N1673U

On June 19, 1998, about 1925 Alaska daylight time, a wheel equipped Cessna 207 airplane, N1673U, sustained substantial damage during a forced landing, about 3 miles north of Holy Cross, Alaska. The airplane was being operated as a visual flight rules (VFR) on-demand passenger flight to Bethel, Alaska, under Title 14 CFR Part 135, when the accident occurred. The airplane was operated by Inland Aviation Services Inc., Aniak, Alaska. The certificated airline transport pilot, and the three passengers, were not injured. Visual meteorological conditions prevailed. VFR company flight following procedures were in effect. The flight originated at the Grayling Airport, Grayling, Alaska, about 1840.

During a telephone conversation with the National Transportation Safety Board (NTSB) investigator-in-charge (IIC), on June 19, 1998, at 2030, the pilot reported he was in cruise flight about 2,200 feet mean sea level (msl). About 25 miles from Holy Cross, the pilot noticed the engine oil pressure began to drop below the normal operating range, and the engine oil temperature began to rise. The pilot decided to divert to Holy Cross. About 10 miles from Holy Cross, the oil temperature continued to rise, and the engine propeller rpm increased to maximum. The engine began losing power, and then quit running. The pilot selected an emergency landing area on a sand bar along the Yukon River. The sand bar contained several willow trees. During the landing, the airplane's nose gear was sheared off, and the wings received damage from the trees. The pilot reported he did not notice any external signs of damage to the engine.

The engine accrued 1,491.1 hours since being overhauled on April 15, 1991. Since the overhaul, the number three, and number five engine cylinders were changed after the engine accrued 808.9 hours. At 1,132.1 hours since overhaul, the number one cylinder was changed. At 1,241.5 hours, the number three cylinder was again changed. At 1,273.5 hours, the number two cylinder was changed. At 1,340.1 hours, the number four, and number six cylinders were changed. During the most recent 100 hour inspection of the engine on May 25, 1998, 50.8 hours before the accident, no metal was found in the engine oil.

On July 27, 1998, a Federal Aviation Administration (FAA) airworthiness inspector, Anchorage Flight Standards District Office, examined the engine during a tear down inspection. The inspection revealed the crankshaft was broken along the aft, lower edge of the number two cheek journal, at the forward radius of the number two main bearing journal. The number two main bearing was broken and flattened. The number two main bearing saddle was flattened and distorted. The left and right engine case halves mating surface exhibited galling/fretting at the number two main journal bolt holes. The surface of the number two main bearing journal exhibited circumferential discoloration and scoring.

On August 21, 1998, the NTSB IIC examined the accident engine crankshaft. The fracture line was straight and flat, through the full dimension of the number two cheek journal, and was oriented perpendicular to the longitudinal axis of the crankshaft. The forward edge of the fracture surface adjacent to the number two main bearing was curled aft and inward. The fracture surface of the number two cheek journal exhibited gouging and distortion in the center of the cheek, corresponding to about the dimension of the main journal diameter.

The number two rod bearing journal exhibited blue discoloration along the aft edge of the bearing surface, at the aft radius of the cheek. The number two rod bearing exhibited spalling of the inner surface. The number two rod bearing oil galley was plugged with metal.