Book 5 Module 7A
CATEGORY B1 B2
ELECTRICAL CABLE MAINTENANCE
EWIS
SOLDERING
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AUTHORITY
It is IMPORTANT to note that the information in this book is for study/training purposes only.
When carrying out a procedure/work on aircraft/aircraft equipment you MUST always refer to the relevant aircraft maintenance manual or equipment
manufacturer’s handbook.
You should also follow the requirements of your national regulatory authority (the CAA in the UK) and laid down company policy as regards local procedures, recording, report writing, documentation etc.
For health and safety in the workplace you should follow the regulations/guidelines as specified by the equipment manufacturer, your company, national safety
CONTENTS
Page Aircraft electrical cables – maintenance practices 1
Storage 2 Handling 3 Installation 3 Failures 5 Stripping 6 Crimping 7 AMP crimping 7
Powered crimping machines 13
ERMA hydraulic crimping machine 13
Testing of crimped joints 14
Plugs and sockets 15
Connector pin removal/insertion 17
Co-axial cable installation 21
Fibre optic cables and connectors 24
Wiring protection techniques 25
Protective sleeving techniques 27
Shielding 29
Cable changing 31
Electrical wiring interconnection system (EWIS) 33
HOW TO TACKLE THIS BOOK
This book deals with syllabus item 7.7 (the Electrical Cables and Connectors element) and item 7.15(a) (Soldering Methods) of the EASA Part 66 syllabus.
Note that as from September 28th 2010 syllabus item 7.7 of EASA Part 66 is re-titled ‘Electrical Wiring Interconnection System (EWIS)’. The contents are essentially the same as under the old syllabus title but with the addition of EWIS installations, wire types and damage tolerance.
EWIS maintenance will be in chapter 20 of the AMM.
It should be read in conjunction with the appropriate book in module 6, which gives description and identification of aircraft electrical cables and types, and the book in this series dealing with electrical test instruments.
Most of the book is straightforward. It is recommended that you have a close look at the bonding and cable system on your aircraft, checking on the manufacturer, the type of cable and identifying the codes. Check the wiring Manual/wiring diagrams and make sure you understand the maintenance that can be carried out from visual inspections through to complete cable replacement/testing and bonding checks. Soldering of cables has mostly given way to crimping and is not often carried out on aircraft – there are safety concerns such as burns from hot electrical irons and fire risk from the same source as well as electrical supplies to the iron. It would be of benefit if you can observe any soldering processes.
The requirements for the installation and maintenance of electrical cables and aircraft bonding are laid down in British Civil Airworthiness Requirements (BCAR’s) D, K and G (old system), now in EASA CS25 (large aircraft), EASA CS27 and 29 (helicopters), EASA CS23 (small aircraft), EASA1 and CAP562 Civil Aircraft Airworthiness
Information and Procedures (CAAIPs) (CS = Certificate Specification).
It is recommended that you check on the latest amendments to the regulations in CAP562 (CAAIPs) (electrical equipment). This section deals with aircraft electrical cables and systems and can be read on the net at www.caa.co.uk.
AIRCRAFT ELECTRICAL CABLES – MAINTENANCE PRACTICES Reliability is of prime consideration for aircraft cables since the performance and safety of the aircraft and its occupants is dependant on electrically operated systems. Care, therefore, must be exercised during the manufacturer and maintenance of cables, looms and circuits and these must be fully tested after
manufacture/replacement/disturbance.
Listed below are a number of qualities that an aircraft cable should possess.
Minimum Weight and Size. Cables should be of minimum mass and size but at the same time provide maximum insulative properties for the insulation cognisant with the voltages to be carried and low resistivity coefficients for the conductors.
Resistance to Fluids. Aircraft cables should be able to withstand the effects of water, engine oils, fuels, hydraulic oils, solvents, etc.
Non-inflammability. Wiring is often in high fire risk areas such as engine nacelles, APU bays etc. Such wiring should not cause any fire to spread and for this reason the protective covering should be made of a self-extinguishing material. During flight some cables could experience a large range of temperatures and must remain flexible at all times.
Resistance to Abrasion. Aircraft cables must be resistant to abrasion that might be induced by vibration. Cables should be strong enough to support their own weight and easily workable (have high ductility and be malleable).
Electrical Requirements. The conductor must have a low volts drop per unit length (low resistance) and the insulation must have a high resistance (good insulation) consistent with the voltage rating of the cable.
Current Rating. The normal current rating of a cable can be defined as: ‘The amount of current it will carry without sustaining a temperature rise sufficient to cause the value of insulation resistance to deteriorate to an unacceptable level or without exceeding a specified voltage drop’. Earlier cables either had the current rating stamped on the outer sheath or had a colour identification related to the current rating.
However, because a cable’s current carrying capacity is influenced by a number of factors other than just electrical load current, it is nowadays the practice of cable manufacturers to use a classification based on the American Wire Gauge (AWG). Modern aircraft cables have a wire gauge number stamped on the outside. The aircraft electrical designer will take into account the factors listed below before choosing a cable for a particular job:
* The electrical loading of the cable.
* The amount of heat generated by neighbouring cables.
* The number of cables in the loom (the more cables the less current each cable can carry).
* The ambient temperature of the surrounding air.
* Whether the cable is enclosed in a conduit or in free air. * The thermal conductivity of the cable.
Note. Cables have a specified maximum continuous operating temperature, which is caused by the combination of ambient temperature and temperature due
to
I
2R
(power in watts) losses in the cable. In general it is undesirable tocontribute more than 40°C rise by electrical heating.
Plating is employed on copper, copper alloy and aluminium conductors to improve resistance to corrosion and to assist termination connection techniques. Often it is the plating which will determine the temperature rating of a given cable. Typical temperature values for continuous use are:
Tin plated copper 135°C. Silver plated copper 200°C. Nickel plated copper 260°C. Nickel clad copper 260°C.
Nickel clad copper is used in place of nickel plated copper on fire resistant cables to provide a thicker nickel coating. Plating used on crimped terminal ends must be compatible with the conductor plating on the cable – information on this is obtained from the termination manufacturers.
Deterioration. Aircraft cables are designed to provide the best possible combination of resistance to deterioration caused by extremes of temperature, mechanical damage and contamination by fluids, and in general, are suitable for installation without additional mechanical protection.
Working conditions and the environment, however, may necessitate the provision of extra protection in those places where the cables are exposed to the possibilities of local damage or conditions that could cause deterioration. These areas can include: wheel bays, landing gear bays, engine bays, toilet systems etc.
STORAGE
After manufacture and prior to delivery the cable is given a quality control check that will normally include an insulation test and a continuity check. The ends are sealed to prevent ingress of moisture. The cable is wound on drums, suitably labelled and documented (EASA form 1). The drums are protected to prevent damage during transit and storage. Smaller sizes of cable may be supplied in wrapped coils.
On receipt the cable should be inspected as far as possible for any signs of damage or moisture ingress. If any is found the whole drum/coil should be returned to the supplier together with all documentation. If the condition of the packing, as received, gives rise to doubt regarding the soundness of the cable, it should be returned. Note. Check the cable part number/batch number and confirm its identification
against its documentation/stores release certificate (EASA form 1).
