ANC97LA042

The pilot reported a partial loss of engine power during departure from a rural village. He said the airplane was approximately 600 feet agl when he heard a loud 'clunk-clunk' and felt an engine vibration. He reduced power, and turned downwind to land on the same runway he had just departed. On the turn from base to final, he said the engine continued to lose power, and he was unable to maintain sufficient altitude to reach the runway. He made a forced landing in deep snow. Postaccident inspection disclosed a fractured connecting rod bolt. Metallurgical examination at the NTSB laboratory revealed a fatigue failure of the rod bolt. The engine was rebuilt approximately 1,642 service hours prior to the accident. The engine manufacturer recommends an engine overhaul at 2000 hours.

On March 27, 1997, about 1031 Alaska standard time, a wheel equipped Cessna 207 airplane, N1747U, sustained substantial damage during an off airport forced landing at Grayling, Alaska. The airline transport certificated pilot and one of the two passengers aboard the 14 CFR Part 135 Yute Air scheduled commuter flight reported no injuries. The remaining passenger complained of a whiplash injury. The flight operated in visual meteorological conditions, and was departing Grayling, en route to Aniak, Alaska.

In his report to the NTSB, the pilot noted he departed runway 33, and was passing through about 600 feet agl when he heard and felt a loud, mechanical "clunk-clunk" type noise, accompanied by an engine vibration. He reduced power to approximately 20 inches of manifold pressure, and turned downwind in preparation for a precautionary landing on runway 33. During the turn from base to final, the pilot noticed the engine RPMs began to drop, and there was no response from the engine when he advanced the throttle. He was unable to maintain the desired glide path to the runway, and made a forced landing short of runway 33 in 3 to 4 feet of snow on the bank of the Yukon River.

A Federal Aviation inspector traveled to the accident site and noted substantial damage to the airframe, and a hole in the engine casing adjacent to the number two cylinder.

The engine was examined by an NTSB investigator and parties to the investigation on April 10, 1997, at Sea Air, Lake Hood, Anchorage, Alaska. Parties to the investigation were the FAA, Yute Air, and Continental Motors.

Inspection/tear down of the engine disclosed a hole in the left engine case above the number two cylinder with the number two cylinder connecting rod protruding through the hole and contacting the left magneto. There was a bulge and a crack on the right side of the engine case adjacent to the number one cylinder. Further disassembly revealed the number one and two cylinder rods had separated from the their respective crankpins. The number one and two cylinder rod caps were found loose in the oil sump pan. Both caps were distorted and gouged. All four of the rod cap bolts were fractured and retrieved from the oil pan. Two of the bolts had their respective nuts and cotter pins still attached, and the shanks ends near the fracture surfaces were necked down and stretched. A third rod cap bolt was missing a nut, its threads were missing, and its head was separated about 1/2 inch from the top of the bolt. A fourth rod cap bolt was discovered which displayed less damage then the other three. It was broken about mid shaft, and the nut and cotter key was still attached. The bolt's shanks next to the fracture surfaces did not display any apparent necking or stretching. It was not possible to determine if this bolt came from the number one or number two rod cap. This bolt was sent to the NTSB metallurgical lab for further testing and analysis.

The NTSB metallurgist who examined the bolt and its fracture surfaces, discovered extensive fatigue cracking and fretting in the area of the fracture.

According to company maintenance records, the engine, a Teledyne Continental Motors IO-520-F, serial number 286311-R, was rebuilt by Penn Yann Aero Service, Inc., Penn Yann, New York, on October 27, 1995. The engine had accumulated approximately 1,642 service hours since it was rebuilt. The factory recommended time between overhauls for this engine is 2,000 hours.

The rod bolt examined by the NTSB metallurgical lab was returned via registered mail by lab personnel to Mr. Daryl Carlson, Yute Air, on August 18, 1997.