On January 7, 1990, a Mooney M20B (registration N74520) operated by Robley D. MC Cants was involved in an aviation accident near Santa Ana, CA. Investigators recorded the probable cause as: The shearing of the bolts retaining the idler shaft, which resulted in a change in the internal timing and failure to drive the left magneto. This summary draws on records from the U.S. National Transportation Safety Board (NTSB) historical archive.
A Mooney M20B experienced an engine failure while on short final approach. Investigation revealed sheared bolts retaining the idler shaft, which caused a change in internal timing and failure to drive the left magneto.
What happened
The engine of a Mooney M20B failed while the aircraft was on short final approach. The event occurred during the landing phase of flight.
The investigation
A post-accident examination of the engine revealed that the bolts retaining the idler shaft had sheared. This failure allowed a change in the internal timing and resulted in the failure to drive the left magneto. Both bolt heads, the lockplate, and the cotter keys were found in the oil sump. The ignition springs on the electrical harness had evidence of arcing.
Findings
The mechanical failure was attributed to the shearing of the bolts retaining the idler shaft. This caused a change in the internal timing and failure to drive the left magneto. The ignition springs on the electrical harness had evidence of arcing.
Probable cause
The shearing of the bolts retaining the idler shaft, which resulted in a change in the internal timing and failure to drive the left magneto.