📖 The lesson
Primary and secondary flight controls
Chapter 1 explained what each surface does. This chapter is about the systems that move them: the primary controls — ailerons, elevators (with the trimmable stabiliser), rudder and roll spoilers — and the secondary controls — trim, flaps, slats, speed brakes and ground spoilers. Sub-module 11.9 is the most heavily examined in Module 11A (13 questions for B1.1, at level 3).
Manual (mechanical) control systems
- Cables — flexible steel or corrosion-resistant steel cables (typically 7×19 construction for primary controls) running in closed loops; one cable pulls for each direction. They pass over pulleys (with guards to stop the cable jumping off), through fairleads (guides that allow no more than a few degrees of deflection) and through pressure seals at bulkheads.
- Turnbuckles adjust cable length and tension, and must be locked with clips or safety wire.
- Cable tension regulators keep tension constant as temperature changes: the aluminium airframe expands and contracts more than the steel cables, so without regulation tension would fall in the cold and rise in the heat.
- Push-pull rods (with adjustable rod ends), torque tubes, bellcranks and quadrants/sectors change the direction of motion and transmit it to the surface horn.
- Stops — primary stops at the surface limit its travel; secondary stops at the cockpit control prevent the cables being overloaded.
- Inspection: run a cloth along cables to find broken wires (never a bare hand), look for corrosion, wear at fairleads and pulleys, pulley alignment and freedom, and correct tension. Cables with broken wires beyond limits are replaced, not repaired.
Powered controls and artificial feel
- On transport aircraft the air loads are too great for the pilot, so hydraulic power control units (PCUs) move the surfaces. The pilot's linkage (or, in fly-by-wire, an electrical signal) moves the servo valve; the feedback (follow-up) linkage closes it again when the ram reaches the commanded position.
- The PCU is irreversible: air loads on the surface are not felt at the controls. Important surfaces have two or three PCUs supplied by different hydraulic systems, and each PCU can be bypassed or put into a damping mode if it or its system fails, so the others can still move the surface (and flutter is prevented).
- Some aircraft retain manual reversion: with all hydraulics lost, the pilot moves the surfaces (or their servo tabs) directly through the cables — heavy but possible.
- Artificial feel gives the pilot forces that increase with deflection and speed:
- Spring feel units — springs and cams that centre the control and resist deflection (aileron and rudder feel-and-centring units).
- Q-feel — the elevator feel computer uses dynamic pressure (q, from pitot and static pressure) to vary hydraulic pressure on a feel actuator, so the stick force rises with airspeed, as it would on an unpowered aircraft. Stabiliser position may also be used as an input.
Trim
- Light and unpowered aircraft use trim tabs (Chapter 1) driven by cables or electric actuators from a trim wheel or switch.
- Powered aileron and rudder trim moves the neutral point of the feel-and-centring unit with a trim actuator: the surface and the cockpit control take up a new zero-force position. No tab is needed because the PCU holds the surface.
- Pitch trim uses the trimmable horizontal stabiliser (Chapter 5) — driven by electric or hydraulic motors through a gearbox to a ball-screw jack, controlled from switches on the control wheels, the autopilot, Mach trim and speed-trim functions, and the manual trim wheels (cable-driven on many Boeings). A no-back brake stops air loads back-driving the jack.
- Protection: stabiliser trim cut-out switches isolate a runaway; trim brakes stop movement if the column is moved against the trim; a take-off configuration warning sounds if the stabiliser is outside the green take-off band when thrust is set.
High-lift devices: actuation and protection
- Flaps (and often slats) are driven by a central power drive unit (PDU) — hydraulic motors (sometimes with an electric alternate motor) driving through a differential gearbox with brakes — and torque shafts running along each wing to rotary or ball-screw actuators at every track. A single drive keeps both wings together mechanically.
- A flap/slat control computer (for example SFCC or FSEU) compares the selected position with feedback from position pick-off units. At the outboard ends, asymmetry pick-offs detect any difference between the wings, a runaway or overspeed, and the computer then applies wing-tip brakes that lock the torque shafts. A locked system cannot be reset in flight on many types.
- Flap load relief retracts the flaps automatically to a smaller setting if the airspeed exceeds the placard speed for the selected flap, and re-extends them when it falls.
- Some aircraft move individual flaps or slats with independent hydraulic actuators; Krueger flaps are usually actuated hydraulically. An alternate (electric) drive allows extension if the main hydraulic supply fails.
Spoilers, lift dumpers and speed brakes
- Each spoiler panel has its own hydraulic actuator; a spoiler mixer (mechanical or in the computers) combines roll commands from the control wheel with the speed-brake lever position.
- The speed-brake lever has an ARMED position: at touchdown (main-gear wheel spin-up or compression, with radio-altimeter and thrust-lever logic) all spoilers extend automatically as ground spoilers / lift dumpers; they also deploy during a rejected take-off and retract if the thrust levers are advanced for a go-around.
- In flight, spoilers may be inhibited in some configurations (for example with flaps fully extended on some types) and some panels are locked out at high speed.
