On September 1, 1996, about 1505 Alaska daylight time, a wheel equipped Piper PA-28-161, N4366Y, crashed during a forced landing about 7 miles east of Skwentna, Alaska. The airplane was being operated as a visual flight rules (VFR) local area sightseeing flight under Title 14 CFR Part 135 when the accident occurred. The airplane, registered to and operated by Vernair Inc., Anchorage, Alaska, sustained substantial damage. The certificated commercial pilot and one passenger were not injured. One passenger received minor injuries. Visual meteorological conditions prevailed. A VFR flight plan was filed and VFR company flight following procedures were in effect. The flight originated at Merrill Field, Anchorage, at 1440.

The operator/pilot reported that he was in cruise flight about 4,000 feet mean sea level when the engine developed a vibration. He switched fuel tanks and applied carburetor heat. The engine condition did not improve and began to run rough. The engine then quit and oil spray was deposited on the windshield. The pilot maneuvered the airplane toward a sand bar adjacent to the Yentna River. During the touchdown on the sand bar, the main gear encountered soft mud. The airplane received damage to the main gear and fuselage. After the emergency landing, the pilot noticed that the entire propeller, propeller flange and the outboard end of the engine crankshaft were missing from the engine. The missing propeller and crankshaft segment were not recovered.

On September 11, 1996, an engine examination was conducted at Sea Air Inc., Anchorage, Alaska. The examination revealed the crankshaft was broken about even with the front edge of the engine case. The crankshaft displayed a diagonal fracture and crack oriented in an aft direction on a 45 degree angle from just forward of the oil slinger flange.

The engine crankshaft was submitted to the National Transportation Safety Board (NTSB) Materials Laboratory for examination. The examination revealed crack arrest positions indicative of fatigue cracking emanating from the inner diameter surface of the crankshaft bore. The crack progressed between the forward edge of the oil slinger in a forward direction through 1/3 of the radius of the crankshaft, forward of the oil slinger. Examination of the crack origin revealed a 0.01 by 0.01 inch corrosion pit. A second fatigue crack origin was noted at the intersection of the first crack and the forward edge of the oil slinger. This crack progressed in an aft direction along a 45 degree angle around and through the number 1 main bearing journal.

Numerous other corrosion pits were found adjacent to the fracture origin. The inside diameter surface of the crankshaft bore was covered by heavy accumulations of sludge deposits.

Examination of the maintenance records revealed the engine was overhauled on October 8, 1992. The operator reported the engine accrued a total time in service of 8,775.0 hours, 1,879.9 since the last overhaul. The operator noted a previous major engine overhaul as "rebuilt" in June, 1988. The crankshaft had been ground 0.006 inches undersize.

The engine manufacturer reported the engine was built and shipped from the factory on April 13, 1981. They did not have any record the engine was ever returned to the factory for an overhaul or remanufacture.

The engine is the subject of a Lycoming Engine Mandatory Service Bulletin number 505A, dated October 18, 1994. The service bulletin addresses the inspection of the inside diameter of 4 cylinder engines for the presence of corrosion. The bulletin requires an initial inspection of the crankshaft..."For new, remanufactured and overhauled engines shipped from Lycoming prior to and including 1984, the initial inspection must be conducted within the next 200 hours of operation or 1 year from date of this Service Bulletin, whichever comes first. For new, remanufactured and overhauled engine shipped from Lycoming after 1984, the initial inspection must be conducted at the next overhaul or engine disassembly or within 10 years of the original ship date, whichever comes first."

The operator's company operations manual, chapter XIII, states in part:..."13-1 Aircraft Inspection and Maintenance:...b. It shall be the responsibility of the Director of Maintenance to ensure that adequate equipment, A.D. notes and current manufacturers service bulletins that are mandatory are complied with."

A review of the engine maintenance records did not reveal any notation that the operator complied with the service bulletin.

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