What is the smallest size of cable allowed in the primary control system of an aircraft?

• A. 1/4 inch diameter
• B. 1/2 inch diameter
• C. 1/8 inch diameter
• D. 1/16 inch diameter

Answer: (C)

Primary control cables are critical for safe flight; they must carry the forces from the pilot’s controls to the control surfaces with a built‑in safety margin. Because of this, there’s a minimum diameter that ensures enough strength and predictable performance under load, gusts, and repeated use.

One eighth inch in diameter is the smallest size that meets those strength and safety requirements for primary control cables. Cables smaller than that would have too little tensile strength and would elongate more under load, increasing the risk of control loss or sluggish response. Cables larger than that are allowed and can be used, but they aren’t the smallest option, which is what the question asks for.