Cable Systems PDF

Cable Systems Introduction to Cable Systems The traditional method of moving aircraft flight control surfaces was by use of flexible control cables. Even though many modern aircraft now utilise digital flight control technology, control cables are still the primary means of moving flight control sur...

Cable Systems Introduction to Cable Systems The traditional method of moving aircraft flight control surfaces was by use of flexible control cables. Even though many modern aircraft now utilise digital flight control technology, control cables are still the primary means of moving flight control surfaces in commuter and general aviation aircraft. The two most common materials used to construct aircraft control cables are carbon steel and stainless steel. Stainless steel cable is more expensive and has a slightly lower tensile strength, but its longer life in harsh environments makes it the better of the two materials where corrosion may be a problem, such as in agricultural aircraft and seaplanes. Control cable system operating poser actuators 2023-11-24 B-07b Maintenance Practices CASA Part 66 - Training Materials Only Page 475 of 498 Cable Construction Three types of steel cable are used for aircraft control systems: Non-flexible 1 × 7 or 1 × 19 Flexible 7 × 7 Extra-flexible 7 × 19. Non-flexible Cable It may be of the 1 × 7 or 1 × 19 types. The designation means that the 1 × 7 cable is made up of one strand comprised of seven individual wires. The 1 × 19 cable is made up of one strand comprised of 19 individual wires. Non-flexible cable may be used only for straight runs where the cable does not pass over any pulleys. Flexible Cable Flexible cable is of the 7 × 7 type and is made up of seven strands, each of which has seven individual wires. Flexible cables are used only for straight runs or where pulleys are large. Extra-flexible Cable Extra-flexible cable is of the 7 × 19 type and is made up of seven strands, each of which has 19 individual wires. It is used where cables must change direction over relatively small-diameter pulleys. Extra-flexible cable is the most commonly used in aircraft flight controls and trim systems. © Jeppesen Aircraft control cable construction 2023-11-24 B-07b Maintenance Practices CASA Part 66 - Training Materials Only Page 476 of 498 All aircraft cable is pre-formed, that is, the wires are shaped in their spiral form before the cable is wound and they will not spring out when the cable is cut, as shown in the illustration below. Pre-formed cable Cable breaking strength is shown below. 2023-11-24 B-07b Maintenance Practices CASA Part 66 - Training Materials Only Page 477 of 498 © Jeppesen Breaking strengths Aircraft control cables should be cut using either a parrot-beak wire cutter, which preserves the cable shape and does not deform it, or a sharp, cold chisel on a hardwood block. Cable Termination 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. Both of these systems were time-consuming and involved great skill to accomplish properly. They produced a termination that was certified for only 75% of the cable strength. Woven splice 2023-11-24 B-07b Maintenance Practices CASA Part 66 - Training Materials Only Page 478 of 498 Nicopress Process Copper Nicopress sleeves may be compressed into a cable using a special crimping tool which reduces the diameter of the sleeve during the press and forms an extremely strong bond. The cable is put through the sleeve and around the thimble, and its end is passed back through the sleeve so that about 1/8 in. is visible beyond the sleeve. The centre compression is made first, then the compression nearest the thimble and finally the compression at the end of the sleeve. A Nicopress termination produces 100% of the cable strength. Nicopress termination After all three compressions are made, they are checked with a special Nicopress gauge. If the compressions are made properly, the sleeve fits snugly in the slot. © Jeppesen Nicopress gauge 2023-11-24 B-07b Maintenance Practices CASA Part 66 - Training Materials Only Page 479 of 498 Running splices may be made to join two lengths of cable using two Nicopress sleeves. When two sleeves are used correctly, the splice is approved as having the full cable strength. Nicopress splice technique A safety feature to assist during inspection of Nicopress sleeves is a band of paint applied to the cable and sleeve. If the paint band is ever broken, it is highly likely that the cable has slipped and further inspection is required to determine the cause. Testing the Assembly Place a spot of paint on the cable against the sleeve. Connect the cable to the test apparatus and apply the recommended tension. If a gap appears between the cable and sleeve, the cable has slipped. The splice must be rejected. Nicopress sleeve testing 2023-11-24 B-07b Maintenance Practices CASA Part 66 - Training Materials Only Page 480 of 498 Swaged Terminals The vast majority of aircraft control cables are terminated with swaged terminals. To install one of these terminals, 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, or until it is almost flush with the end of the hole if the terminal is drilled all the way through. © Jeppesen Terminal swaging procedure Swaging is a process that requires special training and certification for aircraft cable termination. Using the proper equipment and a certified operator, compress the sleeve onto the cable. A special before-and-after gauge is used to determine that the terminal is properly