What happened
On August 10, 2007, a Jak 11, registration OK-JZE, was conducting a flight from LKKB to LKTB, with the ultimate destination being an airshow at LHKE in Hungary. Approximately 10 minutes after takeoff, while flying over the Bečváry area, the aircraft experienced a sudden decrease in engine RPM, which stabilized at 150 and caused the propeller blades to move to a coarse pitch setting.
The pilot identified the issue as a propeller malfunction based on engine parameter checks. Due to the situation, the pilot decided to perform an emergency landing at the LKCV military airfield. Although the airfield was closed at the time due to approaching thunderstorms, the pilot declared an emergency and received permission to land. The aircraft landed safely on runway 14 with no injuries to the crew and no damage to the aircraft.
The investigation
The ÚZPLN investigation focused on the mechanical failure of the powerplant. Technical inspections of the engine revealed that the drive linkage for the R-7E propeller governor, located at the front of the engine casing, had become disconnected. This linkage consists of a cable system wrapped around a pulley, which is attached to the governor control shaft via a hub using a four-rivet steel connection.
While the rivet connection was found to be the original factory installation, the investigation noted that the Jak 11 maintenance manual does not specifically mandate inspections of this particular assembly point.
Findings
- The primary cause of the engine RPM drop was the disconnection of the R-7E propeller governor control linkage due to operational wear.
- The pilot was appropriately qualified for the flight profile.
- The aircraft was airworthy and held a valid Special Certificate of Airworthiness at the time of the incident.
- Weather conditions and airfield status did not contribute to the mechanical failure.