Incident Overview
An airplane experienced a total loss of engine power during cruise flight, prompting the pilot to execute a forced landing. Subsequent examination of the engine focused on the crankshaft and attached rod components to determine the cause of the power loss.
Examination Findings
Inspection revealed that both nuts on the number three connecting rod had backed off from their original installed positions. Additionally, one nut on the number four connecting rod had also backed off but had not failed. The separated number three rod and the damaged number four nuts could not be measured.
Torque Analysis
Bolt preload is related to installation torque but is influenced by variables such as lubrication, thread deformities, variations, rough surfaces, and contamination. Torque tests indicated a variance of 18–25 percent between tightening and loosening torque values. While the breakaway torque values fell within the range of test values for properly torqued nuts, the uncertainty of the readings prevented any conclusion regarding the adequacy of the original installation torque.