compressed. © Jeppesen Before-and-after gauge 2023-11-24 B-07b Maintenance Practices CASA Part 66 - Training Materials Only Page 481 of 498 The sleeve should fit into the ‘before’ portion of the gauge before it is swaged; it should then fit snugly into the ‘after’ slot of the gauge after it has been swaged. When the swaging is completed and certified, apply a ring of brightly coloured paint around the end of the terminal and cable to indicate any subsequent slippage. A number of different swage terminals are available, the most popular being the fork end, clevis end, turnbuckle end, and balls with either a single or double shank. Swage terminal types Proof-Load Test After the cable is completed with the terminals properly installed, checked and marked with paint, it should be tested by loading it to 60% of its breaking strength. The load should be applied gradually and held for at least 3 min. 2023-11-24 B-07b Maintenance Practices CASA Part 66 - Training Materials Only Page 482 of 498 Cable Inspection Cable Inspection Practices Since cables are such a vital part of an aircraft control system, they should be carefully examined at each scheduled inspection. 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. To inspect for broken wires or strands, hold a clean, dry rag in your hand and wipe every portion of the cable within reach. The rag snags on any broken wires or strands that stick out of the cable. Move the controls through their full range and check the cable as near the pulleys as you can reach. If there is any possibility of corrosion, disconnect the cable and bend it into a loop as shown below. Cable wear check CAUTION: Be very careful not to bend the cable too sharply or try to twist it opposite its natural lay orientation, called ‘bird-caging’. Permanent damage may result. Bend the cable with a large radius so that broken wires and strands will stand out, and use a torch and magnifying glass to check for corrosion. The FAA publication, AC 43.13–1B, paragraph 7-149 Cable System Inspection, recommends the following procedure for inspecting aircraft control cable: 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. 2023-11-24 B-07b Maintenance Practices CASA Part 66 - Training Materials Only Page 483 of 498 Close inspection of these critical areas must be made by passing cloth over the areas to snag on broken wires. This will clean the cable for a visual inspection and detect broken wires if the cloth snags on the cable. A very careful visual inspection must be made since a broken wire will not always protrude or stick out but may lie in the strand and remain in the position of the helix as it was manufactured. Broken wires of this type may show up as a hairline crack in the wire. If a broken wire of this type is suspected, further inspection with a powerful 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. Both seaplanes and agricultural aircraft operate in an environment that is highly conducive to corrosion in control cables, and both of these types of aircraft utilise corrosion-resistant stainless steel cable. It is a good idea, however, to wipe these cables with LPS-3, Boeshield T-9 or Dinitrol ML. These corrosion-preventative products protect and lubricate the cables without producing any negative effects. Grumman Ag Cat seaplane 2023-11-24 B-07b Maintenance Practices CASA Part 66 - Training Materials Only Page 484 of 498 Grumman Ag Cat crop spray External wear patterns will extend along the cable equal to the distance the cable moves at that location and may occur on one side or on the entire circumference. Replace flexible and non-flexible cables when the individual wires in each strand appear to blend together (outer wires worn 40%–50%). Wire wear 2023-11-24 B-07b Maintenance Practices CASA Part 66 - Training Materials Only Page 485 of 498 As wear is taking place on the exterior surface of a cable, the same condition is taking place internally, particularly in the sections of the cable which pass over pulleys and quadrants. Worn condition is not easily detected unless the strands of the cable are separated. This type of wear is a result of the relative motion between inner wire surfaces. Under certain conditions, the rate of internal wear can be greater than that occurring on the surface. Replace control cables when they become worn, distorted, corroded or otherwise damaged. If spare cables are not available, prepare exact duplicates of the damaged cable. Use materials of the same size and quality as the original. Standard swaged cable terminals develop the full cable strength and may be substituted for the original terminals wherever practical. However, if facilities and supplies are limited and immediate corrective action is necessary, repairs may be made by using cable bushings, eye splices and the proper combination of turnbuckles in place of the original installation. 