Cables should be stored in a clean, well-ventilated store. They should not be stored near chemicals, solvents or oils and, if necessary, protection should be provided against accidental damage.
Loose coils, wrapped or not, must not be stored so that a heavy weight is placed on them. This may cause distortion/damage to the insulation or the conductor core. The ends of cables should be sealed by the use of waterproof tape or sealing compound.
HANDLING
Cables should be handled carefully at all stages of storage and installation.
When taking lengths of cable from a drum or reel, the cable should not be allowed to come in contact with rough or dirty surfaces. The drum should be mounted so that it can rotate freely and the cable pulled off using little or no force and avoiding kinks.
Made-up Cabling
Most looms are made up in a Loom or Cable Bay prior to being fitted to the aircraft. Separate cables are laid out on a smooth bench, cut to size and end fittings attached – all in accordance with an approved drawing (in some bays a full size plan of the loom is laid out on the bench it-self).
Each cable in the loom will have printed on its insulation at regular intervals its aircraft wiring code – at the same time the printing machine will automatically carry out an insulation check.
Cable looms and cable runs made-up on the bench should be inspected before installation in the aircraft to check that:
a) Cables, fittings, crimped joints etc, are of the correct type, have been obtained from an approved source, have been satisfactorily tested before making up and are not damaged in any way.
b) Connectors and cable looms conform to the AMM, Wiring Diagram
Manual or Modification Drawing in respect of terminations, length, angle of outlets and orientation of contact assemblies, identification and
protection of connections.
c) Crimped and soldered joints have been made in accordance with the AMM, Wiring Diagram Manual or Modification Drawing, are clean and sound.
d) That cable loom binding and strapping is secure.
e) Continuity, resistance and insulation tests have been carried out.
f) Cables should be identified using the aircraft wiring code (iaw the wiring diagram).
INSTALLATION
Guidance on the factors requiring special attention during the installation is given in the following paragraphs – but always check the AMM.
Contamination. To prevent moisture from running along the cables and seeping into the associated equipment, the cables should be so routed as to run downwards away from the equipment. Where this is not possible, the cable should incorporate a
Where conduits, tubes or ducts are used, they should be installed in such a way that any moisture accumulating in them will be able to drain away. Cables that are routed through such fittings should be capable of withstanding any such moisture that may be encountered.
Interference. Interfering magnetic fields may be set up by electrical equipment, electrical currents in the cabling, or the aircraft structure and also by magnetic materials. Cables must be installed so as to reduce electrical interference to a minimum and avoid interaction between the different electrical services.
For example, cables carrying dc should be positioned at least 2ft (600mm) away from the aircraft compass. If supply and return cables run close together the magnetic effects are insignificant.
Note. Requirements for the avoidance of compass and radio interference are given in Chapter J4-1 of British Civil Airworthiness Requirements. (Now EASA CS23 – light aircraft, CS25 – large aircraft, CS27 & 29 – helicopters).
Protection of Cabling. The cables are required to be protected from abrasion,
mechanical strain and excessive heat and against the damaging effects of fuel, oil and other aircraft fluids, water in both liquid or vapour form and the weather.
Cables should be fitted away from the skin of the aircraft so as to reduce the effect of the high skin temperatures, likely to be reached in the tropics, from affecting the cable. The cables should not be run near the hot parts of an engine or other hot components unless a cooled air-space or heat barrier is provided.
Where cables are routed through metal fittings or bulkheads etc, the edges of the holes through which they pass must be radiised and smoothed and fitted with an insulated bush or sleeve. Cables which are drawn through holes or tubes must be an easy fit requiring only a moderate, steady pull. Care being taken to keep the cables parallel to one another and to avoid the formation of kinks (which may cause fracture).
Conduits, ducts and trunking used for carrying cables should have smooth internal surfaces and have sufficient diameter to allow the cable to be pulled through using only a moderate steady pull. The designer should also take into consideration cable cooling (or lack of) when placing cables in ducts.
Cables fitted through pressure bungs (pressure bulkheads) should be fitted into the correct size holes for the size of cable, to ensure sealing. Use the correct cable threading tool to avoid damaging the bung.
Support of Cabling. The cabling must be adequately supported throughout its length and a sufficient number of cable clamps must be provided for each run of cable to ensure that the unsupported lengths will not vibrate unduly. The minimum bend radius should not be less than that stated in the appropriate specification (eg SAE specifies – for cable groups - 10x the largest cable diameter in the group).
For single cables 10x the diameter is given by the same standard.
At terminal blocks, where the cable is suitably supported at each end of the bend, the minimum radius may be 3x (CAAIPS).
The same standard for co-axial cables states a minimum bend radius of 6x the outside diameter (for flexible cables) and 10x for semi rigid cables.
Cables must be fitted and clamped so that no tension will be applied in any
circumstances. Any loops or slackness will not occur in any position where the cables might be caught and strained by normal movement of persons or controls or moving equipment in the aircraft, or during flying, maintenance or adjustment.
Where it is necessary for cables to flex in normal use, eg connections to retractable landing gear etc, the amount and disposition of slack must be controlled so that the cable is not stressed in the extended position and that the slack will not be fouled, chafed, kinked or caught by anything during movement in either direction.
Cables should normally be supported independently of, and with maximum practicable separation from, all fluid and gas pipelines.
To prevent contamination of the cables in the event of leakage, cables should be routed above rather than below liquid carrying pipelines.
FAILURES
Typical failures might be:
Wet arc tracking. An Airworthiness Notice (AN 12) - now cancelled, highlighted the problem of fluid contamination where there is insulation damage. The cable in this case had been hot stamp printed (for identification purposes) but the stamp had penetrated the insulation. Fluid from a leaking toilet waste system
contaminated the cable which caused electrical arcing to occur. This caused rupture of the cable and also others nearby.
Cable looms are particularly vulnerable to liquid contamination because they can provide a drainage path and also once wet can remain so for a long time. Correct routing, as mentioned earlier, is important.
Dry arc tracking. Occurs when two cables short circuit together, which will cause intense local heating which may cause damage to other cables in the loom.
Abrasion. This may occur due to cable rubbing on cable or cable rubbing on the structure of the aircraft or equipment. Careful inspection of cable loom clipping and tying is necessary to check on this problem.
Conductor ‘knuckling through’. Some earlier cables had this problem of the conductors ‘knuckling’ (bending) and penetrating the insulation. This was due to applying excessive pull-through forces, so great care must be taken not to put the cables under tension.
Red Plague. Cables with silver plated conductors can exhibit the aptly named ‘Red Plague’ if the plating has been damaged and then exposed to moisture. Silver- plated conductors are generally unsuitable for use in unpressurised areas.
Glycol Fires. If de-icing fluid contaminates silver-plated conductors an electrical fire can result. These conductors must therefore be kept away from areas where de-icing fluids can be present.
STRIPPING
This is the stripping away of the insulation to leave the conductor exposed. Carried out in preparation for soldering or crimping or compression joining of the cable to an end fitting/termination.
