ANC18LA059

The private pilot was conducting a cross-country flight when he demonstrated a momentary zero-G maneuver, shortly after which the engine lost partial power. The pilot was unable to restore engine power and subsequently conducted a forced landing to tundra covered terrain, during which the airplane nosed over, resulting in substantial damage. Postaccident examination of the airframe and engine revealed no anomalies that would have precluded normal operation; however, one engine exhaust valve exhibited signatures consistent with uneven excessive heat exposure, but no mechanical failure of the valve, therefore would not have resulted in a loss of power. It is unlikely that a momentary zero G maneuver would result in a partial power loss that could not be restored.

On July 31, 2018, about 2300 Alaska daylight time, a Cessna 185 airplane, N185KB, sustained substantial damage when it was involved in an accident near White Mountain, Alaska. The private pilot and three passengers were uninjured. The airplane was operated as a Title 14 Code of Federal Regulations Part 91 personal flight. The pilot reported that he was flying with a friend in the front seat and his two sons in the rear seat. Earlier that day, he fueled the airplane with 55 gallons, which resulted in 84 gallons of usable fuel for the 120-mile round trip. The flight to the destination was uneventful. The pilot departed on the return flight that evening and proceeded to the coast to avoid rising inland terrain and low clouds. The pilot-rated passenger in the front seat reported that, while maneuvering toward the coast, the pilot demonstrated an "up down" maneuver for his sons in the rear seats. He accelerated the airplane and gently pulled and pushed the nose over so that they could feel momentary weightlessness. About 45 seconds to 1 minute later, the pilot noticed that the propeller rpm had reduced from 2300 to 1920 rpm. He advanced the propeller control lever forward, but there was no response. He then pushed the throttle and mixture controls full forward, with no corresponding response. He activated the auxiliary fuel pump switch; however, full engine power was not restored, and the pilot performed a forced landing to terrain. The passenger stated that the landing appeared normal, and she was surprised when the airplane nosed over onto its back during the landing roll. The left wing, both lift struts, vertical stabilizer and rudder, and fuselage sustained substantial damage. The occupants egressed and the pilot called a family member, however; diminishing visibility prevented rescue that night. They sheltered inside the airplane for the night with one sleeping bag and some survival equipment and were rescued the following morning. The pilot and passenger each stated they regretted not having more sleeping bags, rain gear, and a hand-held aviation VHF radio on board. Examination of the airframe and engine revealed no anomalies with the exception of one exhaust valve, which exhibited green crescent signatures consistent with exposure to excessively high temperatures; however, there was no sign of valve failure, such as fractures or restricted movement of the valve. All fuel samples were clear and fuel was present in the fuel lines to the engine.