Accident Overview
During climb out, the instructor experienced a sudden reduction in engine rpm. After checking the engine and electrical systems, he took the controls and executed a forced landing to a dry flood control canal. His attempts to advance the throttle resulted in a further loss of power. During the last portion of the approach, he saw utility lines and attempted to avoid them, but the aircraft struck an overhead cable. Witnesses reported hearing popping sounds and seeing smoke trailing the aircraft.
Engine Examination
An examination of the engine revealed a broken exhaust push rod. Further analysis determined that the push rod failed due to the manufacturing process in which it was artificially aged. This process made the push rod susceptible to stress corrosion cracking (SCC) with a lower stress threshold. The ASM handbook notes that the preferred method is to maintain the material at the aging temperature for a longer period of time.
Cause of Failure
The interference pins inserted in the ends of the push rod most likely produced hoop stress at a level sufficient to initiate SCC once the push rod is exposed to an environment of humid and/or salty air. SCC most likely began after manufacture, but before the push rod was placed into service during a period in which it was exposed to adverse environmental conditions.