Secondary Flight Controls

What counts as secondary

A secondary flight control changes the aircraft's configuration, performance or trim rather than steering its flight path moment to moment: the aircraft remains controllable without it, though flying may become harder work. The secondary flight control surfaces are:

lift-augmentation devices — the trailing-edge flaps and leading-edge slats that re-camber and enlarge the wing for take-off and landing;

speed brakes — surfaces raised symmetrically in flight to add drag (and, when they are wing spoiler panels, to cut lift) when the aircraft must go down or slow down;

flight and ground spoilers — the wing's upper-surface panels, and the same physical panels the previous lesson listed as roll spoilers: the classification follows the function, not the panel. Rising asymmetrically for roll they are primary; rising symmetrically as speed brakes in flight, or all together on the ground to kill lift and plant the weight on the wheels for braking, they are secondary;

trimming devices — trim tabs on manual aircraft, and the trimmable horizontal stabiliser (THS) of the previous lesson's powered designs.

How they are driven

Secondary surfaces are driven by many actuation methods, from more than one power source:

flaps and slats — the heavy sets, usually moved by central power drive units (hydraulic motors, often one per hydraulic system on a shared gearbox) turning torque shafts that run along the wing to screw jacks or rotary actuators at each surface panel: one drive, mechanically synchronised surfaces;

spoiler panels — individual hydraulic actuators;

trim — electric motors or its own hydraulic circuit;

electric standby motors — on many types, backing up the flap and slat drives at reduced speed when hydraulics fail.

The screw jack in that chain carries its own subtlety. Where hydraulic motors drive a screw jack, a mechanical lock (a no-back brake) is fitted: when the motor stops, the lock prevents the air loads on the surface from back-driving the jack and winding the flaps in or out on their own. The motor moves the surface; the lock owns every position in between and after; the drive is made irreversible mechanically, just as the previous lesson's actuators were made irreversible hydraulically.

Speed limits

Every secondary surface carries limiting flight speeds, because each extended device and its mechanism are sized for a limited dynamic pressure: flap and slat settings each have a maximum speed (the VFE ladder of the flap placard), the landing gear had its VLO and VLE, and even the speed brakes have limits and load reliefs on some types. The structural logic is the familiar one: an extended flap at excessive speed feels loads its tracks, jacks and skin were never certified for.

Three protections

Load relief (auto-retraction) — if speed exceeds the limit for the selected flap setting, a load-limiting system retracts the flaps one step (or lets them blow back against a relief arrangement) and re-extends them when the speed is back inside the limit: the system trades configuration for structure, automatically.
Flap/slat asymmetry protection — the nightmare case is a torque shaft failure with one wing's panels driving on while the other's stand still: lift and drag then differ wing to wing, and the rolling moment can exceed the ailerons' authority. The protection compares position sensors across the two sides and, at the first disagreement, applies brakes that freeze the whole drive. The implication is accepted deliberately: the flaps are then stuck, symmetrically, at whatever setting they had reached, and the crew lands at that configuration with the appropriate speed corrections, because a stuck-but-symmetric wing is flyable and an asymmetric one may not be.
Auto-slat — on some aircraft the slats extend further automatically when the aircraft approaches the stall: at high angle of attack the system drives the slats from their sealed take-off position to the fully open gapped position, buying stall margin exactly when it is needed. It is a stall-protection device wearing a secondary control's clothes.

Blow-back

Control surface blow-back is the general concept behind several of these systems: aerodynamic forces overruling hydraulic ones. An actuator holds its surface with finite authority, and where the design sizes that authority deliberately low, a high enough dynamic pressure pushes the surface back from its commanded position: spoiler panels partially blown down at high speed, flaps easing back against their relief units, and the previous lesson's blow-back rudder limiter. Blow-back is the deliberate exception to the irreversibility rule: authority sized down on purpose, not a strut that failed. Used this way it is a structural protection that needs no sensors: the air itself measures the dynamic pressure and takes back what the structure cannot afford to give.

What counts as secondary

How they are driven

Check you understood

Speed limits

Three protections

Blow-back

What the exam asks