Incident Details
During an aerial application operation, after the third pass, the aircraft's engine developed a severe vibration and subsequently seized. The pilot jettisoned the chemical load, turned the airplane into the wind, and executed a forced landing into a cranberry bog.
Pilot Statements
The pilot reported that during the preflight inspection, he had felt no binding when pulling the propeller through to check for hydraulic lock. Additionally, the pilot stated that the airplane had not seemed to be developing full power for approximately one week prior to the accident.
Engine Examination
Post-accident teardown of the engine revealed a broken connecting rod upon removal of an upper cylinder (number 9). Metallurgical examination of the connecting rod fragments identified well-defined crack arrest positions indicative of fatigue on the smaller fragment. The fatigue region exhibited relatively uneven fracture features, and in several locations, the fatigue crack appeared to have extended due to overstress. No visible metallurgical defects were found in the initiation area. The larger fragment showed features typical of overstress.
Advisory Circular Reference
According to Advisory Circular 65-12A, a partial hydraulic lock could cause a slightly bent connecting rod that might go unnoticed but would be sure to fail later. Upper cylinders on high-wing aircraft are known to be susceptible to hydraulic lock resulting from a leaking fuel primer.