Flight Controls

Flight Controls: Primary Systems, Feel, and Protection

In ATPL theory and aircraft systems training, “flight controls” primarily refer to the ailerons, elevators, and rudder. These primary flight controls govern roll, pitch, and yaw, and are connected to the pilot’s controls through mechanical linkages, cables, hydraulic power control units (PCUs), or digital fly-by-wire systems. Secondary controls (e.g., flaps, slats, spoilers, and trim devices) assist in performance and workload management, but the core handling qualities and stability depend on the primary system design, its power sources, and its protection features.

A key distinction in the question bank is between reversible and irreversible flight controls. In reversible systems, aerodynamic loads feed back to the cockpit, so pilots sense air loads naturally and do not require an artificial feel system. In irreversible systems—typical of hydraulically powered transport aircraft—the actuator isolates the pilot from aerodynamic forces. Therefore, regulations and good design practice require an artificial feel system (feel and centering units or “Q‑feel”) that runs in parallel with the powered actuator to provide realistic, speed‑related control forces. Trimming in such systems is often achieved by adjusting the “zero‑force point” of the feel system: for example, rudder trim shifts the zero‑force rudder position rather than moving a traditional trim tab. Similarly, aileron and rudder trim adjustments in irreversible systems bias the feel unit to relieve pilot workload without relying on aerodynamic feedback.

Transport-category aircraft also rely on redundant hydraulics to preserve control authority following a failure. If a hydraulic leak disables one system, the remaining independent systems take over, with multiple actuators or dual chambers on critical surfaces ensuring continued control. Depending on the type, additional protections such as Power Transfer Units (PTUs) or a Ram Air Turbine (RAT) may support pressure supply, but the core ATPL concept is that control continuity is maintained via system segregation and redundancy. To prevent structural overload and excessive side loads at high IAS, many aircraft incorporate a rudder travel limiter that progressively reduces maximum rudder deflection as airspeed increases; full travel is available at low speed for takeoff and landing, while authority is limited in cruise to protect the fin and tailplane.

Flight control safety features also include trim-runaway prevention. A common measure is a double-switch (dual‑action) logic on the elevator trim, which reduces the probability of an uncommanded trim movement by requiring two simultaneous inputs and providing cutout options. Underlying aviation regulations (e.g., EASA CS‑25/FAA 14 CFR Part 25) set requirements for control forces, stability, and failure tolerance; ATPL students should recognize how artificial feel, trim biasing, rudder limiting, and hydraulic redundancy together ensure compliance with these standards while preserving predictable handling and clear cockpit procedures.

What this question bank covers

• Definition and roles of the primary flight controls (ailerons, elevators, rudder).
• Reversible vs. irreversible control systems and the need for artificial feel in powered/isolated designs.
• Trim systems in irreversible controls, including adjustment of the zero‑force point and rudder trim biasing.
• Hydraulic redundancy and control continuity after a system loss, including typical procedures and protections.
• Rudder travel limiters and speed/Mach‑related deflection limits to protect the structure at high IAS.
• Trim runaway prevention (elevator trim double‑switch logic) and associated cockpit safeguards.
• Regulatory context within ATPL theory, touching on aviation regulations and certification-driven design features and procedures.