Primary Flight Controls

What counts as primary

A primary flight control is one used to control the aircraft's flight path about its three axes: pitch, roll and yaw. Lose one and the aircraft is not merely less comfortable, it is less controllable. The primary control surfaces are:

the elevator — pitch, hinged to the tailplane (or the whole moving stabiliser on some designs);

the ailerons, roll spoilers and flaperons — roll: ailerons at the wing's trailing edge, spoiler panels rising on the down-going wing to help them, and on some types a flaperon, a single surface doing double duty as flap and aileron;

the rudder — yaw, hinged to the fin.

The syllabus also lists the three means of moving them: manual actuation, fully powered (irreversible) actuation, and partially powered (reversible) actuation, and the whole lesson hangs on understanding those three.

Three ways to move a surface

Manual controls — the pilot's muscles do all the work: control column and pedals connected to the surfaces by cables, pulleys, push-rods and levers. The pilot feels the genuine aerodynamic hinge moment, which is honest feedback and also the limit: when the surface loads outgrow the pilot's arms, manual control has reached its ceiling.

Fully powered controls (irreversible) — a hydraulic actuator does all the work: the pilot's input moves only a control valve, the actuator drives the surface, and a feedback linkage closes the valve when the surface reaches the commanded position. Irreversibility means the path only works one way: the actuator can move the surface, but air loads on the surface cannot drive the system backwards, because the incompressible fluid locked on both sides of the actuator's piston holds the surface like a rigid strut. Gusts and buffet stop at the actuator instead of shaking the controls, and no hinge moment, however large, reaches the pilot.

Partially powered controls (reversible) — the halfway house: the actuator boosts the pilot's input, but the linkage remains mechanically continuous, so a share of the surface's real hinge moment still comes back through it to the pilot's hands.

Feel, manufactured

Irreversibility solves the strength problem and creates a new one: the pilot now feels nothing real. A fully powered control system needs an artificial feel system because moving a valve takes the same negligible effort at 150 knots as at 350, and a pilot given forceless controls has lost both the cue that speed is high and the natural protection against commanding a structure-bending deflection. Feel units push back with springs, and on the surfaces where it matters most the push is scaled by dynamic pressure (q-feel), so the controls stiffen with speed exactly as honest cables would. A partially powered system needs no feel system: its linkage is continuous and reversible, so a real share of the aerodynamic load already reaches the pilot, doing naturally what the feel unit does artificially.

Trim, when the surface is powered

Trim changes too. A manual aeroplane trims with tabs: small surfaces deflected opposite the desired movement, whose hinge moment flies the main surface to a new float position. But an irreversible surface does not float, it sits where its actuator holds it, so a tab on it would be furniture. Powered trim therefore takes two forms:

Pitch trim — a stabiliser trim: the whole horizontal stabiliser is driven to a new incidence, usually by a screw jack, carrying the steady trim load so the elevator can return to neutral with its full travel available for manoeuvring.
Rudder and aileron trim — shifting the feel unit's zero-force point: the trim selector repositions the datum at which the artificial feel pushes with no force, the linkage follows it, and the surface takes up and holds the corresponding small deflection. Nothing about the surface changed; the system was simply told that a new position is now neutral.

Inside the system

Whatever the architecture, the parts list repeats, and the syllabus asks the function of each:

Actuators — the hydraulic (or electric) muscles driving each surface, usually two or three per surface on a transport;
Control valves — the metering spools that turn an input displacement into ported hydraulic flow: the actual recipient of the pilot's effort in a powered system;
Cables — the classic transmission from cockpit to actuator or surface: light, flexible around corners via pulleys, needing tension maintenance;
Electrical wiring — the transmission's modern form, carrying command signals where cables once ran;
Control surface position sensors — transducers reporting where each surface actually is, feeding indication, warnings and the computers of the coming lessons.

Redundancy and the jam

Redundancy in a large transport's primary controls is layered: each surface is driven by two or three actuators fed from different hydraulic systems (the hydraulics module's independence doing its work), critical axes carry multiple surfaces (left and right elevators, ailerons plus roll spoilers, sometimes split rudder panels), and the mechanical or electrical runs to them are separated and duplicated. No single hydraulic failure, actuator failure or severed run takes an axis.

The danger of control jamming is that a mechanical system's strength becomes its trap: a seized bearing, a foreign object or ice can fix a run solid, and the axis dies with it unless the design can amputate. The means of retaining control are breakout and split: the captain's and first officer's controls are joined through a breakout device that a determined force overrides, separating the two sides so the unjammed side flies its own surfaces; split surfaces on separate runs mean a jammed left elevator still leaves a live right one; and trim, differential engine thrust and the autopilot's independent servos remain as coarser hands on the same axes.

Gust locks and rudder limits

Two more protections close the lesson. On the ground, gusts can slam unpowered surfaces against their stops, so controls are locked: internally, by gust locks pinning the runs or surfaces, or externally, by fitted clamps. And because taking off with locks engaged is likely to be unsurvivable, the locks carry warnings:

internal lock levers — interlocked with the thrust levers or triggering the configuration warning;
external locks — wearing long red flags;
the before-take-off check of full and free controls — the human backstop.

In the air the rudder needs a different guard. The structures module showed what large or alternating rudder inputs can do to the fin, so a rudder deflection limitation (rudder limiter) reduces the available rudder as speed rises, by one of three means:

a rudder ratio changer — the gearing between pedal and surface is reduced with airspeed, so full pedal commands ever less deflection;
variable stops — the physical travel limits of the rudder (or the pedals) are wound inward as speed increases;
blow-back — the deliberately sized actuator can be overpowered by aerodynamic loads, so at high speed the airflow itself pushes the rudder back off its commanded deflection.