The following precautions should be followed when stripping any type of cable: a) Use approved tools only. Never use a knife for stripping.
b) Ensure blades of cutting tools are sharp and free from nicks.
c) For size 8 or larger cables a knife may be used to make cuts lengthwise through the outer covering and insulation. The insulation should be bent back and cut off with side cutters or scissors. The cable must be examined for any damage to the conductor strands. No strands should be damaged or missing.
d) Ensure that the blade of the cable stripper is square to the cable and the outer covering and insulation is clean cut, with no frayed edges.
e) When using hand strippers to remove lengths of insulation longer than 19mm (0.75in) the stripping should be carried out in more than one operation – taking off a short length of insulation each time.
f) If the lay of the conductor strands is disturbed in the stripping action, the strands should be re-laid with a light twist.
g) When stripping aluminium cables extra care must be taken as the individual conductor strands break easily if nicked.
h) Bending of aluminium cables can cause work hardening, resulting in failure of strands sooner than in cables with copper conductors. i) Aluminium, when exposed to the atmosphere, forms an oxide film,
which acts as an insulator. If left untreated it can cause corrosion at connecting joints and it also increases its thickness as heat is generated by current flow, and thus increases the electrical resistance of the joint. Treatment when crimping is by using an inhibiting compound to
specification DTD 5503 (50% by weight zinc oxide in white petroleum jelly).
j) When setting up and adjusting cable strippers/pliers it is advisable to first try the stripper setting out on a scrap piece of cable of the same type and gauge as the actual piece to be worked on.
CRIMPING
A crimped connection is one in which a cable conductor is secured by compression to a termination so that the metals of both are held firmly together in close contact. A typical crimp termination (figures 1 and 3) has two principal sections, crimping barrel and tongue, together with, in some types, a pre-insulated copper sleeve that mates with the crimping barrel at one end and is formed, during the crimping process, so as to grip the cable insulation as well as the cable conductor to give increased support.
The barrel is designed to fit closely around the cable conductor so that after crimping a large number of points of contact are made. The crimping pressure is applied with a hand or hydraulically operated crimping tool fitted with a die or dies shaped to give a particular cross-sectional form to the completed joint. The precise form of the crimp is determined by such factors as the size and construction of the conductor, the
materials, the dimensions of the termination and the type of crimping system used. It is, therefore, important that only the correct type of die and crimping tool should be used, and that the necessary calibration checks have been made to the tool.
There is a vast range of terminations available, many of which are colour coded and suitable for use only with specific types of aircraft cable and crimping tools. It is important, therefore, that the appropriate crimping equipment manufacturer’s instructions regarding the use of cables and terminations are followed.
Figure 1 shows two typical terminations: (a) a ring tongue termination - note the insulation grip portion (C) and the conductor grip portion (B) and (b) an inline splice.
Fig. 1 TYPICAL CRIMP TERMINATIONS
There are many types of hand crimping tools and the following notes are related to the AMP crimping tool. In general most of the information given will apply to most tools. Figure 2 shows the AMP crimping tool which uses AMP terminations.
AMP CRIMPING
The tool used for crimping AMP terminals has several design features to ensure a consistent quality of completed crimp joint to include:
1) Crimp ratchet. 2) Locator.
3) Insulation adjusting pins. 4) Colour and dot coding.
Crimp ratchet. This ensures the bottoming of the die jaws before the jaws can be opened again. The tool cannot be released until the jaws have been completely closed. It is important with crimp ratchets that the correct size cable; correct size termination and correct size dies/crimping tool is used. If not, it is possible to jam the tool as the crimp will not complete and the ratchet will not release.
Locator. The locator holds the terminal in the correct position in the die jaws and allows the conductor strands to protrude 0.8mm from the terminal barrel when the wire is fully inserted.
Fig. 2 TYPICAL CRIMPING TOOL – EXAMPLE AMP
Insulation Adjusting Pins. To allow for small variations in wire size and to ensure optimum mechanical strength of the joint the insulation die head has three degrees of adjustment:
1) Tight. 2) Medium. 3) Loose.
Colour and Dot Coding. The wire size range is stamped on the face of the tool, the colour of the handle is related to the wire size range and the terminals are also of the same colour, eg for a red handled tool you use red coloured terminals. There are various colours - related to wire size, eg blue, green and yellow.
During the crimping operation a code marking related to the size of the tool is impressed on the terminal insulation.
This is a dot as indicated in figure 3. The number of dots is related to the size of the tool. Therefore on inspection after crimping the operator is able to ascertain that the correct tool was used.
The AMP terminals are known as Pre-Insulated ‘Diamond Grip’ (PIDG) terminals. We shall now look at a crimping operation on a ring tongue terminal.
Fig. 3 TYPICAL CRIMP
Preparation – General
Note. It is advisable to set up the crimping process using scrap pieces of wire of the same type and gauge as that to be used on the ‘real thing’. This means that, for example, if the tool is used with a terminal size that is too big it is not able to close properly and the ratchet will not release. This means that the cable will have to be cut to remove the tool (and crimp) from the aircraft for special
attention to operate the release. The cable may now be too short for further work to proceed and a new length of cable will have to be used.
1) Check the wire is the correct specification and not damaged; has the correct coding imprinted (can be carried out by a machine automatically or by hand) and has been insulation and continuity tested.
2) Ensure correct size of wire. Strip so that stripped conductor extends to 0.8mm (1/32”) (AMP) beyond the correct size terminal.
3) Ensure correct tool, check: a) Calibration date.
b) Ratchet handle operation.
c) Inspection of dies for cracks, dents etc.
d) Use GO, NO-GO gauges to ensure die closure is correct. 4) Ensure correct position of insulation grip adjustment pins (AMP) as
follows:
a) Insert insulation adjustment pins into the number 3 position. b) Locate terminal in crimping jaws.
c) Insert a test length of un-stripped wire into the insulation grip portion of the terminal.
d) Close handles slowly and fully until crimp ratchet releases. e) Open handles, remove terminal and check insulation support as
follows: Bend wire back and forth once, terminal sleeve should retain grip on wire insulation. Check that end of the conductor element is level with the end of the barrel and gripped firmly. f) If wire pulls out set insulation adjustment pins in next tighter
position, (No 2) and re-crimp. Repeat test until desired insulation grip is obtained. Ensure that both adjustment pins are in the same position.
The Crimping Process
1. Insert correct terminal into the jaws of the tool ensuring the termination barrel butts flush against the locator.
2. Squeeze handles until terminal is held firmly in place, do not deform
terminal. Once ratchet is engaged, the handles cannot be opened. 3. Insert stripped cable into terminal.
4. Hold wire in position and close handles until ratchet is free to release. 5. Inspect crimp and check:
a) Barrel insulation is in firm contact with wire insulation. b) Correct dot code/colour combination.
c) Crimp is centred in barrel.
d) Conductor is protruding 0.8mm (1/32”) from end of barrel of crimp.
e) Joint for correct formation, freedom from fracture, rough edges or excessive flash or damage to insulation.
