Name three mechanical methods by which flight control systems may be actuated.

• A. Cables, push-pull rods, and torque tubes.
• B. Pneumatic cylinders and servo motors
• C. Hydraulic pistons and electric motors
• D. Electronic actuators only

Answer: (A)

Actuation by mechanical linkages means moving a control surface solely through physical connections that transmit force from the pilot’s input, without using fluid power or electric motors. The three classic mechanical methods are cables, push-pull rods, and torque tubes.

Cables run from the cockpit controls to the control surfaces, routing through pulleys and fairleads. When you pull or release the control, the tension in the cable moves the surface. They’re lightweight and simple but require careful tensioning and regular maintenance to prevent slack, wear, or stretch affecting control feel.

Push-pull rods are rigid tubes with an inner rod that can push or pull. They provide a direct, stiff transmission over relatively short distances, giving precise and predictable movement with minimal flex. They’re well-suited where crisp control response is needed and the geometry allows a straight path.

Torque tubes are shafts that rotate to carry the pilot’s input along a wing or tail axis, often connected to bellcranks or other linkages that convert rotation into the desired surface deflection. This setup keeps motion organized over longer spans and maintains synchronized movement between surfaces.

The other options involve fluid power or electric actuation (pneumatic cylinders, hydraulic pistons, electric motors, or electronic actuators). Those are not purely mechanical linkages, so they aren’t the three mechanical methods described here.