What happened
During flight, the pilot observed a decrease in manifold pressure and elected to perform a precautionary landing. While executing the base leg of the landing pattern, heavy smoke and fumes filled the aircraft cabin. The conditions were severe enough that the pilot had to look out of the side vent window to complete the landing safely.
Once the aircraft came to a stop on the runway, an engine compartment fire was identified and subsequently extinguished. There were no injuries reported in this incident.
The investigation
An examination of the engine revealed that the left intermediate exhaust pipe (Lycoming part no: LW-15811) had partially separated at the flange. It was noted that the engine crossover exhaust assembly had previously undergone modifications in accordance with AD 89-12-04 and Textron Lycoming Service Bulletin #484, both of which were designed to prevent this specific type of failure and separation.