What happened
On September 19, 2018, at approximately 17:45 LT, an WSK PZL-Mielec AN-2, registration YU-BRK, was conducting a scheduled agricultural spraying mission over the Jagodina region. The flight, operated by STS Avijacija d.o.o., was intended to treat mosquito populations. While flying at an altitude of approximately 150 meters near the village of Novo Lanište, the crew noticed intense engine vibrations.
Following the vibrations, oil began leaking from the engine onto the left side of the windscreen, rapidly obscuring the pilot' and crew's visibility. Thick black smoke and intermittent flames appeared beneath the engine cowling. Due to the severe loss of visibility, the pilot initiated an emergency landing in an unpopulated, overgrown field. During the approach, the aircraft's left wing struck trees, causing the plane to rotate around its vertical axis and skid across the ground for nearly 180 degrees. The aircraft sustained heavy damage to the fuselage, wings, engine, and propeller, and the landing gear was broken during the impact.
The investigation
The CINS investigation focused on the mechanical failure of the engine and the operational history of the crew and aircraft. Investigators examined the engine components, specifically the cylinder and connecting rod assemblies, and analyzed the oil and fuel systems. The investigation also reviewed the maintenance logs and flight records of the crew members.
Technical analysis of the engine revealed that the internal walls of the cylinders showed no signs of overheating, suggesting that the cooling baffles were functioning correctly. However, investigators identified evidence of a prior hydraulic lock event. The investigation also noted discrepancies in the flight logs, where certain flight hours for the crew members could not be verified due to inconsistent or missing documentation regarding flight assignments and pilot roles.