What happened
On May 5, 2015, a Savannah S was conducting a flight from EPOM to Zofianów. The flight plan required the aircraft to maintain an altitude of 3,500 feet and pass through the LIMA waypoint in the EPLL airspace. Approximately 50 minutes and 40 seconds into the flight, while near the eastern boundary of TMA A, the pilot noticed a drop in engine RPM.
Upon increasing the throttle to compensate for the power loss, the pilot experienced intense vibrations from the powerplant. These vibrations were noted to occur specifically when increasing engine speed, rather than at idle.
In response to the mechanical instability, the pilot decided to perform an emergency landing in an unprepared field. The pilot notified FIS Warszawa of the intention to land and selected a field positioned with the wind at the aircraft's back. The landing was performed on soft ground, and the pilot subsequently reported to FIS that the landing was completed without injuries or damage to the aircraft.
The investigation
Following the incident, the engine cowling was removed to inspect the powerplant. The investigation established that a return spring for the throttle of the left carburetor had broken. This failure caused the throttle positions between the left and right carburetors to become misaligned, which was the source of the vibrations when the pilot attempted to increase power.
Findings
- The primary cause of the engine power fluctuations and subsequent vibrations was a broken throttle return spring in the left carburetor.