2023-11-24 B-07b Maintenance Practices CASA Part 66 - Training Materials Only Page 486 of 498 The Critical Importance of Correctly Swaging Control Cable Ends A recent investigation of a fatal aircraft accident discovered that the rudder cables had failed by pulling through the copper sleeve at the thimble ends. Whether this caused the accident has not yet been determined; however, the poor swaging is obviously of serious concern. The failures occurred because the 1/8-in. Nicopress copper sleeves had been swaged (compressed) with a 5/32-in. die onto the 1/8-in. diameter cables. This was the result of human error. Subsequent tests of such incorrectly made assemblies showed the cable would pull out of the sleeve at low load. The manufacture of replacement control cables is covered in Chapter 7 of AC 43.13-1B, paragraph 7148. Mechanically Fabricated Cable Assemblies. These cable assemblies are critical components. Special care and attention are needed to ensure that the correct materials and tools are used and that the tools are in good condition and adjusted correctly. When using Nicopress or similar sleeves, the following precautions are vital: 1. Ensure the size of the sleeve to be used is correct for the cable diameter. The sleeves are not individually marked, so do not assume the size is correct because it came from a marked packet or bin. 2. Ensure that the correct die is used and that it is adjusted correctly. 3. Ensure that the sleeve has been compressed correctly using the appropriate go/no-go gauge. 4. Proof-load the cable to 60% of the minimum breaking strength of the cable (as required by AC 43.13-1B, paragraph 7-146). Note: A description of the Nicopress process, the size information and the die adjustment is included in paragraph 7-148. The data may also be obtained from the Nicopress organisation. The same precautions and care apply when using other swaging processes. Ensure that you have appropriate manufacturer's data to achieve a reliable result. Cable assemblies approved for manufacture in situ require a representative assembly to be manufactured under similar conditions and tested before the aircraft installation is certified. 2023-11-24 B-07b Maintenance Practices CASA Part 66 - Training Materials Only Page 487 of 498 Cable Installation Practices Fairleads Aircraft manufacturers route the control cables in the most direct manner possible, and at each point where a change of direction is required, they use a pulley. At any point within a run of cable where the cable is likely to contact the structure, it is run through a fairlead made of some form of relatively soft plastic or fibre, such as Nylon® or Tufnell®. The cables may touch or rub on a fairlead, but fairleads must never be used to change the direction of a cable. © Jeppesen Fairlead types Pressure seals are used at each location where a control cable passes through the pressure bulkhead in a pressurised aeroplane. These seals are filled with grease, which allows the cable to pass freely, yet prevents the leakage of pressurisation air as illustrated below. 2023-11-24 B-07b Maintenance Practices CASA Part 66 - Training Materials Only Page 488 of 498 © Jeppesen Pressure bulkhead pressure seal 2023-11-24 B-07b Maintenance Practices CASA Part 66 - Training Materials Only Page 489 of 498 Pulleys Control cable pulleys are installed to keep the cable run correctly aligned within the aircraft structure. They allow cables to change direction. Pulleys must be carefully aligned with the cable so that the cable rides squarely in the pulley groove. Pulley Inspect pulleys for roughness, sharp edges and foreign material embedded in the grooves. Examine pulley bearings to ensure proper lubrication, smooth rotation and freedom from flat spots, dirt and paint spray. During the inspection, rotate the pulleys, which only turn through a small arc, to provide a new bearing surface for the cable. Maintain pulley alignment to prevent the cable from riding on the flanges and chafing against guards, covers or adjacent structures. Check all pulley brackets and guards for damage, alignment and security. Control pulleys must be carefully aligned with the cable so that it rides squarely in the centre of the pulley groove, and all of the pulleys must be free to rotate through their full travel. All pulleys must be equipped with guards to prevent the cable jumping out of the groove when the cable is slack. The guard pin is usually a clevis pin secured with a split pin. Ensure a washer is fitted under the split pin to prevent damage to the pulley frame or structure. 2023-11-24 B-07b Maintenance Practices CASA Part 66 - Training Materials Only Page 490 of 498 © Jeppesen Guard pin Guard pins are installed to prevent cable jumping out of the pulley groove when the cable is slack. It is good practice at each inspection to rotate any pulley that does not normally turn through its full rotation with normal cable movement so the cable will contact a different portion of the pulley groove, as shown below. © Jeppesen Pulley wear patterns 2023-11-24 B-07b Maintenance Practices CASA Part 66 - Training Materials Only Page 491 of 498 Travel Adjustment and Cable Tension The 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. Be sure to follow the AMM’s instructions completely and check each stage of the process before moving to another process. Control cable rigging requires a Duplicate or Independent Inspection at the completion of the cable tensioning procedure and when all paperwork has been completed. The Independent Inspection is an integral part of the certification stage of the task. When cables are rigged excessively tight, it creates a great deal of strain on the cable system and the aircraft structure because as the day warms up, the structure expands or ‘grows’ at a different rate to that of the control cables. Aluminium has a greater coefficient of linear expansion than carbon steel or stainless steel. In extreme cases, the fuselage structure buckles and deforms if cables are overtightened during rigging. In addition to this, control movement is made more difficult as the cable tension increases. Most aircraft use rig pins to lock the