Durham, CA, USA
N234SJ
Bell UH-1B
The helicopter landed hard and rolled over during an autorotation following a loss of engine power. The pilot was demonstrating crop dusting turns and maneuvers for school children. He had completed several simulated passes over a field. He returned to the middle of a field, and turned into a light wind in preparation for landing. The helicopter lost power and entered an unexpectedly rapid descent. He was unable to prevent a hard landing and rollover. The teardown and examination of the engine disclosed that the power turbine shaft retaining bolt had failed in overload, allowing the shaft to float in a fore and aft manner and eventually damaging the blades on the power turbine disk. The type and degree of damage to the turbine section shafting and related components was indicative of the separation of the bolt at some undetermined point in time prior to the accident flight.
On October 16, 2003, at 1324 Pacific daylight time, a Bell UH-1B, N234SJ, collided with terrain near Durham, California. Porter Flying Services, Inc., was operating the helicopter under the provisions of 14 CFR Part 91. The commercial pilot, the sole occupant, was not injured; the helicopter sustained substantial damage. The personal local demonstration flight departed a private helipad about 1300. Visual meteorological conditions prevailed, and no flight plan had been filed. A Federal Aviation Administration accident coordinator interviewed the pilot. The pilot stated that he was demonstrating crop dusting turns and maneuvers for school children. He had completed several simulated passes over a field. He returned to the middle of a field and turned into a light wind in preparation for landing. The helicopter lost power and entered an uncommanded and unexpected rapid descent. He was unable to prevent a hard landing and rollover. He said that he did power checks every day. During preflight, he looked up the tail pipe, and did not see any blades missing. He said that all needles were in the green, and the engine was running perfect when it suddenly quit. He had not detected any vibrations or increases in temperatures. The engine was a Honeywell Model T53-L-11D turboshaft engine, serial number L-E12979. The National Transportation Safety Board investigator-in-charge oversaw examination of the engine at the Honeywell Investigation Laboratory, Phoenix, Arizona, on December 4, 2003. The aerospace business unit of Honeywell, Inc., prepared a report. Pertinent parts of their report follow. Disassembly of the engine revealed that the power turbine (PT) shaft retaining bolt was loosely in place in the aft stub shaft of the PT rotor disk with the threaded end of the bolt separated. Further disassembly found the threaded end of the bolt in the aft end of the PT shaft. Material analysis found heavy smearing on the fracture surface with light surface corrosion on and surrounding the area. Honeywell personnel felt that the separated threaded end displayed features typical of overload. The function of this bolt is to maintain the forward/aft position of the PT shaft by retaining it into the forward stub shaft of the PT wheel assembly. Both ends of the PT shaft are splined. The aft spline engages the mating splines in the forward stub shaft of the PT rotor disk. The forward spline engages the inner splines of the sun gear shaft in the reduction gearbox. With the PT shaft retaining bolt separated, the PT shaft would be allowed to float forward and aft within the compressor rotor assembly. They said that this was indicated by the rotational scoring on the forward and aft tapered sections of the PT shaft with corresponding rotational scoring on the inside of the forward and rear compressor shafts. The point in time in which the bolt separated could not be determined. The PT shaft retaining bolt, once the threaded end separated, would be contained within the aft shaft of the PT rotor disk. The flange on the aft end would keep that end of the bolt relatively centered in the PT aft shaft, but the forward end would be allowed to move away from the centerline until the flange contacted the inner surface of the PT Rotor Disk forward stub shaft. They said that the rotational scoring on the forward flange of the bolt and the scoring on the inner surface of the separated stub shaft adjacent to the separation point indicated this. This scoring caused a stress concentration due to the thinning of the shaft material over time that ultimately resulted in the separation of the stub shaft from the PT rotor disk in overload. As the shaft separated, the forces resulted in the displacement of the wheel away from the centerline, engaging the PT blades into the PT cylinder. Once the blades contacted the PT cylinder, the blades separated in overload. The material analysis noted that one blade had a small amount of fatigue near the trailing edge. Their metallurgist determined that the fatigue was not the cause of the separation, as the remainder of the fracture surface indicated overload. These forces also resulted in the partial separation, as well as rotational and aft movement of the center portion of the turbine support assembly relative to the support struts.
Separation of the power turbine shaft retaining bolt resulting in extensive power turbine area damage and a loss of shaft output power.
Source: NTSB Aviation Accident Database
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