Always follow the manufacturer’s instructions regarding the use of the crimping tool.
Fig. 4 CRIMP LOCATION – RING TONGUE TERMINAL
Note. Only aluminium or bimetal (AlCu) terminations should be used to terminate aluminium cored cables and the cable should be stripped immediately prior to making the joint. An inhibiting compound must be applied before crimping takes place.
Inline Crimping
Sometimes called Inline Splicing and used when a new electrical cable is to be spliced into an existing cable run. The inline crimp is crimped to the end of the existing cable and to the end of the new insert cable – this is done at both ends so a new section of cable is spliced in. Splicing sections of cable into an existing cable run is to be
avoided – it is better to replace the complete length of cable. Where inline crimps are used this should only be carried out if the AMM chapter 20/Wiring
Diagram/Modification document gives authority. The following are general guidelines:
a) No splices under clamps or supports.
b) No splices installed inside connector backshells. c) No splices installed inside a conduit.
d) No splices allowed in areas where the wire bundle flexes, eg
instrument panels (on anti-vibration mountings) and hinged doors. e) Stagger splices on cables in looms to avoid an increase in wire bundle
diameter.
f) Cables should be replaced if the number of repair splices exceeds three. g) Splices shall not be installed in critical circuits, ie stabiliser trim, fire
detection and extinguishing and fuel control valves.
Crimping In-Line Splices (figure 6)
1) Select the required in-line splice and a tool of the same colour coding. 2) Adjust the insulation crimping adjustment pins as detailed above and
carry out a pre-trial test splice.
3) Insert in-line splice into crimping jaws until properly located. 4) Squeeze handles until splice is lightly gripped.
5) Insert prepared stripped wire into terminal barrel. When inserted the conductors should be visible in the inspection window.
6) Hold wire in position and complete the handle squeeze operation. 7) Release ratchet and remove tool.
8) Inspect for correct formation of completed crimp.
9) Place any necessary sleeving onto cable and push out of the way. 10) Insert other end of in-line splice into jaws until properly located. 11) Complete crimping operation by repeating items 4 to 8 above. 12) Pull sleeving into place over crimp and heat shrink.
Fig. 6 IN-LINE CRIMPING
Crimping in High Vibration Areas
Special precautions are needed when crimping cables in high vibration areas such as wheel wells, engine bays etc. Besides the problems associated with vibration, chaffing, loosening of connections etc, there is also the attendant problems associated with debris damage (wheel wells), heat damage (wheel wells [brakes] and engine bays), fuel and lubricant contamination (engine bays) and moisture ingress (wheel wells). In general when splicing/crimping in these areas the following applies:
1. Consult chapter 20 of the AMM for any special procedures. 2. Use special moisture proof (low temperature) splices.
3. Protect the splice with special insulation, eg tape wrap the splice 3 times followed by 2 spiral wrapped tapes in opposite directions, with 50% overlap over the entire splice.
4. Observing the necessary safety precautions heat shrink a sleeve over the entire covering and if necessary fit an abrasion protection sleeve.
5. Tie cable/cable loom using the correct spacing (chapter 20). Ties should normally be made every 6 inches (152mm) and tied using a clove hitch and square knot. Plastic ties should not be used.
6. Check routing of cable – ensure clearance from hydraulic, fuel and other system pipelines. Also check clearance from heat sources and any
moving parts (landing gear).
7. Check that there is sufficient slack to allow for structure flexing, temperature changes and any movement between components. 8. Where clamps are used only Ribbon Cushion type clamps should be
used – but not on coaxially cables.
9. Consideration must be given as to whether a drip loop is required (to allow any collected moisture to drip thus preventing moisture ingress into moisture sensitive areas).
10. Check all systems for correct operation, clearance etc – the electrical system, landing gear retraction mechanism, steering mechanism etc. Note. The general guidelines for ordinary in-line crimping also apply.
POWERED CRIMPING MACHINES
When crimping large size cables the force required to squeeze the terminal is too high for manual operation so various machines are available that are power operated. The following is a description of the hydraulic ERMA type.
ERMA HYDRAULIC CRIMPING MACHINE
This machine (figure 7) is supplied as a kit containing eight sets of dies for cable size from AWG 6 to AWG 0000. An Allen key is supplied for fitting the dies to the machine. The crimp formed is a regular hexagon shape and has two code letters impressed on it by the dies during crimping. These code letters are HG and HH and are the same as those marked on the cable lugs by the manufacturer.
Preparation of the Machine
Check the calibration date. Check that there are no leakages, damage and that the control valve closes and releases and the pump works correctly.
Fig. 7 ERMA CRIMPING MACHINE
The machine operating handles should be screwed firmly into position and the code letters stamped on the dies checked for correct size.
If the dies are to be changed:
a) Select the two matched dies bearing the correct code letters for the size of cable in use (check machine manufacturer’s instruction manual). Check that the lugs to be used have the same code letters marked on the terminal palm. Inspect dies for cracks, corrosion, dents, burrs etc. If damaged in any way do not use.
b) Remove the upper die adapter by sliding it from the dovetailed head of the tool. This leaves the slotted head of the tool open to allow the lower die to be fitted against the ram. Insert the spigot on the upper die into the hole in the die adapter until it is held in position by the spring-loaded steel ball.
c) Close the hydraulic valve. Pump the handle to move the ram forwards to show the hexagon socket screws which hold the lower die. Slacken these screws using the Allen key provided with the kit. Fit the lower die into the ram so that the screws fit into the recesses on either side of the die. Tighten the screws to hold the die, ensuring that they are below the surface of the ram body. Open the hydraulic valve to retract the ram. d) Slide the upper die adapter, complete with die, into the dovetailed
grooves until it is located centrally by the spring-loaded steel ball.
Crimping Operation
In general carry out pre-crimp checks on the cable as described earlier with the hand operated tool.
Check that the two-letter code on the cable lugs and on both dies is correct for the size of the cable to be terminated.
a) Close the hydraulic valve. Place the terminal lug centrally between the dies and pump the handle until the lug is lightly gripped.
b) Strip the cable insulation so that when the cable is inserted into the lug the insulation lies flush against the end of the barrel and the conductor projects slightly from the other end. Place any insulation sleeving
required over the cable and push out of the way.
c) Insert the conductor into the barrel of the lug and pump the handle until the dies are fully closed. When fully closed a safety valve will operate with an audible click and pressure on the pump handle is reduced.
d) Open the hydraulic valve to allow the ram to retract. The crimped termination can then be removed from the machine.
e) Inspect crimp for correct formation, dents, cracks, flashes and
codification, pull insulation sleeving down over the joint and shrink fit as necessary.
TESTING OF CRIMPED JOINTS
Local regulations will determine the inspection periods required for crimping tools. AMP recommends 3 months or at intervals of 1000 crimps whichever is the smaller. This test will include checks on ratchet operation, dies for dimensions and damage, security of handles, dies and adjuster pins.
In order to ascertain that the crimping tool is functioning correctly a test crimp must be made and this is subjected to a mechanical tensile test and millivolts drop test as laid down by the manufacturer. If the test specimen fails, then all the crimps carried out using the tool would have to be inspected and the crimping tool would be
withdrawn from service.
