Cable System Components

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
• Pulleys

End-Fittings

Previously, whilst cables were ‘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 and ball end-fittings (refer to figure below) 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.

FIGURE

Turnbuckles

Turnbuckles are devices which are attached (via internal or external threads) to appropriately designed end-fittings of aircraft cables and are used to join lengths of cables and to adjust the tension of those cables. Cable runs that are too tight will make the controls stiff to operate and conversely, cables that are too slack will make the controls sloppy and unresponsive.

Turnbuckles are adjusted by the use of a ‘left-hand’ thread in one end of the turnbuckle and a ‘right-hand’ thread in the other end (refer to figure below). When the centre part of the turnbuckle is rotated, its length will increase or decrease and so it will adjust the cable tension.

The groove around one end of the turnbuckle barrel indicates the ‘left hand thread’. Once the correct tension has been obtained and confirmed (using a cable Tensiometer), the turnbuckle is checked for safety and the device is then securely locked.

The spring type of locking clip can only be inserted into the turnbuckle when the corresponding longitudinal grooves in the barrel and end fittings are aligned.

FIGURE

Cable Tensioning Devices

Although the cable tension can be correctly adjusted on the ground, which set tension may alter once the aircraft is in flight. This can be due to the large temperature differentials involved (particularly with larger aircraft and the consequences of an expanding, contracting and flexing airframe. To overcome these problems, a tension regulator is installed in some control runs.

As previously stated, engineers will use a Tensiometer to set and check the tension of a cable. The tension regulator is a device, which has springs, incorporated within the mechanism to ensure that the cable tension remains constant, regardless of the flexing and temperature changes of the airframe.

Cable Tension Regulators can be very dangerous, when disconnecting cable runs. So it is important to ensure that they are locked or snubbed in accordance with the AMM, before any work is done on the controls.

Cable Fairleads

The cables of a control run must be supported otherwise they may foul the airframe structure. They are supported by fairleads, which are usually made from fibre. These fairleads should not be lubricated as this will collect dirt and dust, which will cause extra wear on the cable and fairlead. 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.

The fairleads simply allow the cable to pass through the bulkheads without chafing. However, if the bulkhead is the divider between the pressure cabin and the outside air pressure, then the fairlead will be designed to be an airtight seal, as well as a cable guide.

Pulleys

Cables that run from the flight deck to the control surfaces require the ability to change direction. If the cable needs to change direction to another angle, the conventional method of a pulley allows this change with little friction. The example of the elevator flying control run of a simple aircraft has pulleys that can change the direction of the cable through a large range of angles.