WPR15LA248 · Sikorsky S 61A

The commercial pilot reported that they had picked up 4,000 lbs of water from a pond for a helicopter firefighting operation and were starting a climbing left turn. About 20 ft above the ground, the No. 2 (right) engine lost power, and the rpm dropped. The pilot dumped the water from the bucket and attempted to gain airspeed and altitude as the helicopter entered a small valley. The pilot also attempted to release the longline and bucket to no avail. The second pilot used the emergency throttle, but the helicopter continued to lose engine and rotor rpm. Unable to gain altitude and with the terrain rising, the pilot landed the helicopter, which subsequently rolled over and came to rest on its right side. Before touchdown, the longline and bucket had impacted brush and trees and the bucket became entangled in a fence, which likely hindered the pilot's ability to successfully land the helicopter.

On-scene examination revealed that the fuselage belly hook released the longline as intended; however, one of the two hydraulic quick-disconnect couplings failed to release the longline. A safety wire was used to connect the fitting on the quick-disconnect mechanism to the coupling. The use of the safety wire did not allow the quick-disconnect mechanism to function as intended and release the longline.

The examination of the No. 2 (right) engine revealed metallic debris on all four magnetic plugs. The front frame accessory drive was manually rotated, and no concurrent rotation of the centrifugal fuel purifier drive splines was observed. Removal of the radial driveshaft revealed scallop-shaped wear marks around the shaft, which matched the mating female splines of the pinion assembly. In addition, about 0.10 inch of material wear was observed on the front frame housing consistent with pinion gear assembly contact. Removal of the pinion gear assembly from the accessory gearbox revealed that the pinion gear, which mates with the bevel gear, exhibited wear on the gear teeth and some metal smearing along the tips, which is consistent with gear disengagement. When the bevel and pinion splines disengaged, the fuel and oil pumps were no longer being driven, so fuel and lubrication to the engine were cut and the engine lost power.

Disassembly of the pinion support assembly revealed roller ball, bearing cage, and race damage to the upper support bearing and wear to the bevel gear teeth consistent with that observed on the mating pinion. Aluminum oxide particles were embedded in the cage pocket and inner races of both bearings and likely contributed to the bearing wear that eventually caused bevel and pinion gear disengagement.

According to the manufacturer, aluminum oxide is not present in this engine's bearing or any component within the oil lubrication system pathway. However, aluminum oxide can be introduced into the engine during the overhaul/repair process if measures are not taken to maintain a clean environment, including management of air flow when bearings are exposed. Aluminum oxide is abrasive and, once embedded into the bearing, will typically result in uneven wear and accelerated failure. The last engine light overhaul was completed about 1 year before the accident, and it is likely that the aluminum oxide was introduced at that time.

Verbatim NTSB analysis and probable cause from the NTSB dataset