The pilot was conducting flights to transport supplies and people about 4 miles between a lower altitude landing zone (LZ) and a high-altitude LZ. He was flying down-canyon to lose altitude and set up for a steep approach to the lower LZ. As he was maneuvering and slowing the helicopter to land, a low rotor rpm warning occurred. He saw the rotor rpm was about 95% and aborted the landing. He radioed ground personnel that he had a partial engine failure and was looking for a place to land. As he continued to fly down the canyon, he raised the collective and the low rotor rpm warning occurred again. He attempted to land in a clearing but overshot it and landed in trees. He said he did not check the throttle position during the event sequence, but he had maintained 100% main rotor rpm during each offload at the upper LZ.The engine control unit captured data consistent with main rotor impact. The data showed that during the 12 seconds before the fault, the power lever angle (PLA) readings were between 48° and 50° and the engine was operating consistent with those settings. The normal PLA reading when the throttle is in the FLY position is 70°, and full engine governing is available down to 62° PLA. The pilot’s operating handbook states that the throttle should be in the FLY position for takeoffs and landings. It also cautions that failure to position and maintain the throttle in the FLY detent position during normal flight operations can limit available engine power.There is a detent in the throttle that is engaged by a ball plunger when the throttle is rolled to the FLY position from either the Idle or Max Ng positions. The throttle was manipulated by hand during postaccident examination and, when the throttle was rolled down from the FLY position towards Idle, there was perceivably less tactile resistance than when the throttle was rolled from FLY to Max Ng. Testing of the throttle determined the ball plunger would disengage from the FLY position detent when 4 lb of the force was applied to turn the throttle. The helicopter maintenance manual states 14 to 15 lb of force should be required to move the Page 2 of 9 WPR22LA333throttle through the FLY detent position. Disassembly of the collective assembly revealed the detent where the ball plunger in the throttle engages at the FLY position was worn, which likely reduced the force required to move the throttle out of the FLY position towards IDLE. No other preimpact anomalies were found with the engine or airframe systems.Maintenance records indicted the throttle friction should have been verified in accordance with the manufacturer’s recommended inspection schedule and maintenance procedures. The procedures directed maintenance personnel to set throttle friction to the appropriate amount by adjusting the ball plunger on the throttle. Although the ball plunger disengaged from the FLY position detent in postaccident testing with less force than specified by the manufacturer, it could not be determined if maintenance personnel did not properly adjust the ball plunger in accordance with the manufacturer’s specified procedures, or if the worn detent reduced the ability of the ball plunger to maintain the correct torque after being set properly. Although the maintenance procedures did not require inspection of the throttle detent, if the specified throttle friction forces were unable to be achieved during the inspection, this should have provided the opportunity necessary for maintenance personnel to identify and correct the worn throttle detent.The accident is consistent with the pilot not ensuring the throttle was in the FLY detent position as the helicopter made an approach to land which resulted in reduced engine power, a low main rotor RPM condition, and subsequent impact with terrain. The worn throttle detent bracket likely contributed to the pilot unknowingly rolling the throttle towards idle prior to commencing the approach to land and subsequently not recognizing that the throttle had moved.