What type of cable is used in primary control systems?

The correct answer is the 7 x 19 cable, which is commonly used in primary control systems due to its flexibility and strength. This type of cable consists of 7 strands, each containing 19 wires, allowing it to provide a high degree of flexibility while still maintaining excellent tensile strength. This is particularly important in control systems where the cable needs to withstand repeated bending and movement without breaking or degrading.

In primary control systems, the requirement for reliability and durability is critical since these systems directly affect the aircraft's ability to perform safely. The 7 x 19 construction helps ensure that the cable can endure the dynamic loads placed upon it during operations.

Other cable types listed could have specific applications but do not offer the same combination of flexibility and strength as the 7 x 19 cable. For instance, 7 x 7 cable is stiffer and not as well-suited for applications requiring significant bending. Single strand wire lacks the strength and flexibility needed for control surfaces, and the 2 x 7 cable does not provide enough cross-sectional area or redundancy to meet the safety and operational requirements of primary control systems. Thus, the choice of 7 x 19 cable aligns with the performance demands of these critical systems in aviation.