What happened
On April 20, 2006, at approximately 11:45 UTC, a Z 37A Čmelák, registration OK-CJV, was performing aerial chemical spraying operations at low altitude (approximately 50m AGL) near the village of Lom u Tachova. During the return flight to the Tachov-Oldřichov airfield, the engine suddenly stopped during flight.
The pilot evaluated the situation and determined that an emergency landing was necessary. The aircraft was successfully landed in a suitable area of clay-based terrain north of Lom u Tachova. There were no injuries to the crew, and the aircraft sustained no damage during the landing.
The investigation
The ÚZPLN investigation focused on the mechanical failure of the engine and the interruption of fuel delivery. An inspection of the aircraft revealed that both fuel tanks contained approximately 60 liters of gasoline and the engine oil level was at 10 liters. No significant fluid leaks or contamination of filters were observed during the initial inspection.
Technical analysis of the fuel pump drive mechanism (702 ML) revealed that the mechanical connection between the vertical shaft and the pump housing had failed. Specifically, the investigators examined the threaded connection of the housing and the vertical shaft. An expert analysis of the fractured tabs of the split pin (safety wire) indicated that the failure was caused by a fatigue fracture. The fracture likely initiated at a sharp corner created where the tab was pressed against the nut.
Findings
- The engine shutdown was caused by an interruption of fuel delivery to the carburetor.
- This interruption resulted from the loosening of the threaded connection between the pump housing and the vertical shaft.
- The loosening was caused by the fatigue failure of the split pin tabs.
- The fatigue fracture was likely initiated by a sharp notch created during the securing process of the pin.
- The pilot responded correctly to the emergency in accordance with the flight manual.