Gust locks and gust-lock systems
On the ground, wind can slam control surfaces against their stops, damaging hinges, linkages and actuators. Light aircraft use external locks (padded clamps or battens fitted over the surfaces) or internal control locks (a pin through the control column). Larger unpowered or partly powered aircraft have gust-lock systems operated from a cockpit lever, which lock the surfaces mechanically. To prevent a take-off with the locks engaged, the lock lever usually restricts throttle movement to a low power setting, and external locks carry red streamers. Aircraft with irreversible hydraulic PCUs need no separate gust lock: trapped fluid and the actuators' damping hold the surfaces, sometimes assisted by gust dampers.
Stability augmentation and protection
| Function | Problem it solves | How it works |
|---|---|---|
| Yaw damper | Dutch roll — the coupled yawing/rolling oscillation of swept-wing aircraft — and turn coordination | A yaw-rate sensor (gyro or IRS) and a filter tuned to the Dutch-roll frequency command small rudder movements through a series actuator, usually without moving the pedals; the filter "washes out" steady turn rates so the damper does not fight a turn |
| Mach trim | Mach tuck — the nose-down pitch as the centre of pressure moves aft in the transonic range | Automatically applies nose-up elevator or stabiliser as a function of Mach number, keeping a stable stick-force gradient |
| Rudder limiter | Overloading the fin at high speed | Reduces rudder travel (variable stop, ratio changer or pressure limiter) as airspeed rises (Chapter 1) |
| Stall warning | Approaching the stall unnoticed | Angle-of-attack sensing drives the stick shaker and aural warning (Chapter 7) |
| Stall protection | Entering the stall — especially a deep stall on T-tail aircraft | A stick pusher pushes the column forward at a set angle of attack; fly-by-wire aircraft use angle-of-attack protection and, on Airbus, alpha floor (automatic take-off/go-around thrust) |
| Active load control | Wing bending loads in manoeuvres and gusts | Manoeuvre and gust load alleviation: accelerometers trigger fast, symmetric upward deflection of the outboard ailerons and spoilers, moving lift inboard and reducing the wing-root bending moment — allowing a lighter wing |
Electrical and fly-by-wire systems
- In a fly-by-wire (FBW) system the pilot's sidestick or control wheel drives position transducers (RVDTs or LVDTs); flight-control computers apply the control laws and send electrical commands to electro-hydraulic servo valves on the actuators, which report their positions back.
- Redundancy: several computers, often of two different designs with different software (to avoid common faults), multiple power supplies and hydraulic systems, and cross-monitoring that disconnects a faulty channel. The A320, for example, uses ELAC, SEC and FAC computers; the B777 uses three primary flight computers and four actuator control electronics units.
- Control laws: in normal law the computers provide stability, turn coordination and flight-envelope protection (pitch attitude, bank angle, load factor, high angle of attack and overspeed). With failures the system degrades to alternate (secondary) law with fewer protections, and finally direct law, where surface deflection is proportional to stick deflection.
- Mechanical back-up: some types keep cable control of the rudder and stabiliser trim (A320) or of selected spoilers and the stabiliser (B777) to cope with a total electrical failure.
- Benefits: less weight, envelope protection, built-in stability augmentation, common type ratings across a family and extensive built-in test. More-electric aircraft (A380, B787) add electro-hydrostatic (EHA) and electro-mechanical (EMA) actuators powered by the electrical system.
- Electrical systems on conventional aircraft also drive trim actuators, flap motors (electric alternate drives), spoiler and autopilot servos.
- Pneumatic actuation is used for a few functions on some aircraft, but hydraulic power dominates flight controls (Chapter 14).
Balancing and rigging
Rigging sets the control system so that each surface has the correct neutral, range of travel, cable tension, stops and sense of movement. A typical sequence from the AMM:
- Lock the cockpit control, bellcranks and quadrants at neutral with rig pins; check that every pin slides in freely.
- Set cable tension with the turnbuckles, using a tensiometer with the correct riser for the cable size and correcting the figure for the ambient temperature; lock the turnbuckles (no more than three threads exposed).
- Adjust push-pull rods so the surface sits at neutral, checked with rigging boards/templates, a protractor or an inclinometer.
- Remove the rig pins and set the stops: primary stops at the surface first, then the secondary stops at the cockpit control with the specified clearance.
- Measure the travel in each direction against the AMM limits, then check for full and free movement and the correct sense (e.g. wheel right → right aileron up).
- Check tab travel and free play, and the static balance of any surface that has been repaired or painted (Chapter 5).
- Have the work independently inspected (duplicate inspection) before certification — flight control maintenance is a critical task where errors have caused accidents; the second person checks the whole disturbed system, including locking and sense of movement.
The classic error
Crossed cables give the wrong sense of movement — the surface moves the opposite way to the input. It has caused fatal accidents, and it is exactly what the "correct sense" check in the independent inspection is there to catch. Always watch the surface while someone else moves the control.