ANC24LA064

The pilot was conducting an air tour flight when the airplane’s propeller rpm decreased and the propeller subsequently stopped producing noticeable thrust. The pilot cycled the propeller but the propeller rpm continued to decrease.. The pilot performed a forced landing, which resulted in substantial damage to the fuselage and wings. The pilot reported no mechanical issues with the airplane other than the loss of propeller pitch control and subsequent loss of thrust. Postaccident examination and testing of the propeller and overspeed governors revealed that they performed within their respective manufacturer’s specifications, and no abnormalities were observed that would have prevented normal operation. The engine was completely disassembled and all damage noted was consistent with impact. The cause of the propeller rpm becoming uncontrollable could not be determined.

On July 12, 2024, about 1700 Alaska daylight time, a turbine-powered DeHavilland DHC 2T (Beaver) airplane, N535KT, sustained substantial damage when it was involved in an accident near Petersville, Alaska. The pilot and eight passengers were not injured. The airplane was operated by the pilot as a Title 14 Code of Federal Regulations Part 135 air tour flight. The pilot of the accident airplane was conducting an air tour flight within the Denali National Park and Preserve. The airplane departed from the Talkeetna Airport (TKA) about 1614 for a flight over the Tokositna Glacier before returning to TKA. The propeller rpm was set at 3000 and the engine torque was set at 82%. The pilot reported that, while operating over the upper elevations of the Tokositna Glacier, the engine torque indications decreased from 80% to 40% and the propeller was no longer producing any thrust. The pilot reached out to other pilots flying in the area to help in troubleshooting the problem. He cycled the propeller, and the torque indication went from 40% down to 30%. The propeller never went into the full feather position. The pilot performed a forced landing to an area of hilly, tree-covered terrain. He shut down the engine and turned off the fuel about 20 to 30 ft above the ground. The airplane sustained substantial damage to the fuselage and wings. The pilot reported no mechanical issues with the airplane other than the loss of propeller pitch control. The pilot had flown several flights in the accident airplane the day of the accident. Postaccident examination of the wreckage revealed propeller control continuity from the cockpit controls to the propeller governor. The engine chip detector was checked and no chips were observed. The reduction gear box filter screen was clean, as was the oil. The propeller governor screen was clean. The propeller governor was removed and the propeller blades were released from the “start locked” position. When the blades were released, high pressure oil was discharged from where the governor was removed. The discharge of oil verified oil was being supplied to the propeller governor and hub at the time of impact. The beta ring micro switch position rod was jammed in the beta ring assembly. The rod had pulled out of the rear bushing and was jammed against the bushing. All electrical components associated with the beta backup system were tested and operated as expected. Operation of the power lever and propeller feather systems caused all relays and switches to activate and deactivate as expected. Inspection of the circuit breaker system found that the circuit breakers functioned properly and were wired to the correct bus. No oil leaks were observed in the engine compartment or along the fuselage. The propeller and overspeed governors were tested and performed within their respective manufacturer’s specifications; no abnormalities were observed that would have prevented normal operation. The NTSB Materials Lab examined the beta switchassembly and was unable to determine if the jammed beta position rod existed before impact. The engine was completely disassembled. There was no buckling damage on the gas generator case or the exhaust duct. The gas generator case was found with one fractured bolt at one of the engine mounting pads. The power turbine shroud housing and the power turbine disk assembly were found with blade tip and path rub. There were no fractured flanges on the engine accessories. All damage to the engine was consistent with impact damage.