5.4 Flight Control Systems, Rigging, and Travel

Primary, Secondary, and Auxiliary Controls

Flight controls are grouped by what they do:

The three primary surfaces move the aircraft about its three axes; remember the matchup of axis to motion. Trim tabs (and balance, servo, anti-servo tabs) reduce pilot effort. Flaps (plain, split, slotted, Fowler) increase lift and drag for takeoff/landing; slats/slots energize airflow at high angle of attack; spoilers dump lift and can assist roll. Differential and Frise ailerons reduce adverse yaw — knowing the type explains the rigging and the inspection focus.

The Mechanics of a Control Run

Controls are moved by cable systems or push-pull (torque) tube systems (and on transports, hydraulic actuators). A cable run includes:

• Control cables — usually 7×19 (flexible, over pulleys) or 7×7 (less flexible) corrosion-resistant or carbon steel wire rope.
• Turnbuckles — adjust cable length/tension; have a barrel with right- and left-hand threads.
• Pulleys and fairleads — change cable direction and support it; check for worn flat spots, frozen bearings, and cable rubbing.
• Bellcranks, quadrants, and push-pull rods — convert and transmit motion.

Inspect a control run as a system: frayed/corroded cable (run a cloth along it to snag broken wires — replace per the broken-wire-per-inch limit), worn pulleys, loose rod-end bearings (check rod-end thread engagement via the inspection hole — a wire that won't pass means enough threads are engaged), and proper safetying. Cable wear, a seized pulley, or a loose rod end can cause binding, lost motion, or jamming that only shows up under full deflection and load.

Setting Cable Tension

Cable tension is measured with a tensiometer fitted with the correct riser for the cable size; the dial reading is converted to pounds using the instrument's calibration chart. Because steel cable expands and contracts with temperature, tension must be set using a temperature-correction (rigging) chart — you read ambient temperature and cable size to find the target tension, so a system rigged on a cold morning is not over-tight at noon.

Procedure for tensioning: support/neutralize the surface, adjust the turnbuckles to bring tension to the charted value at the given temperature, keep the two cables of a pair balanced, then safety the turnbuckles (clip type or double-wrap wire). A turnbuckle is in tolerance when no more than three threads are exposed outside the barrel on either end — more than that means the cable assembly is the wrong length. Over-tension causes drag, pulley/bearing wear, and high control forces; under-tension causes slack, lost motion, and cable jumping a pulley.

Rigging and Verifying Travel

Rigging sets each surface to its correct neutral position and travel:

1. Place flight controls at neutral/streamline using the manufacturer's rig pins, blocks, or index marks in the cockpit and at the surface.
2. Adjust push-pull rods/turnbuckles so neutral lines up at both ends simultaneously.
3. Set control stops so the surface reaches — but does not exceed — the specified degrees of travel up/down or left/right.
4. Measure travel with a universal propeller/control protractor or a rig fixture, comparing against the maintenance manual's degree limits.
5. Confirm cable tension at temperature and recheck for full, free, correct movement.

The single most important verification is direction of movement: with the stick moved right, the right aileron goes up and the left aileron goes down; pulling back raises the elevator (pitch up); right rudder pedal moves the rudder right. A reversed control — easy to create by crossing cables or installing a pushrod backward — is catastrophic, so direction is checked independently by a second person or against the manual before flight. After any control-system work, a required operational check confirms full travel, free movement, correct direction, and no binding through the entire range — never just at neutral.

Cable Inspection Limits and Troubleshooting

Control cables are a graded inspection item with hard limits. Run a cloth slowly along the cable to snag broken wires; a cable is rejected when broken wires exceed the allowable count (commonly stated as a maximum number per a given length, such as per inch), and it is automatically rejected at critical fatigue and fairlead points where wear concentrates. Look for corrosion (internal corrosion is hidden — twist the cable open gently at suspect spots), kinks, bird-caging, flat spots, and wear where the cable rides pulleys.

Pulleys must turn freely (a flat-spotted or frozen pulley wears cable fast and signals a seized bearing), and the cable must ride in the groove — a misaligned pulley or worn fairlead lets the cable saw against structure.

Troubleshooting a control complaint follows the system. A stiff or binding control points to over-tension, a seized pulley/bellcrank bearing, a dragging fairlead, or interference; excess play or sloppy feel points to under-tension, worn rod ends, or stretched cable. A control that reaches a stop early or late is a rigging or stop-adjustment problem, not a strength problem.

Because each fault has a different cause, the technician confirms tension at temperature, free pulley rotation, secure stops, and correct travel and direction together. A worked check: if measured aileron travel is short of the manual's degrees, verify the surface truly reaches the control stop (not a cable hitting a fairlead), confirm rig-pin alignment at neutral, and only then re-adjust the pushrod or stop — never force more travel by loosening a stop beyond its limit, which would let the surface overtravel and load the structure abnormally.

Reassembling, re-rigging, and re-checking as a system is the safe habit.