Figure 8 shows the principle of carrying out a millivolts drop test on a crimped terminal. A normal rated current is passed through the cable and crimp and a
millivolt meter is placed as shown. The manufacturer will give the maximum millivolts drop that can be allowed, but a general figure is 5mV/10amps flowing. Note that the amount of exposed conductor is shown exaggerated.
Fig. 8 CRIMPED JOINT MILLIVOLT DROP TEST
PLUGS & SOCKETS
Each wire in most cable looms have the ends crimped to a pin and that pin fitted into a plug which will normally have many pins fitted. The plug fits into a socket
receptacle. These two connecting devices may be fixed or free, that is, may be in a component or the socket fitted externally to equipment/bulkhead. Typically the connection method is by a screw-thread but a bayonet coupling may be used. To ensure that each wire in the plug is aligned to its correct wire in the socket the following precautions/design features are incorporated:
* Each wire will have a unique aircraft ident code printed on (or fitted with a sleeve with the code on - old) and the pin crimped to it will be fitted into a specified hole in the plug (these are numbered, normally clockwise out from the centre).
* The wiring manual/AMM/modification leaflet will specify the wiring code and the plug location number to be used.
* When assembling the plug (together with all the fitted pins) to the socket there is an alignment lug on the plug shell that aligns with a corresponding grove in the socket shell to ensure correct orientation.
Note the detail in figure 9 including colour and alpha/numeric coding for each wire. The connector is of the square type.
Pins for insertion into plugs and sockets are attached to cables by soldering (rare) or (usually) crimping. Once the pin is crimped into position on the cable, it is then inserted into its respective hole in the plug/socket.
Fig. 9 EXAMPLE – TAKEN FROM AN AIRCRAFT WIRING MANUAL
Fig. 10 TYPICAL PLUG & SOCKET
Fig. 11 PIN/HOLE LOCATION IDENTIFICATION
Unwired contacts are usually fitted with a sealing plug and if appropriate any unused holes in the moulding should be fitted with an approved filler plug. If the contact is not installed then a sealing rod is installed in its place.
Figure 10 shows the ‘front’ view of a typical mating pair plug and socket. Note the terms used and the shell locator lug and grove. Figure 11 shows the ‘rear’ view of a plug/socket indicating the pins/holes identification system.
There are many types of plugs and sockets in use and figures 12 and 13 show two examples of LRU socket connectors. Square and rectangular plugs and sockets will have some form of orientation lug/corner to prevent incorrect assembly. The ARINC 600 family of connectors have a high pin density (many pins per square inch) with index pins to ensure correct orientation.
Fig. 12 SQUARE TYPE PLUG/SOCKET CONNECTORS
Fig. 13 ARINC RACK MOUNTED CONNECTORS
CONNECTOR PIN REMOVAL/INSERTION
There are two basic types of contact retention used in plug and socket connectors on aircraft, one with the contacts being released for removal from the rear of the contact insert and the other from the front. Each system requires the use of different types of insertion/extraction tools. It is essential that the correct procedures and tools are used for a particular type of plug or socket.
Remember, always ensure power is off and the circuits, C/Bs, fuses etc tagged before removal/fitment of plugs, sockets and contact pins.
Rear Release Connectors
The insertion/removal tool is usually plastic and may be of the type shown in figure 14. This tool is expendable and is failsafe in that mishandling will result in damage to the tool rather then to the connector or termination. The insertion end is coloured (according to wire/connector size), eg Canon size 20 contact Red/White, size 16 contact Blue/White, size 12 contact Yellow/White. The white end being the removal end.
Fig. 14 INSERTION/REMOVAL TOOL
Figure 15 shows a connector with a rear release. Note the construction of the pin and the retention clip. The removal tool is inserted from the rear of the plug (the left hand side in figure 17) to spring the retention clips out and release the pin rearwards (to the left in the drawing).
Fig. 15 PIN RETENTION – REAR RELEASE
Contact/Pin Removal
1. Hold plug/socket connector with the rear end facing you.
2. The extraction tool (white end) is positioned over the cable connected to the contact pin to be removed.
3. Slowly slide the tool along the wire into the connector until a positive resistance is felt. The retention clip will now unlock.
4. With the cable held against the extraction tool the contact should be removed by pulling the cable and tool together from the connector. Do not twist or tilt the removal tool as damage can occur to the pin.
Contact/Pin Insertion
1. Remove the backshell or other accessory from the rear of the plug/socket connector and move up the cable loom out of the way.
2. Ensure that the correct hole in the connector has been selected to insert the pin – check the hole numbering system on the front face of the plug/socket and the cable ident code and the wiring diagram.
3. Snap the coloured end of the appropriate insertion/removal tool onto the wire. When inserting the wire into the tool use the thumb and not the thumb nail as this could damage the insulation. Position the tool
forward onto the contact shoulder, except in the case of size 22 contacts, in which case the tool should be positioned on the back of the crimp bucket.
4. Holding the connector with the rear insert facing you, slowly push the contact straight into the connector. A positive stop will be felt when the retention clip locks in the contact.
Fig. 16 USE OF INSERTION/REMOVAL TOOL
5. The cable should then be released from the tool and the tool removed by pulling it back out of the plug mould.
6. If the contacts are to be inserted into holes near the edge of the insert, the open side of the tool should always face the edge of the insert, this avoids excessive strain on the insert.
7. The proper size contacts and sealing plug should be fitted into any vacant contact hole.
Front Release Connectors
Figure 18 shows a front release connector and its retention clip, note the construction of the pin and retention clip.
Fig. 18 PIN RETENTION – FRONT RELEASE
Contact/Pin Removal
1. The extraction tool should be positioned over the contact to be removed from the front of the connector.
2. The central plunger of the tool should be held back.
3. Push the tool into the connector to release the retention clip from the front shoulder.
4. Pushing forward the tool will eject the contact rearwards out of the connector.
Contact insertion is similar to the insertion of rear release connectors.
Testing of Connectors
Test probes used should be of such a size that the contacts are not damaged or bent and small enough to allow for contact on one pin only.
On socket contacts test probes should be of the same size as (or less than) the mating plug contact.
Any bent pins must be replaced. Trying to straighten them is likely to cause a fracture and possible failure at a later date.
CO-AXIAL CABLE INSTALLATION
Co-axial cable is used for the transmission of low power data signals which might get corrupted from external electromagnetic activities. They consist of an inner copper conductor; a dielectric insulator; an outer copper braid (to shield the inner conductor from any interference) and an outer plastic protective covering. Any work carried out on the cable must be in accordance with the AMM as it is important that any connections made do not interfere with signal transmission causing signal attenuation. End fittings are attached to allow the cable to be fitted to various items of equipment.
