ANC09LA065

The airline transport pilot was on a Title 14, CFR Part 135 passenger flight. The pilot said during cruise flight he heard a loud bang, and the engine started running rough. He said he diverted to the nearest airport, but the engine quit completely, and he was unable to reach the runway. The airplane subsequently collided with terrain, sustaining substantial damage to both wings and the fuselage. An examination of the engine revealed that a locking screw had backed out of one of the anti-vibration counterweights on the crankshaft, scoring the interior back surface of the engine case. The unsecured counterweight then moved from its position in the crankshaft, and was likely struck by the engine's master rod, which shattered the weight, and liberated it from its normal location. The liberated counterweight was struck by internal moving parts, creating several component failures, and ultimately an engine seizure.

On July 11, 2009, about 1300 Alaska daylight time, a Noordyun UC-64A (Norseman) airplane, N225BL, sustained substantial damage following a loss of engine power and an emergency landing about 300 feet short of runway 1 at the Akiachak Airport, Akiachak, Alaska. The airplane was being operated as a visual flight rules (VFR) passenger flight under Title 14, CFR Part 135, by Renfro's Alaska Adventures, Bethel, Alaska. The pilot and sole passenger were not injured. Visual meteorological conditions prevailed, and company flight following procedures were in effect. During a telephone conversation with the National Transportation Safety Board (NTSB) investigator-in-charge (IIC) on July 12, the pilot said he was flying from Bethel to Tuluksak, Alaska, when the engine started to run rough. He said he diverted to Akiachak which was near their position. The pilot said the engine quit completely, and he was unable to reach the runway. The airplane collided with terrain about 300 feet short of runway 1. The pilot said the engine had been rebuilt about 100 flight hours prior to the accident, after developing problems following a previous rebuild. He said that the airplane sustained substantial damage to both wings and the fuselage during the accident. An internal examination of the engine by the NTSB IIC on November 6, showed that a locking screw had backed out of one of the two anti-vibration counterweights on the crankshaft, and scored the interior back surface of the engine case. The locking screw was not located. The engine case and oil passages had fractured metal pieces from piston skirts, piston rings, and ground pieces of connecting rods. A large part of the crankshaft counterweight and half of bolt that normally secures the counterweight halves were found in a piston cylinder. The other half of the counterweight was found in the crankcase; it was fractured into three large and many smaller pieces. The second counterweight was still bolted in its proper place. Piston heads were found in the crankcase, and the engine's master rod was bent.