control system in place when rigging the controls. The control surfaces and major quadrants/bell cranks are locked in place with the rig pins, and the cables are tensioned in accordance with the maintenance manual. Tension is checked with a cable tensiometer, which must be placed in a clear area of each section of cable, midway from any quadrants, bell cranks or surfaces. The tensiometer is equipped with a series of risers which correspond to the cable diameter: 3/32 in., 1/8 in., 5/32 in. and 3/16 in. are the most common diameters of control cable, so four risers of corresponding size are usually adequate. The cable is placed between the two anvils and the appropriately sized riser, the trigger is closed, and the reading on the scale is noted. A pointer lock locks the reading if necessary. The reading is not the cable tension – this is an arbitrary reading for the particular riser. The tension in pounds is read from a chart which converts the reading for the riser being used. Cable tensiometer 2023-11-24 B-07b Maintenance Practices CASA Part 66 - Training Materials Only Page 492 of 498 Most large transport aircraft are fitted with a cable tension compensator assembly, which compensates for expansion and contraction of the airframe and regulates control cable tension accordingly. The diagram below shows such a device. Cable tension regulator and compensation device The device incorporates two quadrants mounted on a common fulcrum and held in place by powerful springs and an indicator scale. The aircraft control cables are fitted and locked in the two quadrants. As the aircraft expands, the cable tension increases, causing the quadrants to rotate and compress the springs. This, basically, maintains a fairly constant cable tension in response to changes in aircraft structural dimensions. A compensator device is required for each set of control cables, usually ailerons, elevator and rudder, as these are most affected by changes in structural dimension. Cable tension charts are incorporated into all maintenance manuals, as shown below. 2023-11-24 B-07b Maintenance Practices CASA Part 66 - Training Materials Only Page 493 of 498 © Jeppesen Cable tension chart - typical Control Spring-Back All aircraft flight controls have a primary stop at the control surface itself and a secondary stop on the actuating control in the cockpit. The controls should be rigged so that when the primary stop is contacted, there is at least a 1/8-in. gap at the secondary stop. When you push the actuating control to its stop, the spring-back is quite apparent and you are sure the control has reached the primary stop first. 2023-11-24 B-07b Maintenance Practices CASA Part 66 - Training Materials Only Page 494 of 498 Turnbuckle Locking After the cable tension has been adjusted with the turnbuckles, they must be checked for the proper amount of thread visible at the turnbarrel ends. There must be no more than three threads visible and the turnbuckles must never be lubricated. © Jeppesen Turnbuckle locking chart 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 as shown in the table and illustration. 2023-11-24 B-07b Maintenance Practices CASA Part 66 - Training Materials Only Page 495 of 498 © Jeppesen Acceptable methods of lock-wiring a cable turnbuckle In both single- and double-wrap methods of lock-wiring, there is the option of straight or spiral wiring. In either method, the wiring must be terminated with at least four wraps around the shank of the turnbuckle. This method of turnbuckle locking should be practised until competence is achieved as it is still used on many commuter and most general aviation aircraft. AC 43.13-1B Chapter 7, Section 10, gives detailed instructions on safetying (lock-wiring) methods for turnbuckles. Many modern aircraft use clip-type locking devices as shown below. These are relatively easy to install; they are inserted into a groove in the cable terminal end and turnbuckle body, and the spring end is clipped through the hole in the barrel. 2023-11-24 B-07b Maintenance Practices CASA Part 66 - Training Materials Only Page 496 of 498 © Jeppesen Turnbuckle locking clips Bowden and Teleflex Control Cables Bowden cables are flexible cables that run inside outer sleeves. The inner cable may be made of piano wire or standard aircraft cable (wire rope). They have hundreds of applications, from light aircraft engine hot air boxes to seat belt inertia reel controls and seat recline mechanisms in the modern transport aircraft. Bowden cables are designed to pull only. Bicycle brake cables are a common example. The cable can be pulled with a lever or knob, but a spring is required to return it to its original position. Inspections and checks should always be made for freedom of movement and smooth operation. Some types of Bowden cables are repairable, but normally they are simply replaced. Bowden cables 2023-11-24 B-07b Maintenance Practices CASA Part 66 - Training Materials Only Page 497 of 498 Teleflex® cables are designed to both pull and push. They are much stronger in construction than a Bowden cable and usually have the cable attached to a rod or shaft inside the outer sheath to give additional strength to the assembly. Teleflex cable can be moved incrementally and locked in any desired position. Uses include engine controls, flight controls and actuator controls. Teleflex® cable terminations Teleflex® cable used as a flight surface trim control 2023-11-24 B-07b Maintenance Practices CASA Part 66 - Training Materials Only Page 498 of 498