What happened
On July 1, 2012, at approximately 09:10 UTC, a Zlín 226MS, registration OK-IFJ, was performing a glider tow flight from Zbraslavice airfield. During the eighth tow of the day, while climbing to an altitude of approximately 250 meters AGL, the pilot experienced a heavy impact from the front of the aircraft. This was immediately followed by an oil leak, engine vibrations, and a significant loss of power.
As the aircraft was flying over the area of Bohdaneč, the pilot determined that the vibrations persisted even after reducing power. To ensure safety, the pilot decided to perform an emergency landing with the engine shut down. The pilot executed the necessary emergency procedures—including turning off the magnetos, closing the fuel valve, and cutting the master switch—and glided the aircraft back to Zbraslavice, landing on runway 33. There were no injuries to the pilot, and the aircraft sustained no significant damage beyond the engine compartment.
The investigation
The ÚZPLN investigation focused on the technical inspection of the M137A engine and the aircraft's maintenance records. The inspection revealed extensive internal damage to the engine. Specifically, the bolts from the rear left engine mount had been torn out, and the left engine mount arm was bent. The engine block was pierced on both sides near cylinder number 6. Further examination of the engine cover showed it had been struck by a broken connecting rod.
Inside the engine, investigators found numerous metal fragments from the connecting rod and engine casing reinforcement. The connecting rod for cylinder number 6 had suffered a longitudinal fracture; the rod eye had split, and the rod shank was split lengthwise, leaving the rod head loose on the crankshaft. The lubrication pathway was found to be clear, and no signs of overheating or seizure were present on the rotating parts.
Findings
- The primary cause of the incident was the fracture of the connecting rod eye in cylinder number 6 due to material fatigue.
- The secondary damage to the engine block, engine mounts, and engine covers was a direct result of the broken connecting rod striking internal components during operation.
- The engine was operating within its approved service life, with approximately 38 hours remaining before its next overhaul.
- The high operational stress associated with glider towing likely contributed to the development of the fatigue crack.
- The pilot was fully qualified and the aircraft was maintained in accordance with all applicable regulations.