What happened
On May 21, 2015, an FK-14 Polaris ultralight aircraft was operating in the vicinity of the Chrcynno landing field, approximately 20 km northwest of the airfield. During the flight, the pilot noticed unusual engine vibrations occurring specifically when manipulating the throttle lever at high RPM settings. Although the pilot assessed the vibrations as not being severe, the instability was concerning enough to prompt a decision to terminate the flight and return to the home base.
The aircraft landed at Chrcynno without further incident or difficulty. Following the landing, a post-flight inspection was conducted to identify the source of the mechanical instability.
The investigation
The investigation, conducted by the user's commission, focused on the mechanical state of the engine control system. Investigators examined the throttle cables and the synchronization of the carburetors. The inspection revealed that the throttle cables were extending unevenly, which caused the carburetor throttles to open at different rates, directly resulting in the observed engine vibrations.
Further examination of the control cables identified that one of the cables had been damaged by overheating. This thermal damage was caused by the cable's routing, which placed it in too close proximity to the engine muffler.
Findings
- The primary cause of the engine vibrations was the damage to one of the throttle control cables.
- The cable failure was a result of overheating due to improper routing near the muffler.
- The uneven movement of the cables led to unsynchronized carburetor throttle openings.