Module 11B.9 — Piston aeroplanes: flight controls, trim, flaps, rigging and balancing (ATA 27)

Levels and scope

For 11B only 11.9(a) primary and secondary flight controls and 11.9(d) balancing and rigging apply: A2 level 1, B1.2 level 3. Actuation and protection (active load control, hydraulic actuation, stall protection) and fly-by-wire are turbine-only (11A). Because B1.2 is at level 3, rigging procedures and fault diagnosis are examined in detail.

11.9(a) Primary and secondary controls

Primary controls

Surface	Typical light-aircraft mechanism
Ailerons	Control wheel/stick → cables via pulleys in the fuselage and wing → bellcrank → push-pull rod to the aileron; differential travel by bellcrank geometry
Elevator / stabilator	Cables or push-pull rods to an elevator horn or stabilator torque tube; stabilator with anti-servo tab
Rudder	Rudder pedals → cables → rudder horn; pedals often interconnected with nose-wheel steering (springs/bungees)
Spoilers	Rare on piston aircraft (some high-performance singles use spoilers for roll)

Mechanical components:

• Cables: flexible stainless or carbon steel (7×7 or 7×19 construction), swaged terminals; inspect for broken wires by wiping with a cloth (never bare hand) especially at pulleys and fairleads, corrosion, wear.
• Pulleys (phenolic/aluminium) with guards to keep the cable on; fairleads (deflection max about 3°); pressure seals where cables pass through pressure bulkheads.
• Bellcranks, quadrants, torque tubes, push-pull rods with rod ends (check thread engagement via witness/inspection hole), control stops (primary at surface, secondary at control column).
• Turnbuckles: barrel with right- and left-hand threads; no more than 3 threads exposed at either end; safetied with safety wire (single- or double-wrap) or clip-locking devices.

Secondary controls

• Trim: cable-operated trim tab (trim wheel → cable → jackscrew/actuator at tab) or electric trim (motor with clutch, split switch to prevent runaway, trim disconnect/circuit breaker); ground-adjustable tabs; bungee/spring trim (spring bias on the control system).
• Flaps: manual (lever with detents, cables/rods), electric (motor driving jackscrew/torque tube, limit switches, position indicator/transmitter) or hydraulic (on some twins). Check symmetrical travel and stop at limits; flap interconnect cables/rods keep both sides together.
• High-lift devices: slotted/Fowler flaps, fixed slots on some STOL types.

Gust locks

Control locks (pin through the control column, external surface locks/battens) protect surfaces from wind damage when parked. An internal control lock must be designed so it cannot be left engaged unnoticed (e.g. covers the starter/master switch or blocks the throttle).

Artificial feel, yaw damper, Mach trim, rudder limiter

Generally not fitted to light piston aircraft (manual controls give natural feel); some higher-performance aircraft have a yaw damper with the autopilot. Down-springs and bob-weights in the elevator circuit modify stick-force gradients on some types.

Stall-warning systems

Reed-type pneumatic horn or electric vane switch on the leading edge (see 11B.5); adjustment of the switch position affects the warning speed — set per AMM and flight test.

11.9(d) Balancing and rigging (B1.2 level 3)

Control surface balancing

• Surfaces must be within static balance limits to prevent flutter.
• After repair or repainting (paint adds weight aft of the hinge), perform a balance check: remove the surface, support it on knife-edges at the hinge line (or use the manufacturer's balance fixture), measure the trailing-edge-heavy moment and compare with the limits; adjust balance weights only as permitted.
• Mass balance weights (lead) must be securely attached — loose weights are a flutter hazard.

Rigging procedure (typical)

1. Lock controls in neutral using rig pins or a control-lock fixture; set surfaces to neutral with rigging boards/templates.
2. Set cable tension using a tensiometer (correct riser for the cable size), corrected for ambient temperature from the chart.
3. Adjust push-pull rods (rod ends) for neutral, maintaining minimum thread engagement.
4. Set travel with a protractor (bubble/digital inclinometer) or travel boards: up/down or left/right degrees per the TCDS/AMM; adjust surface stops.
5. Check secondary stops at the controls — clearance per AMM so that primary stops are reached first.
6. Check full and free movement, friction, correct sense of operation (e.g. moving the stick right raises the right aileron).
7. Safety all turnbuckles, bolts, cotter pins, lock nuts.
8. Independent (duplicate) inspection of the disturbed control system before release.

Wing and fuselage rigging

• Dihedral checked with a dihedral board/protractor, incidence with an incidence board at specified wing stations; adjust via strut fittings (strut-braced wings), wing attach bolts eccentrics, or as per the manufacturer.
• Wash-out set on some wings via strut adjustment.
• After heavy landing or damage, symmetry check — measure from datum points to wing tips, tail tips (equal diagonal measurements).

Typical faults

Symptom	Possible cause
Aircraft flies one wing low with controls centred	Wing rigging (incidence/wash-out) or aileron rigging; can be corrected with fixed aileron tab within limits
Aircraft yaws in cruise	Rudder rigging or fixed rudder tab; fin offset
Excessive control friction	Over-tensioned cables, misaligned pulleys/fairleads, dry/seized bearings
Sloppy controls	Low cable tension, worn rod ends/bushings
Reversed sense	Cables crossed during installation — critical, independent inspection exists to catch it

Key fact: Rigging sequence — lock neutral, set cable tension (temperature-corrected), set travel and stops, check full and free movement and correct sense, safety, then independent inspection.