N6993N

On August 28, 2000, approximately 1050 mountain daylight time, a Cessna T210N, N6993N, nosed over during a forced landing after experiencing a total loss of engine power near Council, Idaho. The commercial pilot and two of his five passengers received minor injuries, and the aircraft, which was owned and operated by SP Aircraft, of Boise, Idaho, sustained substantial damage. The three other passengers were not injured. The aircraft, which was on a 14 CFR Part 135 air taxi flight from Slate Creek Airstrip to Boise, was being operated in visual meteorological conditions. The flight was on company flight following while en route. The ELT, which was activated by the accident sequence, was turned off at the scene.

According to the pilot, while in cruise flight, the manifold pressure of the turbocharged aircraft began to slowly drop from about 27 inches of mercury to less than 24 inches of mercury. Because the engine appeared to continue to run smooth, the pilot thought that the engine turbocharger had ceased to function. Because he was nearing Council, the pilot decided he would land there and "check things out." When he was about four miles from the airport, the engine started to run very rough. It continued running rough for about 30 seconds, and then experienced a total loss of power. Being unable to get the engine to restart, the pilot made a forced landing in an open field to the northeast of Council. Although the touchdown was successful, during the landing roll, the nose wheel sank into an area that had been softened by irrigation, and the aircraft nosed over.

In a post-accident engine teardown inspection it was determined that the crankshaft had failed in the cheek area between the number two main journal and the number two crank pin. It was also determined that the mating surfaces of the main bearing saddles showed signs of fretting. The most severely fretted was the number two main bearing saddle, through which a through-bolt passes. The number one main bearing was deformed, scored and discolored. The number two main bearing was deformed and fragmented.

The crankcase and crankshaft were subjected to a visual and metallurgical inspection at the National Transportation Safety Board's Materials Laboratory. That inspection determined that what appeared to be silk thread and some form of sealant had been applied to the mating faces of the main bearing saddles during the last crankcase reassembly. Further inspection determined that the crankshaft failed due to fatigue, which initiated from the surface at the aft cheek radius of the number two center main crankshaft journal. Shifting of the number two main bearing had produced scoring in the cheek radius, leading to fatigue crack initiation.

A review of the Teledyne Continental Motors 520 series engine overhaul manual current at the time this engine was overhauled (March, 1999) revealed that although aviation grade D silk thread and aviation Permatex are called for in mating the perimeter of the crankcase, instruction 70-20-00 (Crankcase Threading Procedure) and figure 70-20 (Permatex and Threading Procedure) specifically forbid application of any sealing material on the main bearing saddle mating faces.

In a telephone conversation with Continental Motors analytical staff, the NTSB investigator-in-charge (IIC) was informed that application of any material on the mating surface of the main bearing saddles can lead to a loss of proper crankcase clamping force in the area of the through-bolts. If this force is lost, proper bearing crush forces are not maintained, and bearing shift may occur. As a result of bearing shift, the crankshaft my ultimately fail due to scoring and/or improper lubrication.