What happened
On August 10, 2007, a Zlin Z 42M, registration OK-ILP, was performing a flight from LKOL airport with a planned destination of LKPJ. During the climb phase, at an altitude of approximately 300 meters AGL, the pilot experienced a loss of engine thrust accompanied by increased vibrations in the forward section of the aircraft.
Upon checking engine parameters, the pilot found them to be within normal limits and concluded that the issue originated from the propeller. Due to the aircraft's position, the pilot decided to perform an emergency landing in a nearby field near the village of Ústín. During the landing maneuver, the pilot shut down the engine. The aircraft landed on a mown, firm field without any damage to the airframe or injuries to the crew.
The investigation
The ÚZPLN investigation focused on the mechanical integrity of the engine and propeller. While the engine parameters were normal and no fuel or oil leaks were detected, the investigation turned to the propeller assembly.
Upon inspection of the V 503A propeller, investigators found that blade number one could be moved freely by approximately 5-7 degrees. Following the removal of the propeller for repair, the investigation identified a failure within the blade pitch change servomechanism. Specifically, a portion of the carrier arm, which connects to the propeller blade, had broken off.
Findings
- The primary cause of the loss of thrust was a failure of the propeller blade pitch change servomechanism due to metal fatigue.
- Examination of the fracture surface revealed signs of fatigue stress, including progressive crack propagation and corrosion traces.
- The failure occurred because the critical cross-section of the carrier arm had been reduced by fatigue, eventually becoming unable to withstand the loads exerted by the servomechanism during flight.
- The pilot was fully qualified and the aircraft was airworthy at the time of the incident.