EASA module 7 maintenance practices Control Cables.pptx

Aircraft control cablr maintenance

1. Control Cables Chapter 7

2. CONTROL CABLES 2 Cables used in aircraft control systems, comply with a number of British and American Standards and are ‘preformed’ during manufacture. Preforming is a process in which each strand is formed into the shape that it will take up in the completed cable. This makes the cable more flexible, easier to splice and less prone to kinking. Another advantage of preformed cables is that, in the event of a wire breaking, it will lie flat within its strand, so that the cable should be less likely to jam in its pulleys and fairleads. Preformed cables are manufactured from either galvanised carbon steel or corrosion-resistant steel, and are impregnated with friction reducing lubricant during manufacture. Non-preformed single strand cable may be found on some minor aircraft systems. Aircraft control cables are classified by either, the minimum breaking load or nominal diameter. It is very rare for a cable to be manufactured by an operator. They are normally ordered through the aircraft’s IPC, and the aircraft manufacturer supplies the cable fully formed with the necessary end-fittings and to the correct load factor.

3. The types of stability of an airplane 3

4. CONTROL CABLES 4

5. TYPES OF CABLES 5 The construction of the cable is determined by the number of strands it contains, and the number of wires in each strand. For example a cable designated as 7 x 19, consists of 7 strands, each containing 19 wires. The two most common forms of construction are the flexible and the extra-flexible types.

6. CABLE SYSTEM COMPONENTS 6 There are many components associated with cable systems and a selection is presented here merely for information. They include End-Fittings, Turnbuckles, Tensioning Devices, Fairleads and Pulleys.

7. End-Fittings 7 Whilst cables were previously, ‘spliced’ or ‘whipped’, to form end-fittings, the majority of modern cables have a ‘swaged splice’ end-fitting. Most end-fittings, on control cables, are special-to-type and end-fittings such as fork, threaded (internal and external), and ball end-fittings can be found in various locations. The nominal overall length of a cable will depend on the type of end-fitting which is being employed.

8. Turnbuckles 8 Cable runs that are too tight will make the controls stiff to operate, whilst cables that are too slack will make the controls sloppy and unresponsive.   Turnbuckles are devices used to adjust the tension of aircraft control cables and consists of two mating internal and external ‘v’ shaped threads. The adjustment utilises a ‘left-hand’ thread in one end of the turnbuckle, and a ‘right-hand’ thread in the other end. When the centre part of the turnbuckle is rotated, its length will increase or decrease, and so will adjust the cable tension.

9. Cable Tensioning Devices 9 Although the cable tension can be correctly adjusted on the ground, this tension may alter once the aircraft is in flight. This can be due to the large temperature differentials involved, with aircraft that fly at high altitudes and the consequences of an expanding, contracting and flexing airframe. To overcome these problems a tension regulator is installed in some control runs.   The tension regulator is a device which has springs, incorporated within the mechanism, to ensure that the cable tension remains constant, regardless of changes to the airframe.

10. Cable Fairleads 10

11. Cable Fairleads 11 The cables of a control run must be supported to reduce the possibility of fouling, vibration and fluctuations. They are supported by fairleads, which are usually made from fibre materials. These fairleads should not be lubricated as this will collect dirt and dust, which will cause extra wear on the cable and fairlead.   These fairleads allow the cable to pass through the bulkheads without chafing. If the bulkhead is the dividing partition between the pressure cabin and the outside air pressure, then the fairlead will be designed to maintain an airtight seal, as well as a cable guide.

12. Pulleys 12

13. Pulleys 13 Cables that run for long distances inside an aircraft, will need to change direction to allow for the complicated structure. The usual method is by means of a pulley, as this allows the change of direction with little increase in friction.   Aircraft pulley’s are usually made from resin impregnated fibres with a sealed bearing fitted in the centre.   A typical example would be a flying control system, which has pulleys that change the direction of the cable through various of angles. Where a change in direction of the cable is required, a pulley is normally used, due to its low friction in comparison with fairleads. Guards are fitted to pulleys when the risk of the cable riding off the pulley is high.

14. FLEXIBLE CONTROL SYSTEMS 14 Normal aircraft cables are only capable of performing a pulling action, due to their lack of rigidity. Where a two-directional movement of push and pull is required, it would be necessary either to employ the use of rods, with the increased weight penalty or to use flexible control systems. The two most common types of flexible control system are Bowden Cables and Teleflex Control Systems.

15. Bowden Cables 15

16. Bowden Cables 16 The Bowden system of control consists of a stainless steel wire, housed in a flexible sleeve or conduit . This system is intended for unidirectional operation only, with the cable being returned, on release of the control lever by a spring. The transmitting end of the cable is attached to an actuating lever whilst, at the receiving end, the cable is secured to the component to be operated. The flexible cable is made up of several strands of stainless steel wire with connecting end fittings soldered onto the end of the wire. The swaged ends are shaped to fit into the next section of the control run. The flexible conduit consists of close-coiled wire, covered with cotton braiding and a waterproof coating. For long runs or runs not requiring flexibility, the Bowden cable is fed through rigid metal tubing, which can be bent over large radius curves if required.

17. Teleflex Control Systems 17

18. Teleflex Control Systems 18 The Teleflex control system is a Bowden cable that functions in both push and pull without the need for two cables. It achieves this by wrapping a helix wire around an inner Bowden cable to form a screw thread. This flexible transmitting cable operates inside a rigid or flexible metal conduit.   The design of this helix wire, uses a left or right handed pitch and is designed to engage with gear teeth of control units and end-fittings. The main advantages are that it provides a more accurate and positive control throughout the range of movement and the controlled component can be temporarily locked in any desired position. Examples of the types of systems, operated by Teleflex controls, are engine and propeller controls, trimming controls and fuel valves. Teleflex controls can also be used to transmit movement from one place to another, such as in a mechanical Flap Position indicator or as interlocks between controls and throttles during control lock operation.