NTSB Factual Report

On September 30, 2006, about 1620 Alaska daylight time, a float-equipped Cessna TU206A airplane, N206AK, sustained substantial damage during a hard landing in ocean swells, following a complete loss of engine power in cruise flight, about 15 miles west of Juneau, Alaska. The airplane was being operated by Big Foot Air of Anchorage, Alaska, as a visual flight rules (VFR) positioning flight under Title 14, CFR Part 91, when the accident occurred. The commercial pilot and sole passenger were not injured. Visual meteorological conditions prevailed, and a VFR flight plan was filed. The flight departed Cordova, Alaska, en route to Juneau, about 1200.

During a telephone conversation with the National Transportation Safety Board (NTSB) investigator-in-charge (IIC) on October 10, the pilot said during cruise flight he was monitoring the exhaust gas temperature (EGT) of the individual engine cylinders. He said he noted that the number 6 cylinder temperature was several hundred degrees less than the rest of the cylinders. The pilot said he adjusted the throttle and mixture controls to see if the temperatures would stabilize. Due to the rough sea state, the pilot said he elected to continue the flight, and look for a suitable landing site. A short time later, the pilot said the engine started making a tapping sound, which he was able to mitigate by adjusting the throttle and mixture. After another short interval, he felt a vibration in the rudder pedals, and he continued to adjust the mixture and throttle to mitigate the tapping and vibration while looking for a suitable place to land. He said he heard a loud bang, and the engine cowling flew back against the windscreen, and remained there. At the same time, he saw a cylinder exit the side of the engine compartment, and the propeller stopped in the vertical position. The pilot said he tried to glide the airplane into a cove with a beach at the far end. The airplane landed hard in steep swells, damaging a float. He reported that he tried to sail the airplane downwind toward the beach, but the swells coming from the side were pushing the airplane toward the rocks. He stated he was able to restart the engine long enough to beach the airplane.

Under the direction of the NTSB IIC, the airplane was examined by a certificated airplane mechanic. The mechanic said the airplane received structural damage to the fuselage and wings as a result of the ocean landing. He stated that the number 6 cylinder assembly was missing from the engine. The cylinder assembly was attached to the engine case by eight externally threaded studs mounted to the engine case, which protruded through holes in the base flange of the cylinder assembly, and were secured with self-locking nuts. An examination of the eight studs showed that three of the studs had broken at or near their base. The remaining studs had a combination of stripped external threads, or they retained internal threads stripped from the self-locking nuts. The mechanic recovered six of the self-locking nuts from inside the lower engine cowl. Two of the self-locking nuts retained broken mounting studs. The remaining self-locking nuts had stripped internal threads, or retained external threads stripped from the mounting studs. Two of the self-locking nuts, and the number 6 cylinder assembly were not recovered.

An examination of the airplane's engine maintenance logbooks by the IIC revealed that the engine had been overhauled March 13, 1997, at a tachometer time of 2,002.2 hours. On January 16, 1999, at a tachometer time of 2,091.9 hours, the number 5 cylinder assembly was replaced, and the number 6 cylinder assembly was removed and reinstalled. On May 21, 2001, at a tachometer time of 2,200.0 hours, the number 5 cylinder assembly was again replaced. On April 2, 2006, tachometer time 2,832.9 hours, the number 4 cylinder was replaced, and on September 8, 2006, tachometer time 2960.7 hours, the numbers 1 and 3 cylinders were replaced. During an annual inspection on September 9, 2006, at a tachometer time of 2,950.7 hours, with 947.9 service hours, the number 6 cylinder had a compression of 73/80. According to the airplane's Airworthiness Directives compliance report, all directives were complied with as of September 9, 2006.

NTSB Probable Cause

The separation of the number 6 cylinder assembly from the engine during cruise flight, which resulted in a hard forced landing in rough ocean swells. A factor contributing to the accident was the rough ocean swells.