The sequence adopted for attaching an end fitting is as follows (figure 19):
a) The outer protective cable covering is cut back to expose the braided outer conductor, ensure cut is square and at right angles to the cable and the braid is not nicked or cut.
b) If the cable is fixed at the other end, thread the coupling ring and adapter onto the cable.
c) The braid is ‘fanned’ or ‘teased’ out as shown in figure 19 and folded back to fit closely over the adapter.
d) The dielectric insulation is cut back to expose the inner copper conductor. Ensure cut is square and conductor is not nicked or cut. Ensure cutting tool does not crush or deform cable.
e) Screw the sub-assembly to the adapter, this clamps the outer conductor firmly between the two components.
f) Conductors must be visible in the inspection hole of the contact before soldering.
g) Screw the coupling ring on to the sub-assembly by hand.
When routing co-axial cables ensure they are clamped firmly along their entire length. Special cushioned clamps of the correct size may be used as it essential that the cable is not crushed or deformed in anyway.
As these cables are used as transmission lines for aircraft antenna (low signal power) they should not be routed or tied to other cable bundles, the minimum bend radius should be observed (six times the outside diameter – for flexible cables). The length of the cable is also important.
When disconnecting connectors, protect using metal caps which mate with the connector-coupling device. If the correct size cap is not available use snug fitting plastic caps or secure a polythene bag over the connector.
Fig. 19 FITTING AN END CONNECTOR TO A CO-AXIAL CABLE
Testing
The cables should be checked for continuity and absence of short circuits and insulation resistance on completed connector assemblies. Minimum value for insulation resistance is typically 100MΩ.
To determine whether there is an open or short circuit on a transmission line Voltage Standing Wave Radio (VSWR) checks are carried out. This compares the standing voltage maximum to the standing voltage minimum.
Vmax is the sum of the incident (VI and reflected VR voltages and Vmin is the
difference between VI and VR thus VSWR = VI + VR/VI – VR and in theory should be 1, in practice the lowest VSWR is sought.
If there is zero attenuation along a line the VSWR will be infinite when the line is terminated in an open or short circuit. Measurement of VSWR will indicate the existence of reflected waves on the line, which, in turn will indicate the degree of mismatch between the transmitter and the load.
Another test set used to test co-axial cables is the Time Domain Reflectometer (TDR). This uses pulses to test the cable and gives a visual display of cable failures. The test pulses are transmitted along the cable by the test set which receives back the
reflections and shows them on a CRT.
The test set has calibrated controls and can examine up to 100ft (30.5m) of cable and down to 1ft (0.3m) in length.
Low loss cables can be examined up to 2000ft (609m) long at 100 or 200ft for each scale division.
A three number display shows the distance-to-cable failure when the
equipment is set to display horizontally the failures reflected on the CRT
reference line.
The vertical (Y-axis) deflection of the CRT beam is in proportion to the reflected signal plus the incident step.
To test, remove the receiver transmitter from the aircraft rack then, using a locally made test cable, connect the TDR equipment to the antenna cable.
Figure 20 shows an example display of faults that might be found. Figure 21 shows the test of a 350ft (107m) cable and the displays shown on the CRT. It shows a frayed cable at A, impedance rise from B to C, continuity drop at C and an open circuit at D.
Fig. 21 CABLE PROBLEMS
FIBRE OPTIC CABLES & CONNECTORS
Used in the transmission of data using light as the transmission medium.
Care must be taken when working with fibre optic cables as the optic core is easily damaged. Take the same precautions with these cables as with the cables mentioned previously plus those listed below. Fibre optic strands can be damaged by:
a) Excessive pulling - as when trying to pull it through a bulkhead. b) Pinching or crimping - such as stepping on the cable.
c) Bending too sharply. d) Twisting.
Before examining the connector face or glass contacts ensure system is switched off and circuit breaker is tagged. The light from the fibre optic network, although low power, can be intense enough to cause damage to the eyes. Use only approved material to clean the connectors and use protective caps when the connectors are disconnected. Only tighten connectors by hand.
The same visual checks are carried out on these cables as with any other type of cable, but the testing equipment is different.
Testing for Continuity
A simple check is to use a commercially available torch at one end and see if the light can be seen at the other. For a more comprehensive check a test meter is used
injecting a calibrated low power signal at one end and picking it up at the other using an optical receiver.
An Optical Time Domain Reflectometer (OTDR) is used for a more comprehensive test and it functions in a similar way to a TDR except that light is used.
WIRING PROTECTION TECHNIQUES
This part of the book gives general guidelines with regards to wiring protection techniques and it is essential that you follow the procedures laid down in the AMM chapter 20 (Standard Practices – Airframe) and the Wiring Practices Manual for your aircraft.
This part of the book deals with the physical protection of the cables and not the protection of the circuit, which is associated with fuses, C/Bs etc. It is associated with EWIS (see later text).
When using sleeving, tubing, tape or tying materials it is necessary to consider the following: (a) The temperature of the area the material is going to be used in and (b) Whether the material is being used in an area where it is necessary for it to be resistance to fluids chemicals etc.
Cable Looming and Loom Support Cable looms are of three principle types:
a) Open loom. b) Ducted loom. c) Conduit.
The composition of the cable loom is dictated by such factors as: a) Overall diameter.
b) Temperature.
c) Type of current (ac or dc).
d) Interference due to inductive or magnetic effects.
e) Types of cables – essential circuit cables must be protected in the event of short circuits developing in adjoining cables.
Open Loom. Cables are bundled and bound together with cord or with nylon strapping.
Ducted Loom. The bundles of cable are supported in ducts routed through the aircraft structure. The ducts may be made of aluminium alloy or fibreglass reinforced plastic.
Conduit. Used where cables have to pass through areas which may cause
contamination to the cabling due to oil, hydraulic fluids etc. They may be made of plastic
or metal and maybe flexible.
Each loom in the duct is bound with a coloured cord/strap or ident strap to identify the system to which it relates. Figure 22 shows an older method of single cord lacing and the procedure used.
Fig. 22 SINGLE CORD LACING
In high vibration areas special attention must be paid to the support of looms. This again will be laid down in the wiring practices manual but typically on engines tie spacing is every 2” (51mm) and in other high vibration areas 6 to 8” (152mm to 203mm). Plastic ties may be used optionally to lacing tape in pressurised areas only.
Cable Clamps
Cable bundles are secured to the structure by clamps, close enough together to ensure the bundles will not sag or vibrate between supports. The basic clamp is the cushioned clamp as shown in figure 23. It is made up of a metal ‘P’ clip fitted around and within a rubber ‘0’ ring. It is secured to the structure by a nut and bolt (or
similar).
If a bundle is slightly undersize then layers of tape may be used to increase the bundle diameter. If necessary filler rods may be used to build up undersized bundles for a proper fit in the clamp. In high vibration areas ribbed cushion clamps are used, but not for co-axial cables. These have three holes in the clamp with a resilient
bushing giving a choice of sizes. Where cable clamps are fitted with resilient bushings, care must be taken to ensure the bushings used are of such a size that the cables are firmly held in place but do not crush or deform the cable insulation when the clamp is tightened.
Fig. 23 CUSHIONED ‘P’ CLAMP
PROTECTIVE SLEEVING TECHNIQUES
These techniques may be used to provide secondary protection of cables with continuous sleeving, or in some cases wrapping with tape. When sleeving up to a connector, secure the sleeving under the connector clamp.
