ETNA, CA, USA
N204AP
BELL TH-1L
THE 42-DEGREE GEAR BOX FAILED SHORTLY AFTER DEPARTING ON A HELICOPTER LOGGING OPERATION. THE PILOT JETTISONED THE LOAD, BUT WAS UNABLE TO STOP THE HELICOPTER'S ROTATION AND IT CRASHED. METALLURGICAL EXAMINATION OF THE GEAR BOX REVEALED A FATIGUE BANDING PATTERN NEAR THE TOOTH ROOT OF THE INPUT GEAR.
On February 23, 1994, about 1100 hours Pacific standard time, a Bell TH-1L helicopter, N204AP, crashed during an emergency autorotation about 3 1/2 miles north of Etna, California. The pilot was conducting a visual flight rules (VFR) local area flight under Title 14 CFR Part 133. The helicopter, operated by Hiser Helicopters Inc., Redmond, Oregon, sustained substantial damage. The certificated commercial pilot-in-command and the certificated second pilot received minor injuries. Visual meteorological conditions prevailed. The flight originated from a helipad next to the accident site about 0945 hours. The pilot reported in the National Transportation Safety Board Pilot/Operator Aircraft Accident Report, NTSB Form 6120.1/2, that shortly after picking up the 3,200 pounds of logs he felt an audible vibration; the helicopter was between 50 and 60 feet above the ground. The pilot reduced the power and began a descent to return the load to the ground that was followed by a right turn toward the landing area. The helicopter responded normally, but the intensity of the vibration increased. The vibration decreased in about 5 to 8 seconds after the helicopter entered forward flight. During deceleration, as the longline touched the ground, the helicopter began an uncontrollable spin to the right. The ground mechanic told the pilot via radio communications that the tail rotor "had stopped." The pilot rolled-off the throttle after about two or three revolutions to stop the rotation, but was unsuccessful. The helicopter struck the ground as it continued to rotate. The operator told Safety Board investigators that the helicopter's 42-degree gear box tail rotor failed. He also said that the 42-degree gear box had accrued 134 hours of logging activities and a total time of 249 hours since it was overhauled. The previous gear box had accrued 400 hours before the operator prematurely removed it. The operator removed it only to inspect the wear and tear on the gear box. Safety Board investigators sent the gear box to the Safety Board's Material Laboratory Division for metallurgical examination. The metallurgist reported, in part: "...that visual examination of the fracture surface on the input gear revealed the presence of [a] fatigue banding pattern over most of the fracture surface. The fatigue arrest markings emanated from a tooth root.... ...The fracture surface [was] within about 0.1 inch of the tooth root propagated into the tooth, approximately toward the adjacent tooth root. Beyond about 0.1 inch from the root the crack plane changed and the cracking propagated on a relatively flat plane through the web. ...A well defined crack arrest pattern was found in the web of the gear. ...Measurement of the radius from which the fatigue cracking initiated yielded a value of 0.04 inch. The size of this radius is specified as 0.015 inch to 0.025 inch. No defects were found in the fatigue crack origin." The Safety Board's investigator-in-charge returned the 42-degree gear box to the operator on July 6, 1994.
THE FAITUGE OF THE GEAR TOOTH WHICH CAUSED THE FAILURE OF THE 42-DEGREE GEAR BOX.
Source: NTSB Aviation Accident Database
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