Aircraft Control Cables

• Flexible control cables are the traditional method, now accompanied by digital flight control technology, to move aircraft flight control surfaces.
• Carbon steel and stainless steel are the two most common materials to construct aircraft control cables.
• Stainless steel cable provides longer life in harsh environments where corrosion may be a problem.
• Three types of steel cable are used for aircraft control systems: Non-flexible, flexible, and extra-flexible

Non-flexible cable types include:

• 1 × 7 cable, made of one strand comprised of seven wires
• 1 × 19 cable, made of one strand comprised of 19 wires
• Non-flexible cable may be used only for straight runs where the cable does not pass over any pulleys

Flexible Cable

• 7 × 7 type cable, made of seven strands, each with seven individual wires
• Can be used for straight runs or where pulleys are large.

Extra-flexible Cable

• 7 × 19 type cable, made of seven strands, each comprised of 19 individual wires
• Used where cables change direction over relatively small-diameter pulleys.
• Most commonly used in aircraft flight controls and trim systems.
• Aircraft cable has pre-formed wires and they will not spring out when cut.
• Aircraft control cables should be cut using a parrot-beak wire cutter or a sharp, cold chisel on a hardwood block, to preserve cable shape.

Woven splice

• Control cables were originally terminated with a hand-woven splice, using a marlin spike to separate the strands, and using the Army-Navy five-tuck splice or the Roebling roll.
• Achieved only 75% of the cable strength.

Nicopress Process

• Involves compressing copper Nicopress sleeves into a cable utilizing a special crimping tool, producing 100% of the cable strength.
• After all three compressions are made, they involve checking with a special Nicopress gauge.
• When two sleeves are used correctly, the splice is approved as having the full cable strength.
• A band of paint is a safety feature applied to Nicopress sleeves, when broken indicates cable slippage and further inspection is required.
• Place a spot of paint on the cable against the sleeve, then connect the cable to the test apparatus and apply the recommend tension.
• If a gap appears between the cable and sleeve, the cable has slipped and must be rejected.

Swaged Terminals

• Used in the vast majority of aircraft control cable terminations.
• Cut the cable to the proper length and bend it slightly so that it will not slide out of the terminal during the swaging operation.
• Slip the cable into the terminal so it bottoms at the end of the hole.
• Certified operators utilize proper equipment to compress the sleeve onto the cable, with a before-and-after gauge ensures proper compression.
• Complete the swaging and certification by applying a ring of brightly colored paint around the end of the terminal and cable to indicate any subsequent slippage.
• Popular swage terminals include fork end, clevis end, turnbuckle end, and balls with either a single or double shank.

Proof-Load Test

• Load the cable to 60% of its breaking strength after the terminals have been correctly installed, checked, and marked with paint.
• Apply the load gradually and hold for at least 3 minutes.

Cable Inspection Practices

• Cables should be carefully examined at each scheduled inspection, as they are a vital part of an aircraft control system.
• Cables usually wear or break where they pass over pulleys or through fairleads.
• Wear and corrosion are the two most common problems with control cables.
• Hold a clean, dry rag in hand and wipe every portion of the cable within reach, to inspect for broken wires or strands.
• Move the controls through their full range and check the cable as near the pulleys as you can reach.
• Potential corrosion requires disconnecting of the cable and bending it into a loop after bending it into its natural lay.
• FAA publication, AC 43.13-1B, paragraph 7-149 Cable System Inspection, recommends the procedure for inspecting aircraft control cable.

Key inspection definitions:

• A critical fatigue area is defined as the working length of a cable where the cable runs over, under or around a pulley, sleeve or fairlead, or any section where the cable is flexed, rubbed or worked in any manner, or any point within 1 foot of a swaged-on fitting.
• When a broken wire of this (hairline crack) type is suspected, further inspection with a flashlight and a 7-power magnifying glass or greater is recommended.
• Any cable assembly that has one broken wire strand located in a critical area must be replaced.

Other Key Considerations

• Wipe cables with LPS-3, Boeshield T-9 or Dinitrol ML corrosion-preventative products.
• Replace flexible and non-flexible cables when the individual wires in each strand appear to blend together (outer wires worn 40%-50%).
• Replace control cables when they become worn, distorted, corroded or otherwise damaged.
• Critical importance to ensure that correct materials and tools are used, and that the tools are in good condition and adjusted correctly when building replacement control cables.
• When using Nicopress or similar sleeves, ensure the sleeve size is correct, the die is correct and adjusted correctly, that the sleeve has been compressed correctly using the appropriate go/no-go gauge, and proof-load the cable to 60% of the minimum breaking strength of the cable.
• Aircraft manufacturers use fairleads to route the control cables where contact with the structure is likely and use pulleys where a change of direction is required.
• Control cable pulleys are installed to keep the cable run correctly aligned within the aircraft structure.
• Alignment with the cable, pulley groove condition, and bearing lubrication are key points for pulleys.
• Aircraft Type Certificate Data Sheet (TCDS) specifies the required control surface deflection, while the AMM describes the procedure to set this required deflection at a specific cable tension.
• Control cable rigging requires a Duplicate or Independent Inspection at the completion of the cable tensioning procedure.
• Excessive cable tightness can cause strain on the cable system and the aircraft structure because as the day warms up.
• Most aircraft use rig pins to lock the control system in place when rigging the controls.
• Tension is checked with a cable tensiometer placed in a clear area of each section of cable, equipped with a series of risers which correspond to typical cable diameters.
• Large transport aircraft utilize a cable tension compensator assembly, which compensates for expansion and contraction of the airframe and regulates control cable tension accordingly.
• When the primary stop is contacted, there should be at least a 1/8-in. gap at the secondary stop.
• There must be no more than three threads visible after the cable tension has been adjusted with turnbuckles, and they must never be lubricated.
• Most of the primary control cables are at least 1/8-in. diameter and are locked with single-wrapped 0.040 stainless steel wire or double-wrapped 0.040 brass wire, with at least four wraps around the shank of the turnbuckle.
• Many modern aircraft employ clip-type locking devices which install in a groove in the cable terminal end and turnbuckle body, with the spring end clipped through the hole in the barrel.
• Bowden cables are flexible cables that run inside outer sleeves and are designed to pull only.
• Teleflex® cables are designed to both pull and push.