What happened
On August 22, 2015, a Zlín Z 50M, registration OK-TRP, was performing an aerobatic training flight at Karlovy Vary Airport (LKKV). After taking off from runway 11, the aircraft climbed to approximately 900–1200 meters. During the climb, the engine suddenly failed, and the pilot noted an unusual smell resembling burnt oil.
The pilot immediately turned the aircraft toward the airport and attempted to restart the engine, but the starter could not restart the motor. While executing a glide toward the airfield, the pilot identified a gap in the perimeter vegetation near the "Krach" pond to avoid obstacles. During the final approach, the aircraft's landing gear and lower fuselage made light contact with the airport fence. Despite the contact, the pilot achieved a smooth three-point landing on the grass area north of runway 12. No injuries were reported.
The investigation
The ÚZPLN investigation focused on the mechanical failure of the M 137AZ engine. Upon disassembly of the engine, investigators found that the camshaft was not rotating when the engine was turned by the propeller. Specifically, two teeth on the camshaft drive shaft (Sc3757) were damaged.
Further inspection of the camshaft housing revealed that a tooth had broken off from the driven gear (Sc3755) of the oil return pump. This broken piece had become wedged within the pump, causing a sudden blockage of the engine's vertical shaft movement. The investigation also revealed that the oil pump assembly currently in use (assembly no. 4) was different from the one installed during the last overhaul (assembly no. 45), and there was no record of this replacement in the engine logbook.
Findings
- The engine failure was caused by the interruption of the drive for the fuel injection pump and valve gear due to the sudden blockage of the vertical shaft.
- The primary mechanical cause was a broken tooth on the driven gear (Sc3755) of the oil return pump, which originated from a fatigue fracture initiated by a machining groove.
- An unauthorized repair was performed on the oil pump, where the driven gear had been flipped 180 degrees to use the less-worn side of the teeth.
- There was no documentation in the engine logbook regarding the replacement of the original oil pump assembly with the current, unauthorized version.
Safety action
- The ÚZPLN recommends that LOM Praha s.p. issue a service letter containing safety information and recommendations to maintenance organizations, owners, and pilots regarding the cause of engine failures in the M 137 engine series.