Wiring Protection Techniques

There are many ways in protecting cables from damage which range from correct routing and bunching to adding extra protection to the cables insulation or outer sheath. We need to discuss a few as you will have to recognise what is to be used when and how. We shall begin with looming which is bundling of a group of wires to route them through the Aircraft or vehicle in an organised fashion.

The looming of aircraft wires should always be done carefully and in accordance with the Aircraft Wiring Manual. Failure to comply with this fundamental rule can have fatal consequences. for instance, if a fuel tank sensor wire was loomed with the main generator output cables and after time the loom were to chafe and expose a couple of wires on the main generator output cable and maybe just one wire strand on the fuel tank sensor wire, then there is the possibility that a high electrical charge may be passed down the sensor wire, creating a big spark inside of the fuel tank with the possibility that it might ignite the fuel vapour inside the tank and thus i need not say how catastrophic this could be.

ALWAYS PERFORM IN ACCORDANCE WITH THE MANUAL SO THAT THESE THINGS DON’T HAPPEN, IF THEY DO HAPPEN, THE AUTHORITIES WILL CHECK TO SEE IF YOU PERFORMED THE JOB CORRECTLY. REMEMBER YOU ARE AN ENGINEER SO BE PROFESSIONAL.

Now if you still wish to pursue this career then we shall continue. Wire looms are generally large in size so they are generally tied with a bundle or loom tie and then broken down into groups which are tied with a group tie. These ties used to be tied with lacing cord which in the main is being phased out and replaced with plastic cable ties no different than what you may find inside your computer or behind your car dashboard except they are approved for aircraft work. When using lacing cord or cable ties, it is important that the tie is tight enough to prevent movement down the loom but not so tight that it bites into the insulation of the wire as this may aid fraying of the insulation. Lacing cord should be tied and secured with a double knot. Cable ties are self locking for a more professional and permanent fix.

Cable looms may run for long distances through the aircraft and because of this, cable loom supports known as ‘P’ clips are used at distances stated in the aircraft maintenance manual. As a general rule, the loom should be supported so that no wire is stretched during the expansion and constriction due to the hoop stresses endured by a pressurised aircraft structure during normal flight operations. Having said this, it is not permitted that the loom may exceed more than 1/2 an inch deflection between its supports when the clamps are tightened and a moderate hand force is placed on the loom in the middle between the two clamps.

When routing looms near plumbing lines, they should always be level or above the pipeline and it is no closer than half an inch although a six inch gap is preferred where possible. If the gap is less than two inches then a sheathing resilient to the fluid carried in the pipeline should be used especially if it is oxygen or hydraulic fluid.

Obviously it is not preferred that looms are routed near moving components but sometimes it is inevitable. When this is the case then there must be mechanical guards fitted to protect the cable and a distance of at least three inches must be maintained from the components path of travel throughout its entire range of movement.

When securing cables by cable clamps or p clips, the clamp must be secured directly to the structure if it is being used to support the loom, but if it is only to maintain the spacing of the loom between plumbing lines and the loom itself, then providing that the minimum distance spacing is achieved, then a P clip around the loom may be bolted to another P clip located around the plumbing line may suffice.

The bend radius of a loom should be gradual and constant, preferably of approximately ten times the outside diameter of the loom in that area but if the bend must be Tighter then, providing it is adequately supported then a bend radius of approximately Three times the outside diameter of the loom in that area is possible but always check your aircraft standard practice manual.

Shielded or screened cables are cables that are covered in a metal braid. This metal braid should be turned back on itself at the end and secured with tinned copper wire or should be cleanly cut off without damaging the insulation or the wire underneath the braiding. If the wire to be routed is a co-axial cable then it must be routed in the most direct manner as possible. Important note; It is not permitted that an unscreened radio aerial lead be passed any closer than 18 inches to any other unscreened aircraft cable. Heat shrink wrapping of wires is a simple process of slipping over an approved piece of heat shrink of the desired length and diameter just slightly larger than the wire or wire group and heating with a WARM air gun set to the appropriate temperature for that heat shrink. Remember if it is too hot you may damage the wire itself.