📖 The lesson
Cable construction and materials
- Control cable is wound from steel wires into strands, laid around a core. It is described as strands × wires per strand: 1×19 (non-flexible), 7×7 (flexible) and 7×19 (extra-flexible, standard for primary flight controls). Common primary-control sizes are 1/8 in and larger.
- Materials: carbon steel, galvanised or tinned to prevent corrosion, or corrosion-resistant (stainless) steel. Stainless cable is slightly less strong and wears faster on pulleys. Never mix cable types or sizes in a run without approval.
- Cables carry tension only, so each surface needs a pair (or a cable plus a spring) to move both ways.
End fittings
- Swaged terminals — threaded, fork, eye, ball (single or double shank) — are pressed onto the cable with a swaging machine. The swaged shank diameter is checked with a go/no-go gauge. Swaging develops the full cable strength. Paint marks between cable and terminal show any later slippage.
- Nicopress (copper oval) sleeves are crimped with a special tool around a thimble eye. Correct number of presses and order; check with the gauge.
- Older methods: woven splices and wrap-soldered terminals — limited strength, mainly on old types.
Turnbuckles
- A turnbuckle is a barrel with a right-hand thread in one end and a left-hand thread in the other. The left-hand end is identified by a groove or knurl. Turning the barrel draws both terminals in or out.
- After adjustment, no more than three threads may be exposed at either end of the barrel.
- Safety with lockwire (single-wrap or double-wrap spiral method), or with MS21256-type locking clips pushed into the barrel slot and terminal groove. Clips are not reused.
- Cable connectors (quick-disconnect) and cable tension regulators are also found. Regulators automatically keep tension constant with temperature on large aircraft.
Pulleys, fairleads and guards
- Pulleys guide cables and change their direction; they turn on sealed ball bearings. Cable guards stop the cable jumping off the pulley.
- Fairleads (phenolic, nylon) guide a cable through a structure with a direction change of no more than about 3°; they must not be used to bend the cable.
- Pressure seals are used where cables pass through a pressure bulkhead. Rub strips and grommets protect structure.
- Pulley wear patterns show misalignment or tension faults: a worn groove side means misalignment; a flat spot on a seized pulley means a frozen bearing; a pulley too small for the cable or tension too high causes rapid wear.
Cable tension
- Tension is set to the manufacturer's value with a tensiometer (e.g. a T-5 type). Choose the riser for the cable size, read the dial and convert with the calibration card. Measure away from turnbuckles, pulleys and fittings.
- Use the temperature compensation chart: aluminium structure expands more than steel cable, so tension rises in heat and falls in cold.
- After rigging: check range of movement against the stops, correct sense of movement, safetying of all turnbuckles and fittings, and freedom over the full range.
Flexible control systems
- Bowden cables: an inner wire in a flexible outer conduit. They transmit tension (and limited compression) along curved paths — e.g. engine and cabin controls on light aircraft.
- Push-pull (Teleflex) controls: a helically wound inner member running in a conduit. They transmit push and pull, used for engine, trim and fuel controls. Avoid sharp bends; check end-fitting security and smooth operation.
Cable inspection
- Pass a cloth along the cable — broken wires snag it. Pay attention to sections passing over pulleys and through fairleads, where wear is greatest. Bend the cable slightly to expose internal breaks. Do not run bare hands along it.
- Replace a cable with broken wires beyond the limits in the manual (AC 43.13-1B tabulates limits), or any broken wire at a pulley/fairlead. Replace for severe wear, kinks, birdcaging or corrosion. Internal corrosion is found by untwisting the cable slightly.
- Surface corrosion on carbon steel cable may be removed with a coarse cloth or fibre brush and the cable re-protected (corrosion-preventive compound). Never use metallic wool or solvents that remove internal lubricant.