If control cables are adjusted properly but control surfaces vibrate, what might be the probable cause?

The correct choice is related to the condition of the attachment fittings used for the control surfaces. Worn attachment fittings can lead to excessive play or movement in the connections between the control surfaces and the control systems. This added movement can create vibrations when the control surfaces are in motion, especially at certain airspeeds or under specific flight conditions.

When fittings are not in optimal condition, they may not hold the control surface securely in place. This can cause a lack of rigidity and stability, allowing vibrations to occur as the control surfaces are subjected to aerodynamic forces. Therefore, even with properly adjusted control cables, the overall effectiveness and stability of the control mechanism can be compromised if the attachment fittings are worn.

Other factors, like loose cables or incorrect routing, tend to affect the tension or efficiency of the control system directly, while damaged control surfaces would more likely lead to symptoms such as flutter or control response issues rather than simply vibrations.