Incident Details
On 21 April 2013, an AN-2R aircraft departed at 10:00 from an unspecified location for a repositioning flight to Olsztyn-Dajtki Airport (EPOD). The pilot, a professional airplane pilot, was the sole occupant and was uninjured. Prior to departure, a mechanic performed a daily inspection, and the pilot verified the technical documentation and conducted a pre-flight check. About 15 minutes before engine start, the mechanic rotated the propeller several times, noting no resistance indicative of oil accumulation in lower cylinders. The engine started and operated normally during a run-up test.
At approximately 11:20, near Drzewica, about 35 km southwest of Białobrzegi, the pilot detected irregular engine operation and decided to abort the flight, landing at Radom-Piastów Airport (EPRP) around 11:40. After landing, the aircraft taxied to the EADS PZL forest base. Initial visual inspection of the engine revealed no apparent damage, but a more thorough examination by a service team identified internal damage. The engine was removed and replaced; the damaged unit was transported to Mielec for further investigation.
Engine Examination
Disassembly of the engine revealed a complete seizure of the piston in cylinder No. 5, resulting in the breakage of its connecting rod and the onset of piston seizure in other cylinders. According to the report, hydraulic shock—caused by improper preparation of the radial engine for start, such as failure to purge excess oil from lower cylinders after prolonged shutdown—was the very probable cause. Hydraulic shock typically occurs when incompressible oil in lower cylinders creates excessive stresses on the piston-connecting rod assembly during start, leading to buckling of the connecting rod and subsequent uneven piston movement, scoring, and eventual seizure.
Probable Cause
The investigation concluded that the probable cause of the incident was hydraulic shock occurring during previous engine operation.