL TIME).
On June 15, 1993, at about 1402 central daylight time, a Sikorsky S76A, N110AL, lost partial tail rotor control during cruise flight near Cullman, Alabama. The helicopter was not damaged. The airline transport pilot, copilot, and three passengers were not injured. The flight was conducted under 14 CFR Part 91 by Southern Company Services, Inc. of Birmingham, Alabama. Visual meteorological conditions existed at the time, and a visual flight rules flight plan was filed for the business flight to Birmingham. The flight originated in Cullman at about 1400. The pilot reported the following: The flight had just departed from a confined area adjacent to a medical center. At about 200 feet above ground level (agl), he leveled off and the helicopter began to accelerate. As the helicopter reached about 100 knots, a "snap" sound was heard, and the aircraft yawed to the right about 30 degrees. He applied left antitorque pedal, with no effect. He decreased power and added right pedal, and the aircraft yawed further to the right. The second pilot tried to use his pedals, with the same results. The crew read the emergency procedures, and leveled the aircraft at about 2,000 feet mean sea level (msl). The aircraft was maintained in trim with power adjustments. A run on landing was performed at the airport in Birmingham. After touchdown, the helicopter veered to the right as weight was applied to the landing gear. The aircraft came to rest about 90 degrees to the right of the runway center line, with the tail rotor over the center line. He attempted to taxi clear of the runway, however the helicopter would only turn to the right. The aircraft was subsequently towed to the company maintenance hangar. An inspection of the tail rotor section revealed that left hand tail rotor cable assembly, part number 76400-03191-043, had failed about 59 inches from the forward end. The break was located where the cable contacts a pulley. The left tail rotor cable was sent to the NTSB Materials Laboratory for examination. The examination revealed severe wear of the individual wires of the cable strands (refer to Metallurgist's Factual Report, attached to this report). Maintenance records indicate that the failed tail rotor cable was original equipment on this helicopter. Sikorsky maintenance manual procedures dictate that the cable be removed and inspected at 750 hour intervals. The elapsed time since the cable was inspected was about 521 hours. That inspection occurred on September 18, 1992.
THE FAILURE OF COMPANY MAINTENANCE PERSONNEL TO PROPERLY INSPECT THE TAIL ROTOR CONTROL CABLE AT THE LAST 750 HOUR INSPECTION, AND THE SUBSEQUENT TOTAL FAILURE OF THE CABLE DUE TO WEAR.
Source: NTSB Aviation Accident Database
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