Wednesday, February 2, 2011

Aircraft Cable Classifications(Wires)

Listed below are the broad classifications used for aircraft cables.  Regrettably, there is little International Standardization of terminology and it should be noted that the term ‘wire’ is used in the USA where most other countries talk of ‘cable’.
Airframe Cables
Cables designated as ‘Airframe’ are intended to be sufficiently robust to satisfy the requirements of ‘Open’ airframe wiring and the general wiring of Powerplants.  However, in recent years there has been a strong trend towards very thin insulation which is harder and stiffer than insulation such as PVC.  Such ‘stiff’ cables are perfectly satisfactory if the installation is designed to accept but they may very well be quite unsuitable for an older airframe design requiring, say, flexing over hinges.  It follows that even if all the major declared characteristics such as overall diameter and temperature rating are acceptable, the apparently equivalent cables may still not be interchangeable.
There are two basic methods of applying cable insulation, namely wrapping and extruding.  These methods in themselves can produce different ‘handling’ characteristics.  In the USA, the term ‘Medium Weight-Interconnect’ may be used for Airframe Cables.



Interconnect Cables

‘Interconnect’ is a term adopted by the BSI to designate cables which may be used in protected areas of wiring such as the interconnection of equipment within racks.  Such cable would normally be installed within an assembly which would then be positioned into an aircraft.  It would not, therefore, be subject to ‘pulling through’ and other such stressful exercises.
Interconnect cables employ thinner insulation than airframe types, which saves weight and space and increases flexibility, the latter being most important where looms (bundles) are required to turn through small radii into electrical connectors.  However, all the constraints given for airframe cable also apply here.
The term ‘Hook-up’ is commonly used in the USA to designate cables of this general type, and the designation ‘Light Weight-Interconnect’ may also be applied.

Equipment ‘Wire’
 This cable, invariably known as ‘wire’, is intended to be used within equipment and, therefore, is very flexible and suitable for soldering.  It is not designed for use as interconnect wiring but design organizations do, on occasions, select a particular type for use in protected areas of an airframe.  There is a considerable range of such cables which vary in basic construction and performance and they should always be very closely defined. The term ‘Module Wire’ is sometimes used for this class of cable in the USA.

Fire Resistant Cables
This type of cable is required to retain a defined level of electrical insulation in the presence of fire for five minutes, as defined in BCAR Section S1-1  and JAR 1. 
‘Fire Resistance’ should not be confused with ‘high temperature’ and fire resistant types should only be employed where this property is required because other characteristics, such as fluid resistance, will usually be poorer than could be expected from a non-fire resistant high temperature cable.

Fireproof Cables
These cables are required to operate for fifteen minutes in a designated fire as defined in BCAR D6  and JAR 1 and are for use in designated fire zones.As for Fire Resistant types, they should only be used where necessary.

Multicore Screened and Jacketed Cables
Airframe and interconnect cables may be supplied in a multicore form of generally up to four cores, the cores being twisted together.  The multicore may be jacketed (sometimes known as sheath) or it may be screened and jacketed.  The screening is usually a braid which gives 85% surface coverage, but screening to a higher standard may be used, and on replacement of such cables, the standard must not be degraded.  The cores are coloured for easy identification.

Data Bus
Data Bus cables are designed to specific requirements which will not, as a general principle, allow for replacement by any other type other than that specified by the Design Authority for the installation.

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