Engine Crank Case Stud Nut Torquing Procedure
A Cessna T207A powered with a Teledyne Continental TSIO 520M engine, departed for a routine flight and was close to its destination point when the engine began to run rough and all oil pressure was lost. Several attempts to restart the engine were unsuccessful and a forced landing was carried out. The pilot and one passenger received minor injuries, the second passenger was uninjured. The aeroplane was substantially damaged.
The Transport Safety Board (TSB) investigators determined that the forward main bearing of the engine crankshaft had failed. TSB Laboratory Report confirmed the “main bearing had worked loose from its normal position in a rearward direction making contact with the crankshaft”. This can only occur if the engine case halves are loose. Crank case halves can only become loose as a result of improper torque procedures to the case half stud nuts. No other components had failed and maintenance log records had confirmed a recent engine cylinder change.
The Teledyne Continental Motors Maintenance and Overhaul Manual specify the correct procedures and sequence for the application of torque for the engine case stud nuts as seen in figure 1. Information gathered during the investigation found that it is a common practice to apply torque to the nuts only on the side of the engine for the replacement cylinder, and that these nuts are not changed unless obviously worn. The nuts on the opposite side (as seen in figure 3 as thru studs) must also be tightened to the proper torque in conjunction with the application of lubricating oil. The manufacturer has provided information that the non-use of lubricating oil or the failure to apply the proper torque will result in an under tension condition on the thru studs and eventual failure of the engine.
Engine case halves that are loose will allow bearings to work themselves out of place as a result of various action/reaction forces during normal engine operation. This bearing may then move, closing the essential end clearance as seen in figure 2 (view A), and make contact with parts of the crankshaft, resulting in the creation of stress risers and eventual failure of the crankshaft.
Loose engine case halves will not retain bearings in the proper position as a result of various action/reaction forces during normal engine operation. If the essential end clearance as seen in figure 2 (view A) is not maintained, the crankshaft will eventually fail.
Therefore, Transport Canada Civil Aviation emphasizes to all aircraft owners and operators the importance of following the manufacturers’ instructions. In particular even if only one cylinder is being replaced all engine case nuts and thru stud nuts need to be properly retorqued.
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