Tucson, AZ, USA
N6188L
Hughes OH-6A
The engine spooled down during a law enforcement surveillance flight, and the pilot made a hard forced landing in uneven terrain. The crew was performing surveillance at low altitude above an uneven wash, and the pilot was in slow sideward flight. He pulled the collective to gain altitude and airspeed to make another pass. Then he heard and felt the engine spool down. He said that it quit producing power completely, and he attempted to autorotate to the ground. He said that he had insufficient airspeed and altitude to complete a successful autorotation. The helicopter hit the ground hard in a near level attitude. It bounced backwards, and rotated 45 degrees to the right. Metallurgical examination revealed that the governor tee to fuel control PC line fractured as a result of fatigue. The tubes were bent with contours differing from the original manufactured shape, indicative of handling/assembly damage. Disassembly and inspection of the starter/generator revealed conditions indicative of wear and misassembly, including the wrong bearings being installed, which would cause the unit to operate in an out-of-balance condition and induce vibrations.
On October 25, 2004, about 1425 mountain standard time, a Hughes OH-6A, N6188L, landed hard during a forced landing following a loss of engine power near Tucson, Arizona. The United States Border Patrol was operating the helicopter under the provisions of 14 CFR Part 91. The commercial pilot and one passenger sustained serious injuries; the helicopter sustained substantial damage. The public-use flight departed Ajo, Arizona, about 1100, with a planned destination of Tucson. Visual meteorological conditions prevailed, and a company visual flight rules (VFR) flight plan had been filed. The operator reported that the crew was performing surveillance above an uneven wash. The pilot was in slow sideward flight. He pulled the collective to gain altitude and airspeed to make another pass. Then he heard and felt the engine spool down. He said that it quit producing power completely, and he attempted to autorotate to the ground. He said that he had insufficient airspeed and altitude to complete a successful autorotation. The helicopter hit the ground hard in a near level attitude. It bounced backwards, and rotated 45 degrees to the right. It remained upright, and both occupants extricated themselves from the wreckage; however, both sustained serious back injuries. The engine was a Rolls-Royce Allison 250-C20B, serial number CAE 836844. Engine total time was 3,901.6 hours; the last 300-hour inspection occurred on October 21, 2004, at 3,500 total time, and 293 hours since overhaul (TSO). Rolls-Royce examined the engine under the supervision of a Border Patrol investigator, and prepared a report of their findings. Their report indicated that the power turbine governor to fuel control PC line failed 0.4 inch from the governor tee end. The crack initiated from localized fretting damage on the outer surface of the tube, and propagated by fatigue circumferentially before ultimately failing. They also observed fretting damage on the inner surface of the ferrule. The microstructure and composition of the tube portion of the PC line assembly conformed to the material specifications. Photographs of the engine post mishap showed proper clamping. Disassembly and inspection of the starter generator revealed conditions indicative of wear and misassembly, which would cause the unit to operate in an out-of-balance condition. Examination of the starter generator occurred at Flite Components. They found that the armature commutator was worn 0.40 inch beyond minimum limits; the drive end journal was 0.020 inch beyond limits with the commutator end journal on minimums. The drive end bell liner was oversize with excessive wear on the housing, which they said indicated that it was loose in its mounting. They also noted that it had been assembled with incorrect bearings. The terminal end cracked, and the commutator end bell was worn beyond limits. Both Boeing and Rolls-Royce completed metallurgy examinations. The IIC submitted the parts to the Safety Board Materials Laboratory. Materials Lab A Safety Board Materials Laboratory specialist submitted a factual report. Governor Tee to Fuel Control PC Line The line was submitted in sections: two line sections near the fitting on side A (that connects to the governor tee) that includes the fracture, the side A ferrule/nut, a metallurgical mount, and the remainder of the line that contains the opposite fitting on side B (that connects to the fuel control). A section of the line at the bend near side A was missing. Boeing and Rolls-Royce analyzed the hardware prior to examination by the Safety Board Materials Laboratory. On the line, the specialist observed the ink markings "AIR PT," "6870035 C," and "MFR 3T336 062603." Visual examination of the line on side A revealed that the fracture occurred approximately 0.40-inch from the end of the line in an area of heavy fretting. The fretting location corresponded to where the edge of the ferrule is located. Examination of the interior edge of the ferrule also revealed the presence of fretting. Examination of both fracture surface halves revealed that the fracture initiated from the outer diameter (OD) surface in an area of heavy fretting. It propagated through the thickness of the tube, and circumferentially around the tube. The origin area itself contained significant metal smearing, but scanning electron microscopy (SEM) examination of the origin area in undamaged regions confirmed initiation from the OD surface at the base of fretting damage. The fracture features near the origin were flat, normal to the surface, and consistent with fatigue propagation. Moving the ferrule/nut away from the flared end on side B of the line revealed similar fretting at the end of the ferrule as found in side A, though no cracking was visually observed. Previous examination by Rolls-Royce compared a new-make line to the accident hardware, and showed that there was a significant difference in geometry. The comparison showed that side A was bent further outward than the new-make line, whereas side B appeared undamaged. PC Filter to PT Governor Tee PC Line The PC filter to governor tee PC line in the as-received condition contained both fittings, both clamps, and was not cracked. The line was visibly bent near the fitting on side A (that connects to Governor Tee) and in the straight portion of the line just adjacent to the middle bend in the line between the two clamps. On the line, were the ink markings "6876542 D" and "MFR 3T336." Previous examination by Rolls-Royce compared a new-make line to the accident hardware and confirmed that there was a significant difference in geometry. The comparison showed that the long portion of the tube was bent away from side A and the radius near the side A was sharper (in addition to the bending at the fitting itself on side A). No destructive examination of the line was performed.
The loss of engine power due to a fatigue fracture of the PC line from the governor to fuel control, resulting from fretting damage induced by excessive vibration, which was due to the failure of maintenance personnel to install the proper bearings in the starter/generator. A contributing factor was the loss of power at low altitude and airspeed (height/velocity limitation) which diminished the possibility of a successful autorotation.
Source: NTSB Aviation Accident Database
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