What happened
On April 21, 2013, an AN-2R aircraft was performing a repositioning flight to Olsztyn-Dajtki (EPOD). Prior to departure, a mechanic had inspected the aircraft and manually rotated the propeller several times to check for resistance caused by oil accumulation in the lower cylinders; no such resistance was noted. The pilot also performed a pre-flight inspection and a successful engine test run.
During the flight, at approximately 11:20, near the town of Drzewica (about 35 km southwest of Białobrzegi), the pilot noticed irregular engine operation. The pilot immediately decided to terminate the flight and diverted to the nearest airfield, Radom-Piastów (EPRP), landing safely at approximately 11:40. Following the landing, the aircraft taxied to the EADS PZL Forest Aviation Base.
The investigation
While initial inspections of the engine showed no visible external damage, a detailed technical examination by a service group revealed significant internal failure. Upon disassembly of the engine, investigators found that the piston in cylinder number 5 had become completely seized, which had resulted in the failure of its connecting rod. This failure subsequently caused the onset of piston seizing in the remaining cylinders.
Findings
- The investigation determined that the primary cause of the engine failure was a hydraulic lock that occurred during a previous period of operation.
- It was established that hydraulic lock occurs when a radial engine is not properly prepared for startup after a long period of inactivity. If the propeller is not rotated sufficiently to drain excess oil from the lower cylinders, the incompressible nature of the oil creates extreme stress on the piston-connecting rod assembly during startup.
- In this specific instance, the hydraulic lock likely caused the bending of a connecting rod in one of the lower cylinders. This led to uneven piston movement, causing metal shavings to accumulate and eventually leading to the seizure of the piston and the subsequent breakage of the connecting rod.