History of Flight
On August 19, 2015, at approximately 1915 Pacific daylight time, a Sikorsky S-61A helicopter, registration N1043T, experienced a partial loss of power to the No. 2 (right) engine. The helicopter subsequently landed on a mountainside and rolled over about 7 miles west of Ironside, Oregon. The commercial pilot was not injured; the airline transport pilot sustained minor injuries. The helicopter sustained substantial damage to the tailboom. The aircraft was registered to and operated by Croman Corp under Title 14 Code of Federal Regulations Part 133 as a firefighting flight. Visual meteorological conditions prevailed, and the flight operated under a company flight plan. The flight originated from Baker City Municipal Airport (BKE), Baker City, Oregon at 1715.
The commercial pilot reported that after picking up 4,000 pounds of water from a pond, they made a climbing left turn to the east toward the fire. About 20 feet above the ground, there was an engine power loss and a drop in RPM. The pilot dumped the water from the bucket attached to the longline and attempted to gain airspeed and altitude as they entered a small valley. The pilot kept the bucket clear of the ground and obstacles and attempted to release the longline, but it would not fully release from the helicopter fuselage. The second pilot used the emergency throttle, but the engine did not respond; engine RPM and rotor RPM continued to decrease. With the terrain rising, they found the flattest accessible spot and began to slow the helicopter. The pilot landed as level as possible, but the helicopter rolled onto its right side. Prior to touchdown, the bucket impacted brush and trees and became tangled in a wire fence.
Aircraft Information
The accident helicopter was equipped with two General Electric T58-GE-402 turboshaft military engines. These engines feature a ten-stage axial compressor with variable inlet guide vanes, an annular combustor, a two-stage gas generator turbine, and an independent single stage power turbine. Maintenance was performed by Croman Corp. The last engine light overhaul was completed on August 27, 2014, at which time the helicopter had accumulated 903 hours since major overhaul. This was the first of four light overhauls completed at 1,000-hour intervals. After the fourth light overhaul, the TSO is set to zero. The accessory gearbox was separated and examined during the last light overhaul. The last magnetic plug inspection was performed 41.3 hours prior to the accident flight as part of a 35-hour phase inspection. Magnetic plug inspections are required every fourth phase inspection (140 hours).
Wreckage and Impact Information
On-scene examination by a Federal Aviation Administration inspector revealed that the bucket was entangled in a barbed wire fence. The line was tight, off the ground, and stretched around a tree before continuing straight toward the main wreckage. Impact damage was consistent with the helicopter impacting the ground first on its tailwheel, followed momentarily by the main landing gears. The tailwheel was found separated from the airframe, and impact marks indicated the tail rotor impacted the ground. Additionally, marks consistent with main rotor blade impact were observed on the ground near the main wreckage.
Examination of the longline showed that the attachment point on the fuselage indicated the hook released as expected, but one of the two hydraulic quick disconnect (QD) couplings failed to release. The hard-mounted QD system for the hydraulic lines was examined; only one fitting released from the coupling. The longline side of the released fitting was undamaged but had a stretched and fractured safety wire. The other fitting was fracture-separated and did not appear to release from the hard mount system until the helicopter hit the ground. Part of the fitting remained secured to the longline, and the remaining part was still secured to the coupling with a safety wire.
Engine Examinations
Number 1 Engine Examination and Run
Initial visual examination of the number 1 engine revealed minimal damage. The engine was rotated, and continuity was verified through the Ng drivetrain (compressor and gas generator turbine) and Nf drivetrain (power turbine spool). The magnetic plugs were removed; all four exhibited light residue accumulation considered normal for service run engines according to General Electric and Croman Corp. The engine was installed in a test cell, started normally, and idled with normal indications. Initially, smoke was observed emanating from the power turbine area but ceased after a few minutes. The engine was cycled between idle and about 100% Ng several times to ensure the control system functioned properly. The fuel control topping set point was checked and recorded at 101.8% Ng, which is 1.9% Ng below manual specifications.
Number 2 Engine Examination
During examination and disassembly of the number 2 engine, all exterior engine surfaces were intact and in good condition. The magnetic plugs were removed; all four plugs had metal debris accumulation, with the most debris noted on the accessory gearbox and power turbine accessory plugs. The front frame accessory drive was manually rotated at the inlet, and no concurrent rotation of the centrifugal fuel purifier drive splines was observed. The accessory gearbox, fuel control, and oil lube pump were removed. The radial driveshaft was then removed from the front frame; scallop-shaped wear marks were observed around the shaft, matching the mating female splines of the pinion assembly. About 0.10 inch of material wear was observed on the front frame housing, consistent with pinion gear assembly contact.
The pinion gear assembly was removed from the accessory gearbox. The pinion gear, which mates with the bevel gear, exhibited wear on the gear teeth and some metal smearing concentrated along the tips, consistent with gear disengagement. The pinion support assembly was disassembled; the upper support bearing exhibited roller ball, bearing cage, and race damage. The bevel gear exhibited wear on its teeth consistent with the mating pinion.
The accessory gearbox pinion, bevel gears, upper and lower pinion support bearings, and all four magnetic plugs underwent metallurgical examination. Energy dispersive x-ray scan, microstructure analysis, and hardness testing indicated that both the bevel and pinion gears met material specifications. Both gears had excessive wear on 50-75% of the outer tooth span at the gear tooth tip, also consistent with gear disengagement. Cracks were noted on two adjacent bevel gear teeth.
The pinion upper support bearing was disassembled; spalling was observed on the balls, inner, and outer raceways. Wear was mostly concentrated on the upper pinion support bearing inner race land. Energy dispersive x-ray and scanning electron microscopy identified aluminum oxide particles embedded in the cage pocket and inner races of both bearings. The debris accumulation on the magnetic plugs was analyzed and found consistent with bearing and gear material.
According to the manufacturer, aluminum oxide is not present in any T58-GE-402 bearing or component within the oil lubrication system pathway. Historically, aluminum oxide has been inadvertently introduced into the engine during the overhaul/repair process. Aluminum oxide is abrasive and, once embedded into a bearing, can cause uneven wear and accelerated failure.