Module 11A.9 — Flight controls (ATA 27): primary/secondary controls, actuation, fly-by-wire, rigging

Levels

11.9 is one of the most heavily weighted Module 11 topics. A1: level 1; B1.1: level 3 for (a) primary/secondary controls, (b) actuation and protection, (c) electrical and fly-by-wire systems, (d) balancing and rigging.

11.9(a) Primary and secondary flight controls

Group	Surfaces
Primary	Ailerons (roll), elevators/THS (pitch), rudder (yaw), roll spoilers
Secondary	Trim (tabs, trimmable stabiliser), high-lift devices (flaps, slats, Krueger flaps), speed brakes, ground spoilers

System operation — manual (mechanical) controls

• Cables (carbon steel or corrosion-resistant steel, 7×19 for primary controls), pulleys, fairleads (guide cables, max ~3° deflection), pressure seals at bulkheads, quadrants, bellcranks, push-pull rods and torque tubes.
• Turnbuckles adjust cable tension: correct safetying (safety wire or locking clips); maximum 3 threads exposed outside each end of the barrel; inspection hole check.
• Cable tension regulators compensate for thermal expansion of the aluminium structure versus steel cables on large aircraft.
• Stops: primary stops at the surface, secondary stops at the controls — correct clearance at secondary stops (inspection item).

Trim

• Trim tabs (smaller aircraft) or trimmable horizontal stabiliser (THS) driven by a ballscrew/jackscrew with electric or hydraulic motors (large jets), with no-back brake and stabiliser trim brake; manual trim wheel backup.
• Rudder and aileron trim via electric actuators changing the feel/centring position.
• Runaway trim protection: trim cut-out switches, stab trim brake.

High-lift devices

• Flaps and slats driven by a power drive unit (PDU) (hydraulic and/or electric motors), torque tubes and rotary actuators/ballscrews to each surface.
• Asymmetry protection: position pick-offs at each wing tip — on detecting asymmetry or runaway, wing-tip brakes lock the torque shaft.
• Flap load relief retracts flaps if the speed limit is exceeded; slat auto-extension (alpha lock) on some types.

Gust locks and gust lock systems

Prevent wind damage to control surfaces on the ground: internal mechanical locks in the controls (with interlock preventing throttle advance), external control surface locks/battens (with red streamers), or hydraulic actuators that act as dampers when unpowered. Must be removed before flight.

Artificial feel, yaw damper, Mach trim, rudder limiter

• Artificial feel: powered controls give no aerodynamic feedback, so a feel unit (springs, q-feel using pitot/static pressure, or electrical) provides stick force increasing with airspeed.
• Yaw damper: gyro/IRS sensed yaw rate → rudder input to damp Dutch roll and coordinate turns.
• Mach trim: automatically applies nose-up trim at high Mach to counter Mach tuck.
• Rudder limiter / ratio changer: reduces rudder travel with speed to protect the fin.
• Stall warning systems (stick shaker) — see 11.5.

11.9(b) Actuation and protection

Active load control

Load alleviation systems move ailerons/spoilers symmetrically in gusts or manoeuvres to shift wing lift inboard, reducing wing-root bending moment (manoeuvre load alleviation, gust load alleviation).

Lift dump and speed brakes

• Speed brakes (flight spoilers used symmetrically) via speed brake lever.
• Ground spoilers / lift dumpers deploy automatically on landing or rejected take-off when armed and air/ground sensing (wheel spin-up, squat switches) and throttle position logic confirm.

Hydraulic and pneumatic actuation

• Power control units (PCUs) / servo actuators: a mechanical input moves a servo (control) valve, which ports hydraulic pressure to the actuator; the actuator's movement feeds back to null the valve (follow-up). Usually supplied by two or three independent hydraulic systems for redundancy.
• Irreversible controls (no surface load fed back to the pilot) need artificial feel; reversible (power-assisted) controls feed some load back.
• Bypass/damping mode when depressurised prevents flutter.
• Pneumatic actuation is rare for primary controls (used for some secondary devices on older types).

Stall-protection systems

Stick pusher (applies nose-down input at a set angle of attack) or FBW alpha protection; AoA vane inputs; test switches.