Heat Shrinkable Sleeving
This involves applying a high temperature to a length of sleeving which shrinks onto the cable assembly. Points to note are:
1) Heat guns are not considered to be explosion proof.
2) Heater elements in the gun get very hot and may operate in excess of vapour flashpoint (450°F/232°C).
3) Safety regulations apply which may include non-use of heat guns: a) In fuel tanks.
b) Within 100ft (30m) of aircraft when refuelling/de-fuelling of fuel vents, fuel spills and flammable liquid.
A heat gun may be rated at a specific temperature or a selection of temperatures may be available. When using these guns the following procedure provides a general guideline:
a) Allow 10% extra length of sleeving over the area to be covered. b) Protect any wiring insulation or adjacent cables from heat.
c) Pre-heat gun for 15 seconds. Manufacturers usually give minimum shrink temperature, but higher temperatures can be used – consult data on heat shrink tubing for maximum temperature to be used.
d) Hold gun 3” (76mm) from the sleeving and direct hot air at the centre of the area.
e) Apply heat to sleeving until it shrinks into place, usually 5 to 10 seconds is adequate, do not exceed 20 seconds.
Fig. 24 INSULATION METHOD 1 – TAPE WRAP
The following provides details of two methods of insulating a splice assembly:
Method 1
1) Strip splice for at least 1½” (38mm) of outer insulation. Carry out splice. 2) Build up voids between splice and insulation to prevent abrupt changes
using Type B film. Three layers of film are required, each with 50% overlap and extending 1¼” (32mm) + ¼” (6mm) over the outer insulation.
3) Spiral wrap splice with Polymide Tape over the Type B film. Wrap each layer spirally with 50% overlap. Reverse direction of spiral with each layer. Extend tape ¼” to ⅜” (6 to 9mm) beyond Type B film.
Method 2 – Tape and Sleeve Method (figure 25)
1) Select two heat shrinkable sleeves large enough to fit over taped wire areas that will fit tightly over cables when shrunk into place. Cut sleeves long enough to extend 1” (25mm) + ¼” (6mm) beyond taped areas.
2) Select third heat shrinkable sleeve that is large enough to fit over taped splice. Cut sleeves to length that will extend to within ¼” (6mm) + ⅛” (3mm) of ends of the two sleeves.
3) Slide sleeves over the cable out of the way. 4) Complete the cable splice.
5) Clean and insulate and tape wrap as per method 1, steps 1, 2 and 3. 6) Slide small sleeves over tape until they are butted against ends of splice,
heat shrink into place.
7) Centre large sleeve over assembly. Heat shrink into place.
SHIELDING
To prevent interference from external fields originating from electrical equipment such as dc generators, motors, time switches, ignition systems etc affecting other systems, particularly interference on radio equipment, some form of shielding must be used. For generators, motors and time switches, capacitors can be connected across the output. These are in parallel with the output and usually form part of a unit called a suppressor. For prevention of interference to other circuits, cables are enclosed in metal braided sheaths typically made of tinned copper or silver-plated copper, the braid being connected to the earth of the aircraft.
Interference can be due to capacitive and inductive pick-up and crosstalk between adjacent cables. The term ‘pick-up’ means the interfering source is ac power. ‘Crosstalk’ is interference from an adjacent cable.
When two cables are close together, mutual inductance and mutual capacitance exists. To overcome this problem, shielding can be used. Using ‘twisted pair’ cables, ie a pair of wires twisted together reduces interference as the pick-up and crosstalk on adjacent loops cancel each another out.
Radio system cables may be twisted and shielded and in some cases double screened. As mentioned before the screening around a cable must be earthed, but only at one end. If both ends are earthed an ‘earth loop’ is formed and interfering fields may cause a potential difference between the ends of the screen, causing a current flow producing another interfering field.
On newer aircraft, because of a change in regulatory requirements on protection of aircraft systems from the effects of High Intensity Radiated Fields (HIRF) and lightning strikes, both critical and essential systems are required to be protected to levels
greater than on previous aircraft. Protection methods include:
a) Using the metal airframe as a screen against radiated interference including internal structure such as aluminated honeycomb walls, floor and ceiling panels around the flight deck/electrical equipment bays.
b) Using the gold film heater elements of windscreens.
c) LRU’s (Line Replacement Units) containing built-in filters and filter pins, which act to discriminate against certain frequencies and only allow the relevant frequencies to pass.
d) Extensive use of single/double screened cables.
Figures 26 to 29 show details of cable and plug shielding as used on the B777.
Fig. 26 B777 FLIGHT DECK SCREENING
Note the screening of the flight deck, the electronic bay and the electronic racks. Not also the attention to detail on screening methods such as the use of twisted pairs; twisted triplets; tinned copper braiding on individual wires; double layer braiding on bundles. Notice the bonding details of the connectors.
Fig. 28 BUNDLE CONNECTOR – BACKSHELL & SHIELD TERMINATION – PRESSURE HULL – B777
Fig. 29 BUNDLE CONNECTOR – BACKSHELL & SHIELD TERMINATION – WINGS & EMPENNAGE – B777
CABLE CHANGING
The following is a general guide on the procedure to change a cable.
1. If the cable being changed is due to some un-serviceability then the reason for the cause of the damage has to be ascertained and rectified. For example: Fluid contamination – find source of fluid and rectify leak, cable chaffing – check on proximity of moving parts, or if vibration induced, check cable support and/or investigate possible out-of-balance components like propellers, engines etc.
2. Locate cable, using the AMM, Wiring Manual etc and check its
specification. Check that type and size of cable is available from stores or its alternative.
3. Remove power from aircraft, trip C/Bs and tag – checking the AMM for the correct procedure, equipment, test etc.
4. Unscrew the connectors at both ends of the cable, inspect for damage and contamination of the male and female parts of both connectors. Check the general condition of the LRUs to which the cables are connected.
5. Untie, un-cleat, slacken/remove P clips/clamps along the cable/bundle route.
6. The ends of the cable are removed from their respective plugs either by: * Pin connector removal, using the pin connector removal tools
after first unscrewing the backshell – for most electrical cables. * Removing the end fittings of the co-axial cable/fibre optic cable. Note. For some cable lengths a new cable may be pulled through by first
crimping in-line crimp) the new cable onto the old cable, so as the old cable is pulled out it is replaced by the new. This may not work for long lengths as the pulling stress in the cable is too high. This is particularly important for fibre optic cables as they are easily damaged by pulling. 7. Check the specification of the existing cable against that in the wiring
diagram. If it is different find out the reasons why.
8. Check the cable specification on the new cable drum. Calculate the length of the replacement cable and cut the amount required plus an allowance for errors.
Note. The new cable may be made-up in a cable loom shop and laid out on full size layouts on benches. This may require the whole cable loom to be removed from the aircraft to allow the single wire replacement. 9. Check the new cable for damage and contamination. Check that the
manufacturer’s codes are imprinted at intervals along its length.
