Accident Details
The airplane experienced a total loss of engine power while cruising at 3,000 feet mean sea level. The pilot heard a loud bang from the engine, followed by smoke from beneath the left side panel and excessive vibration. The engine lost power, and the pilot executed a forced landing to a field. During the landing rollout, the nose wheel struck a rock, causing the nose gear to collapse. The airplane sustained substantial damage.
Engine Inspection
Post-accident inspection of the engine revealed that a connecting rod had punched a hole in the engine case. The number 2 piston rod, rod bearing, and rod journal were discolored and exhibited heat distress. The crankshaft was broken into two pieces at the number 2 cheek. A fatigue fracture was observed on the fracture surfaces of the number 2 cheek.
The engine was originally a 230 horsepower Continental O-470-R that had been converted to a 270 horsepower O-470-50 under Supplemental Type Certificate (STC) #SE4985NM. The STC required cylinders to be replaced with those used on Continental O-520 engines. The crankshaft, forging number 539664, serial number S401, and heat code E99 (a "wide-rod" crankshaft), was manufactured in the 1960s and originally used on Continental O-470 and IO-470 engines. It had not been replaced since the STC conversion on December 23, 1994, accumulating 1,259.0 hours since conversion.
The engine teardown revealed that the pistons installed were 8.5-to-1 compression pistons used on normally aspirated Continental O-520 engines. The STC specifies 7.5-to-1 compression pistons as proper. On September 24, 2002, P. Ponk Aviation issued Service Information Letter SIL 002, stating that use of the wide-rod crankshaft with any piston higher than 7.5-to-1 compression and a maximum rpm above 2,700 is not authorized and could result in engine failure. No logbook entries indicated any piston changes since the conversion, except for the #3 cylinder changed on April 15, 2000, due to low compression. The engine build-up records from the STC conversion no longer existed.