What happened
On September 6, 2012, a Zlín Z 43, registration OK-COE, experienced a sudden engine failure while performing a flight from Jindřichův Hradec to Strakonice. Approximately ten minutes after takeoff, the pilot noted the engine failure and decided to perform an emergency landing in a harvested cornfield near the village of Horní Kněžeklady.
The pilot identified a suitable landing area within a 60/60-meter stretch of mown field. During the final phase of the landing, the aircraft's tail section made contact with stalks of straw, causing minor damage to the left side of the horizontal stabilizer, specifically affecting the elevator hinge and the trailing edge. There were no injuries to the crew during the incident.
The investigation
An investigation by the ÚZPLN focused on the cause of the engine failure and the subsequent damage to the aircraft. The aircraft's M 337A engine was subjected to a technical inspection at an approved maintenance organization.
Investigators examined the camshaft and found that the camshaft position had shifted relative to the valve lifters. Detailed examination of the camshaft revealed that the shaft had twisted. The inspection also identified damage to a retaining washer (part no. Sc3752) and deformation of the locating pin surface.
Findings
Technical analysis established that the twisting of the camshaft caused the sudden engine failure. The failure originated from a fatigue crack in the camshaft, which initiated at the keyway for the gear (part no. Sc3750).
As the mechanical connection between the gear and the camshaft loosened, the torque was no longer transmitted through the tapered fit of the gear, but instead through the locating key. The inertial forces acting on the connection during engine acceleration and deceleration created localized stress in the keyway area, leading to the fatigue failure.
The investigation determined that the failure of the retaining washer likely occurred during the assembly of the gear during a previous engine overhaul. Furthermore, standard maintenance procedures do not include a specific inspection of the gear-to-camshaft connection, as such a check would require a complete disassembly of the assembly.
Safety action
The engine manufacturer has since implemented measures to ensure the quality of the gear and retaining washer assembly during the manufacturing, overhaul, and repair processes for the M 337A engine series.