Occurrence
During initial climb after departing runway 25 right, the airplane experienced a partial loss of power. About 30 seconds later, the pilot reported poor climb performance to the tower controller and requested to return to the airport.
Ground witnesses, including pilots at the airport, observed the airplane flying low and appearing slower than normal. The engine maintained a constant low rpm sound without coughing, sputtering, or backfiring. The airplane was noted with nose up, wings rocking, and tail moving back and forth. Subsequently, the airplane stalled: the right wing dropped about 90 degrees and the nose pitched nearly straight down. The airplane came to rest behind a building.
Engine Examination
The engine had not been overhauled. Total time recorded at the last 100-hour inspection was 2,762.5 hours. The manufacturer's recommended time between overhauls is 2,400 hours. The last 100-hour inspection included an oil and oil filter change, exhaust system check, spark plug gapping, check for compliance with airworthiness directives, and notation that compression was 70/80 or better.
Post-accident examination determined that only one top spark plug appeared to be within service limits. Disassembly revealed broken rings on two pistons. One piston had a scarf gap that allowed combustion blow-by (blow torching) to consume piston material. False brinnelling was extensive on the face of the cylinder number 1 exhaust camshaft follower and the cylinder number 3 exhaust lobe of the camshaft. All internal engine components displayed signatures consistent with an overextended service life.
The ignition switch was found in the right position. It could not be determined if the pilots left the switch in that position after the before-takeoff magneto check, or intentionally selected it while troubleshooting the lack of power in flight.