What happened
On November 25, 2013, a Zlín Z 226 MS, registration OK-KNN, was performing a ferry flight from Vysoké Mýto to Otrokovice for scheduled maintenance. Approximately one minute after departing from LKKM airport, the pilot experienced intense vibrations originating from the front of the aircraft, accompanied by a significant drop in engine power.
Following the loss of power, the pilot shut down the engine and executed an emergency landing in a field approximately 1 km south of Kroměříž, near road 367. The landing took place on soft, clay-like terrain. The aircraft sustained no damage during the maneuver, and the pilot, a 26-year-old commercial pilot, was uninjured.
The investigation
An investigation by the ÚZPLN focused on the mechanical failure of the M 137A engine. While the aircraft's maintenance records were formally in compliance with regulations, investigators examined the engine's internal components.
Upon inspection of the engine, investigators discovered damage to the crankshaft journal plug and the tearing of the connecting rod eye of the sixth cylinder. Metal fragments from the connecting mechanism were found inside the piston cavity. The investigation revealed that the plug's wall had been weakened by internal corrosion. Additionally, traces of sealant were found on the plug's mating surface, and a sticky, oily sediment was present on the plug's tightening bolt. The investigation also noted that the connecting rod eye bolt nuts had been secured with incorrect safety wires, and a bearing cap retaining pin was missing.
Findings
- The primary cause of the engine failure was the seizure of the connecting rod on the sixth cylinder's crankshaft journal.
- This seizure was triggered by a reduction in oil pressure and volume at the moving parts due to an internal leak.
- The leak originated from a crack in the crankshaft journal plug, which allowed oil to escape into the engine crankcase.
- The crack in the plug was caused by internal corrosion, likely facilitated by the presence of acetate impurities from old sealant residue (dating back to a 1998 inspection) interacting with carbon-oil deposits.
- The pilot responded correctly to the engine vibrations by shutting down the engine and selecting a suitable landing site, preventing further damage or injury.