10. From the wiring manual ascertain the aircraft wiring code to be stamped on the new cable (there are some machines that will do this
automatically). The code is to be placed at intervals along the entire length of the cable. When doing this manually it is important that the insulation is not damaged in any way.
11. Carry out a continuity and insulation test for electrical cables. A continuity test for co-axial cables and light transmission tests for fibre optic cables.
12. Lay up the cable in the aircraft, either by ‘pulling through’ using the existing cable or by manually fitting the cable in. Remember to ensure that its route is that as laid down in the AMM/Wiring Manual.
13. Make sure the cable is free of kinks; not stretched; does not run close to moving objects such as retractable landing gear; does not run close to any heat source and lays correctly in its ties, conduits etc. If it is in a bundle remember to check that it is permissible for the cable to be bundled and that there are the correct number of cables in the bundle. 14. Once in position fit the end connections such as pin connectors, end
fittings etc. Fit pin connectors into the plugs using the insertion tool. 15. Carry out a continuity check (light source check for fibre optics).
16. Assemble the plugs. (Remember if any of the pins are bent they must be replaced – do not try to straighten them – they will almost certainly
fracture). Ensure lenses on fibre optic cable connectors are clean. 17. Fit plugs to LRUs.
18. Reinstate electrical power and carry out tests on the system as laid down in the AMM. These might include duplicate inspections. 19. Go back over the length of the cable and tie-up/re-clamp the cable
making sure not to over tighten and damage the cable.
20. Carry out any paperwork required which would include such things as recording details in the aircraft logbook/record card; recording details from the stores release certificate/EASA form 1 for new cable details; specifying tests carried out; clearing the aircraft for flight/signing the CRS etc.
ELECTRICAL WIRING INTERCONNECTION SYSTEM (EWIS)
Recent amendments to Certificate Specification CS25 (large aircraft) includes the addition of sub part H dealing specifically with EWIS. EWIS contains special maintenance considerations for aircraft electrical wiring systems and has been instigated to overcome shortcomings in present wiring systems and maintenance procedures the failure of which has resulted in aircraft accidents (some fatal). The requirements have been harmonised between EASA and the FAA.
It applies to any wiring/connector/system in the aircraft used to transmit electrical energy or data signals and includes:
* Wires and cables and their insulation. * Splices, conduits, shielding, braiding.
* Termination points on devices such as relays, switches, contactors, terminal blocks, circuit breakers and other circuit protection devices. * Connectors including feed-through connectors.
* Power supply wires, instrument wiring, fire detection wiring etc. * Bus bars.
* Clamps, ties, wiring support, pressure seals and labels/identification systems.
It includes all the above that might be fitted to equipment racks, shelving, distribution panels, junction boxes, circuit board back-planes etc.
EWIS does not apply to:
* Electrical or electronic equipment already qualified to environmental and testing standards acceptable to the CAA.
* Portable electrical devices not part of the aircraft type design, eg laptops and personal entertainment systems.
Each EWIS component installed must meet the following:
* Be appropriate to its intended function and installed according to its limitations specified for the EWIS component without adversely affecting the aircraft or its systems.
* Cables must:
- Be fitted such that there is no strain on any wiring, support etc. - Take into account any deformation of the structure during flight
and its possible effect on the cable run.
- Be of the correct type and size appropriate to the task.
- Be routed away from moving parts and such that the possibility of fluid contamination is minimised.
- Have minimum bend radiuses, for example:
^ For electrical cables 10 times the diameter of the cable (the largest cable if considering a bundle of cables) (standard SAE EN 3197).
^ For coaxial cables 6 times the diameter for flexible cables. For semi rigid cable 10 times (SAE AS 550881).
- Be identified by label sleeving or tags against drawings/manuals, eg WPM (Wiring Practices Manual). The use of hot stamping is discouraged (this has sometimes lead to insulation failure as the hot stamping process burns the code into the insulation and sometimes burns through to the conductor).
* Electrical drawings should comprehensively describe wire routings for the whole aircraft to include: any incompatibilities between
wires/bundles; minimum distances between routes; absolute ban on combining bundles. Identification of routes/bundles must be adequate and may include: labels, tags, placards, coloured ties and bar-codes. Wires and cables should be identified at intervals of not more than 18” (46cm). Wire manufacturers’ idents should not be at the same spacing as the aircraft manufacturers’ idents to avoid the possibility of one ident obscuring all the other idents along the run.
* EWIS components must be separated from other components (or have a barrier) so that any failure will not create a hazard.
* Each EWIS component must be labelled and its function identified. * Any interference produced will not affect personnel/aircraft systems. * The failure of one independent power source will not affect another. * Independent power sources must not share a common ground.
* All EWIS components must have adequate separation (except where they have to be close due to electrical connections etc) from:
- Fuel systems. - Oxygen systems.
- Water/waste systems. Any leaks onto EWIS components must not create a hazard.
- Fly-by-wire systems – using a 2” (51mm) gap for example. * Any hydraulic leaks/anti-icing fluid leaks etc onto EWIS components
Requirements
These apply to electrical systems and to Supplemental Type Certification installation (modifications) on a Type Certified aircraft with a maximum passenger capacity of 30 or more, or a maximum payload of 7500lbs (3402kg) or more.
Part 21 requires that an EZAP (Enhanced Zonal Analysis Procedure) be carried out by a Type Certificate holder on existing aircraft. This is wire focussed and entails a procedure to identify each aircraft zone. These are based, generally, on ATA100 (now called ATA iSpec 2200) zones and are physically based (eg; cabin floor, main spar area etc). Each zone identification must include details of:
1. Any EWIS in the zone.
2. Any zone that contain EWIS and combustible materials.
3. Proximity of EWIS to hydraulic systems and mechanical and electrical control systems. (Author’s note. Special Condition CS25 H-01 does not list oxygen, fuel and water/waste here but does include these systems in EWIS Component Installation Requirements in the same document.)
Action
Equipment must be designed as to minimise possible contamination, damage and wire degradation. Procedures must be instigated to provide regular detailed
inspections and cleaning (to be incorporated into the maintenance programme) of the EWIS:
1. For any contamination (particularly combustible material) and deterioration of systems and equipment.
2. To keep all EWIS equipment clean and free from debris and any combustible material. In the past wire bundles have been prone to become very dirty and contaminated with swarf, liquids and debris. 3. It has been found that wiring is more susceptible to damage and
degradation where it is exposed to pedestrian (maintenance personnel) traffic and special attention must be paid to these areas including cargo bays.
Inspections
The manufacturer is required to provide an inspection schedule and maintenance programme to included detailed inspections of all EWIS equipment in the form of an ICA to meet the requirements of EWIS. The inspections are to be detailed and may be in the form of a:
* DET (Detailed inspection). An intensive examination of a specific item, or assembly to include, as necessary: comprehensive cleaning of the
wiring/ equipment; the use of artificial lighting; magnification aids; mirrors etc. It may require tactile examination and the use of extensive de-paneling/elaborate access procedures etc for access purposes. * GVI (General visual inspection). An examination of a specific item or
assembly to check for obvious damage, failure or any irregularity. Not too unlike a DET but the procedure is not so detailed.
