Module 11C.9 — B3 light aeroplanes: flight control systems, rigging and balancing (ATA 27)

Levels

For B3, 11.9(a) primary and secondary flight controls and 11.9(d) balancing and rigging are at level 2. Actuation/protection systems and fly-by-wire are not in the B3 syllabus.

11.9(a) Primary and secondary controls

Primary control circuits

Control	Typical B3 aircraft arrangement
Ailerons	Control wheel (chain and sprocket to cables) or stick → cables round pulleys → bellcrank in each wing → push rod to aileron; interconnect cable links left and right; differential movement by bellcrank geometry
Elevator / stabilator	Cables or push-pull tubes to elevator horn; stabilator on a torque tube with anti-servo tab (also used for trim)
Rudder	Pedals → cables → rudder horn; return springs; nose-wheel steering linkage or tail-wheel steering springs

Components and their checks:

• Cables: 7×7 or 7×19 stainless/galvanised steel; inspect for broken wires by running a cloth along the cable (especially over pulleys and through fairleads), corrosion, flattening; replace if limits exceeded.
• Pulleys and fairleads: free turning, no flat spots, guard pins in place; fairleads deflect cables by no more than about 3°.
• Push-pull rods with rod ends: thread engagement checked through the inspection (witness) hole; jam nuts tight.
• Bellcranks, horns, torque tubes: cracks, bearing play, bolt condition.
• Stops: surface (primary) and cockpit (secondary) stops.
• Turnbuckles: no more than 3 threads exposed each end; safety wired (single/double wrap) or clip-locked.

Secondary controls

• Trim: manual trim wheel → cables → jackscrew actuator → trim tab (or stabilator anti-servo tab); spring/bungee trim in the elevator circuit on some aircraft; ground-adjustable tabs; electric trim with servo motor and clutch on some aircraft (runaway protection: split switch, disconnect, circuit breaker).
• Flaps: manual lever (cables/rods, detent positions), or electric motor driving a jackscrew/torque tube with limit switches and a position indicator; check symmetry and travel limits.
• Slots/slats: fixed slots (STOL types) or automatic slats on some designs — check freedom of movement.

Gust locks

Control-column lock pins and external surface locks; must be designed/flagged so they cannot be left fitted for flight (red "REMOVE BEFORE FLIGHT" flags; column locks that block the throttle or master switch).

Stall warning

Leading-edge reed horn (sucks air through a reed at high AoA) or vane/lift-detector switch (electrical horn/light). The switch position on the wing sets the warning margin — adjust only per manual and verify (often flight test).

Artificial feel, yaw damper, Mach trim, rudder limiter

Not normally fitted to B3 aircraft — manual controls give natural aerodynamic feel. Feel and stick-force gradients may be modified by down-springs and bob-weights in the elevator circuit.

11.9(d) Balancing and rigging

Balancing control surfaces

• Mass balance prevents flutter; it must be checked after repair, re-covering (fabric) or repainting — e.g. re-covering an elevator with fabric and dope changes its weight aft of the hinge.
• Procedure: support the surface at the hinge line on knife edges/balance fixture, measure the out-of-balance moment (often with a scale at the trailing edge or a sliding weight), compare with the manufacturer's limits, adjust balance weights as permitted.
• Check balance-weight security and corrosion.

Rigging

Step	Detail
1. Neutral	Lock controls in neutral (rig pins/jigs); surfaces aligned with fixed surfaces or set with rigging boards
2. Cable tension	Tensiometer with correct riser for the cable diameter; temperature-corrected value from the manual
3. Travel	Measure surface travel with a protractor/inclinometer or travel board (up/down, left/right) per the TCDS/AMM
4. Stops	Adjust surface stops; check clearance at cockpit (secondary) stops
5. Function	Full and free movement, no binding, correct sense of operation
6. Safety	Turnbuckles, cotter pins, lock nuts, safety wire
7. Inspection	Independent/duplicate inspection of disturbed controls before release

Airframe rigging

• Dihedral and incidence checks with boards/protractors at specified stations; strut-braced wings adjusted at the strut fork fittings.
• Wash-out setting; correcting a wing-heavy condition (wing rigging first, then fixed aileron tab within limits).
• Symmetry check after a heavy landing, ground loop or damage — measured distances from datum points; fin offset set for propeller slipstream effects.

Faults and their causes

Symptom	Likely cause
Heavy, stiff controls	Over-tensioned cables, misaligned or seized pulleys/bearings, fairlead friction
Excessive free play	Slack cables, worn rod ends/bushings
Aircraft flies wing-low hands-off	Wing incidence/wash-out, aileron rigging, fuel imbalance
Yaw in cruise	Rudder rigging, fin offset, fixed tab, airframe distortion
Reversed control response	Crossed cables — must be caught by functional and independent checks

Key fact: After re-covering or repainting a control surface, its balance must be re-checked; after any control disturbance, check sense of operation and have an independent inspection.