Rock Springs, WY, USA
N3276T
BELL UH-1H
During an external load operation involving lifting a drill for repositioning, the pilot heard multiple loud reports (compressor stalls) coming from the rear of the helicopter, followed by an adverse yaw. The pilot jettisoned the drill and pointed the helicopter down a hill to avoid nearby ground personnel. The helicopter landed hard on its skids and then rolled onto its left side. During the engine examination investigators noted no movement of the power turbine during the visual examination and metal pieces were found in the exhaust. The compressor turbine leading edge blades and inlet guide vane trailing edges were damaged. Further examination of the engine revealed that the No. 1 bearing and associated hardware showed evidence of over-temperature exposure, but the exact cause for the over-temperature could not be determined. Pieces of bearing cage were found lying loose at the bottom of the reduction gearbox cavity and the ball bearings were grouped to one side and smeared over. While the manufacturer indicated that a bearing cage failure may have initiated the failure of the bearing, the investigation was unable to conclusively determine the cause of the bearing failure.
HISTORY OF FLIGHT On July 15, 2008, about 1530 mountain daylight time, a Bell UH-1H, N3276T, made a forced landing on sloped terrain after experiencing a loss of engine power in a forested area near Rock Springs, Wyoming. Leading Edge Aviation operated the helicopter under the provisions of 14 Code of Federal Regulations (CFR) Part 133 as a long-line operation. The certificated commercial pilot sustained minor injuries; the helicopter sustained substantial damage. The local flight departed from the accident area at 1330. Visual meteorological conditions prevailed, and a visual flight rules (VFR) company flight plan had been filed. In a written report, the pilot stated that the helicopter was being used to reposition and set a seismic drill rig in a remote area utilizing a 100-foot-long line. The pilot had moved the drill approximately 100 yards uphill and set it down. The pilot reported that the drill was 1,700 pounds. He also indicated that the helicopter was in a stabilized hover about 100 feet over the drill when the event occurred. The driller notified the pilot that he did not like the drill location, and wanted it repositioned a couple of feet away. While initially beginning to lift the drill, the pilot reported that he added power and heard loud pops, followed by a series of compressor stalls. The pilot detached the cable and pointed the helicopter downhill, away from the drill and ground personnel. The pilot stated that the helicopter touched down on uneven terrain on the skids, which broke away. The helicopter rolled onto the left side, which caused substantial damage to the helicopter. According to the ground witnesses, the pilot was relocating the drill when the driller noticed a "bad set on terrain," and signaled to the pilot to lift back up and reposition the drill. The rear legs lifted off the ground and then came back down, with the long line becoming slack. The witnesses looked up and saw that the helicopter was "listing," with a trail of fire, about 10 to 15 feet in length emanating from the rear of the engine. They also heard "knocking" noises as the helicopter continued to list, and then watched the helicopter descend and impact the ground. AIRCRAFT INFORMATION The accident helicopter was a 1969 Bell UH-1H, serial number 69-15911, originally owned and operated by the United States Army. According to the helicopter maintenance logbook, the last 50-hour inspection was performed on July 14, 2008. Aircraft maintenance and flight records filled out for the day of the accident reported the airframe total time as 11,702.8 hours, with 5.9 hours flown on the day of the accident. The helicopter was powered by a Honeywell T53-L-703 turboshaft engine, serial number LE-09126Z. A review of the engine logbooks revealed that the No. 1 bearing was originally installed in 1991, at an engine total time of 1,295.00 hours. The engine was then installed on a UH-1H and operated for a total time of 83 hours between September 2000 and July 2002. When the engine was removed it had a total time of 1,378.00 hours. The engine was then installed on another UH-1H and operated from October 2002 until March 2006. Recorded total time at the time of engine removal was 2,544.00 hours. The reason for removal was due to metal contamination and vibration; maintenance was performed in May 2006. Maintenance personnel indicated the source of the metal contamination was the No. 2 bearing, and the vibration was caused by the exhaust diffuser and bearing housing contact. In July of 2006, the engine was installed in the accident helicopter. At the time of the accident, the engine total time was 2,817.8 hours. The total time on the No. 1 bearing at the time of the accident was 1,522.8 hours. TEST AND RESEARCH During a visual examination of the engine at the owner's facilities, investigators noted no movement of the power turbine and small metal pieces were found in the exhaust. The compressor blades were damaged, as well as the inlet guide vane trailing edges. The engine was boxed and shipped to Honeywell facilities in Phoenix, Arizona, for further examination. An engine inspection was performed by Honeywell personnel under the auspices of the Safety Board. Honeywell personnel reported metallic debris on the engine chip detector, and that fuel was present in the fuel line between the fuel control unit and the fuel flow divider; a sample was collected and submitted for material analysis. An oil sample was collected from the aft gearbox and submitted for analysis. There were no discrepancies noted with either the fuel sample or oil sample that were submitted for analysis (Honeywell's complete test analysis are attached as an appendix to this report). During the external examination of the engine, Honeywell personnel noted oil in the exhaust duct of the engine. The fuel sensing lines remained attached with fuel present in the lines. They were not able to manually rotate the power turbine shaft, and with considerable effort they were able to manually rotate the high speed spool via the starter gear. The reduction gearbox was not disassembled; however, it was free to rotate with debris noted on the oil wetted surfaces. Black debris was also located on the lower most oil transfer tube screen of the planet gear shaft. The accessory drive carrier assembly was intact and undamaged with all gears and bearings free to rotate. The inlet housing assembly was intact with some debris on the oil wetted surfaces. The power shaft bearing retainer assembly was also intact and undamaged. Honeywell personnel checked the No. 1 bearing oil supply nozzle, and verified that the passage was not blocked. Honeywell reported that the No. 1 ball bearing had failed and resulted in the loss of engine power. The degradation of the No. 1 bearing resulted in the inability to maintain axial position of the gas generator spool. The damage to the impeller shroud showed that the high speed spool moved forward, which reduced the clearance between the impeller and the shroud until they made contact. The impeller wore through the shroud wall in two places. The holes in the shroud exposed the flow-path to ambient outside air, which compromised the compressor efficiency and overall engine performance. According to Honeywell, the degradation of the No. 1 bearing along with the decreased engine performance, eventually led to a lower power condition and inability to maintain power turbine speed. Honeywell reported that pieces of bearing cage lying loose at bottom of the reduction gearbox cavity, and balls of the bearings were grouped to one side and smeared over.
A total loss of engine power due to the failure of the No. 1 bearing for undetermined reasons.
Source: NTSB Aviation Accident Database
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