What happened
During a climb to cruise altitude, the pilot heard a loud bang from the Continental O-4SO-U engine, which was immediately followed by intense vibrations. The vibration persisted, making it impossible for the pilot to maintain altitude. In an effort to stop and feather the propeller, the pilot pulled the aircraft into a stall.
The pilot attempted an emergency landing in a parking lot but approached the area with excessive airspeed and altitude. The aircraft touched down on a road and subsequently collided with a berm. There were no injuries reported.
The investigation
An FAA inspector examined the engine and identified two holes in the engine case located above the connecting rods of the forward cylinder. A detailed examination of the powerplant revealed that cylinders number 2 and 6 remained intact, with no signs of heat discoloration on the piston skirts. While combustion deposits were present on the piston crowns, there was no evidence of staining or erosion at the edges of the crowns.
Investigators found that both piston skirts had failed. In the number 4 cylinder, the piston had disintegrated around the pin. This failure resulted in a bent piston rod, though the piston pin remained intact through the rod wrist. No heat distress was noted on either the rod or the pin, and no scoring was found inside the cylinder barrel. The bottom sparkplug for the number 4 cylinder was fouled with metal, and the oil sump contained both oil and large fragments of pistons.
The engine had recorded 2536.1 hours of total time, with 75.2 hours since the last inspection and 806.8 hours since the last overhaul. Maintenance records indicated that six Superior Air Parts, Inc. (part number SA 648029) pistons were installed during the previous major engine overhaul on February 19, 1990.