What happened
During a night instrument instruction flight at 9,000 feet, the aircraft was operating under VFR on top when the left half of the top engine cowling ripped off. The detached material became lodged on the horizontal stabilizer. Following the separation, the airplane became difficult to control and was unable to maintain level flight. The instructor and student diverted to a nearby airport, where they landed not injured.
The investigation
An FAA inspector examined the aircraft and determined that the left cowl pin was missing. The inspection revealed that the pin did not fail mechanically; rather, it was torn away along with the surrounding cowling material.
The manager of the repair shop where the aircraft had been serviced noted that he had previously observed worn side latches and pin grommets. These worn components allowed the upper cowl pin to slide out of the lower pin hole, creating a condition where airflow could rip off the upper cowling. There were no FAA malfunction or defect reports regarding similar failures, and the manufacturer had no records of any previous occurrences of this nature.