What happened
On July 11, 2019, at 07:30 UTC, a flight crew was conducting a training mission at Depułtycze Królewskie (EPCD) airport. The flight was part of a standard training program for single-engine piston (SEP) aircraft. During the execution of a task involving a static stall demonstration, the engine of the AT-3 R-100, registration SP-CLB, spontaneously shut down.
The aircraft was in a climb at an altitude of 3,800 ft and a speed of approximately 55 IAS when the power loss occurred. The student pilot immediately ceased the climb and transitioned to a descent, increasing airspeed, while the instructor attempted to restart the engine. After turning the ignition key, the instructor successfully restarted the engine and decided to terminate the training task. The crew proceeded to land at EPCD at a safe altitude. The landing was completed without incident, and the technical failure was recorded in the aircraft's logbook.
The investigation
The investigation focused on the mechanical causes of the engine failure and the environmental conditions present during the flight. The commission examined the engine's fuel and ignition systems, as well as the operational procedures used during the stall demonstration.
Findings
- The primary cause of the engine failure was carburetor synchronization imbalance, combined with inaccurate spark plug calibration and a fuel flow disruption caused by the momentary activation of the electric fuel pump.
- Atmospheric conditions at the flight altitude may have contributed to the event, as carburetor heating was not active, potentially allowing for carburetor icing.
Safety action
Following the incident, additional maintenance was performed on SP-CLB and other AT-3 aircraft operated by the user. This included a full pneumatic and mechanical synchronization of the carburetors, inspection of the spark plugs, and cleaning of the fuel system. All aircraft passed subsequent test flights.
Furthermore, the Approved Training Organization (ATO) updated its Standard Operating Procedures (SOP) for static stall demonstrations to include:
- Mandatory activation of carburetor heat.
- Activation of the electric fuel pump at least one minute prior to the climbing phase of the demonstration.
- Increased monitoring of exhaust gas and cylinder head temperatures.
- Classification of carburetor synchronization as a "critical task" subject to additional oversight.