History of Flight
On July 15, 2008, about 1530 mountain daylight time, a Bell UH-1H helicopter, registration N3276T, made a forced landing on sloped terrain after a loss of engine power in a forested area near Rock Springs, Wyoming. Leading Edge Aviation operated the helicopter under 14 CFR Part 133 as a long-line operation. The certificated commercial pilot sustained minor injuries; the helicopter sustained substantial damage. The flight departed from the accident area at 1330. Visual meteorological conditions prevailed, and a VFR company flight plan had been filed.
The pilot reported that the helicopter was repositioning a seismic drill rig using a 100-foot-long line. After moving the drill approximately 100 yards uphill and setting it down, the pilot was in a stabilized hover about 100 feet above the drill when the driller requested repositioning. As the pilot began to lift the drill, he added power and heard loud pops followed by compressor stalls. The pilot detached the cable and pointed the helicopter downhill away from personnel. The helicopter touched down on uneven terrain, the skids broke away, and the helicopter rolled onto its left side, causing substantial damage.
Ground witnesses stated that the pilot was relocating the drill when the driller signaled to lift back up. The rear legs lifted off the ground and came back down, with the long line becoming slack. Witnesses saw the helicopter listing with a trail of fire about 10 to 15 feet long from the rear of the engine, heard knocking noises, and 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 by the U.S. Army. The last 50-hour inspection occurred on July 14, 2008. Airframe total time was 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. The No. 1 bearing was originally installed in 1991 at engine total time 1,295.00 hours. The engine had been removed in March 2006 due to metal contamination and vibration, with maintenance performed in May 2006. Maintenance personnel indicated the source of contamination was the No. 2 bearing, and vibration was caused by exhaust diffuser and bearing housing contact. The engine was installed in the accident helicopter in July 2006. At the time of the accident, engine total time was 2,817.8 hours; the No. 1 bearing had 1,522.8 hours total time.
Engine Examination
Visual examination of the engine revealed no movement of the power turbine and small metal pieces in the exhaust. Compressor blades and inlet guide vane trailing edges were damaged. The engine was shipped to Honeywell facilities in Phoenix, Arizona for further examination.
Honeywell personnel found metallic debris on the engine chip detector and fuel in the fuel line between the fuel control unit and fuel flow divider; a fuel sample was collected but showed no discrepancies. An oil sample from the aft gearbox also showed no discrepancies. Oil was present in the exhaust duct. The power turbine shaft could not be manually rotated, but the high-speed spool could be rotated with effort via the starter gear. The reduction gearbox was free to rotate with debris on oil-wetted surfaces; black debris was found on the oil transfer tube screen of the planet gear shaft.
The accessory drive carrier assembly and inlet housing were intact. The No. 1 bearing oil supply nozzle passage was not blocked. Honeywell reported that the No. 1 ball bearing had failed, resulting in the loss of engine power. The degradation of the No. 1 bearing caused inability to maintain axial position of the gas generator spool. The impeller wore through the shroud wall in two places, compromising compressor efficiency and overall engine performance.
Probable Cause
The National Transportation Safety Board determined that the probable cause of this accident was the failure of the No. 1 bearing, which led to a loss of engine power.