7.7.2 Wiring Protection

Question 1

What is the primary function of electrical cables in modern aircraft?

Answer 1

Electrical cables provide the primary source of power, analogue sensor signal voltage, system data, and feedback to the aircraft systems.

Question 2

How are electrical power supplies and data distributed in an aircraft?

Answer 2

They are distributed to various systems Line Replaceable Units (LRU), sensors, and computers via specially designed cables.

Question 3

What is a key consideration for aircraft cables?

Answer 3

Reliability is possibly the prime consideration, as the performance and safety of the aircraft depend on the correct functioning of electrical circuits.

Question 4

What must be minimized to protect aircraft cables?

Answer 4

The risk of mechanical damage and damage from fluids or vapours must be minimized.

Question 5

What precautions should be taken regarding cable installation?

Answer 5

Cables must be installed and protected from heat sources and should not be sharply bent.

Question 6

What is required at the ends of conduits carrying electric cables?

Answer 6

Ends must be prepared or bushed with insulating material to reduce the risk of damage.

Question 7

How should cables connected to apparatus be arranged?

Answer 7

Where possible, they must run downwards from the apparatus or incorporate a download loop before entering.

Question 8

What is a cable loom?

Answer 8

A loom, wire bundle, or harness is a group of wires tied at regular intervals to form a semi-rigid assembly.

Question 9

What are the general requirements for wire bundle forming?

Answer 9

Individual wires must lay in parallel, sufficient spare wire must be left at terminal ends, and cable breakouts must not cross over the main loom.

Question 10

What is the minimum radius of bends in wire groups?

Answer 10

The minimum radius must not be less than 10 times the outside diameter of the largest wire or cable.

Question 11

What must be prevented in cable loom support?

Answer 11

Chafing of cables, mechanical strain, arcing or overheating, and excessive movement in high vibration areas must be prevented.

Question 12

What is the difference between lacing and tying wire bundles?

Answer 12

Lacing secures a group of wires using continuous cord loops, while tying uses individual ties at regular intervals.

Question 13

What precautions should be observed when lacing or tying wire bundles?

Answer 13

Bundles must be tied tightly enough to prevent slipping but not so tightly that they deform the insulation.

Question 14

What is the maximum spacing for ties in high vibration areas?

Answer 14

In engine and pylon areas, the spacing is a maximum of two inches; in other high vibration areas, it is 6-8 inches.

Question 15

What is the purpose of self-clinching cable straps?

Answer 15

They are used for tying and supporting cable assemblies and wire bundles.

Question 16

What checks must be performed during loom assembly?

Answer 16

Individual wires must be checked for continuity point to point following the wiring diagram.

Question 17

What is the recommended slack for wiring installations?

Answer 17

Wiring must have sufficient slack to prevent tension and allow for re-terminations without replacing wires.

Question 18

What is important for cable support at terminations?

Answer 18

Cables must be secured as near as possible to the terminal fitting to guard against movement.

Question 19

What are cable clamps used for?

Answer 19

They are used to securely clamp wire bundles to the aircraft structure.

Question 20

What must be ensured when selecting clamps for wire bundles?

Answer 20

Clamps must have a snug fit without pinching wires and must not allow the bundle to move through the clamp.

Question 21

What should be used between clamps?

Answer 21

Tying can be used between clamps but must not be considered as a substitute for adequate clamping.

Question 22

Where must the back of the clamp be fixed?

Answer 22

The back of the clamp, whenever practical, must be fixed against a structural member.

Question 23

Where should cable clamps be mounted?

Answer 23

Mount cable clamps directly to ‘Z’ members of the structure.

Question 24

What should be used if the structural member is angled?

Answer 24

Use an angle bracket with two mounting screws if the structural member is angled.

Question 25

What is the requirement for clamps on wire bundles?

Answer 25

Clamps on wire bundles must be selected so that they have a snug fit without pinching wires.

Question 26

What must clamps on wire bundles prevent?

Answer 26

Clamps on wire bundles must not allow the bundle to move through the clamp when a slight axial pull is applied.

Question 27

What is the requirement for clamps on RF cables?

Answer 27

Clamps on RF cables must fit without crushing and must be snug enough to prevent the cable from moving freely through the clamp.

Question 28

What can be used to achieve a snug fit for cables?

Answer 28

The cable or wire bundle can be wrapped with one or more turns of electrical tape when required to achieve this fit.

Question 29

What problems can metal clamps cause on co-axial RF cables?

Answer 29

The use of metal clamps on co-axial RF cables can cause problems if the clamp fit is such that the RF cable’s original cross-section is distorted.

Question 30

What must be installed if there is less than 3/8 in clearance?

Answer 30

A suitable grommet must be installed.

Question 31

How should the grommet be cut for installation?

Answer 31

The grommet can be cut at a 45° angle to facilitate installation, provided it is cemented in place and the slot is located at the top of the cut-out.

Question 32

Where should clamps be installed?

Answer 32

Clamps must be installed with their attachment hardware positioned above them wherever practicable.

Question 33

What can be used to increase the size of a wire bundle?

Answer 33

Filler rods or plugs, a length of heat-shrink, or approved insulation tape can be used.

Question 34

What is a common method of supporting cables?

Answer 34

A common method of supporting cables is to raise the loop clamp or ‘P’ clip away from the adjacent structure using a stand-off.

Question 35

What is needed when using plastic or nylon loop clamps?

Answer 35

Washers are normally needed to spread the load around the mounting hole.

Question 36

What is a common method of preventing chafing?

Answer 36

Protective strapping is useful, with spiral or 'SPI.WRAP' being a common method.

Question 37

What is the maximum cable droop allowed?

Answer 37

Cable-droop of a loom between supports must not be more than 0.5 in (1.27 cm) when light pressure is applied.

Question 38

How should wiring be routed to protect against battery acids?

Answer 38

Never route any wires below a battery and inspect wires in battery areas frequently.

Question 39

What is the requirement for wires installed in aircraft bilges?

Answer 39

Wires and cables installed in aircraft bilges must be installed at least 6 in from the aircraft centreline.

Question 40

What should be done if wire without a protective outer jacket can be soaked?

Answer 40

Use plastic tubing to protect it, extending past the wet area in both directions.

Question 41

What must the lowest point of the tubing have?

Answer 41

The lowest point of the tubing must have a 1/8 in (0.32 cm) drainage hole.

Question 42

What is required for wires located in wheel wells?

Answer 42

All wire bundles must be protected by sleeves of flexible tubing, which are securely held at each end.

Question 43

What should be maintained when routing wiring parallel to plumbing lines?

Answer 43

Maintain as much fixed separation as possible (6 in (15.24 cm) or more).

Question 44

What should be done if the separation is less than two inches from plumbing lines?

Answer 44

Use a nylon sleeve over the wire bundle to give further protection.

Question 45

What is the maximum clearance from control cables?

Answer 45

Route the wiring to maintain a minimum clearance of three in from control cables.

Question 46

What is the maximum bundle diameter for conduit installation?

Answer 46

The bundle diameter must not exceed 80% of the internal diameter of the conduit.

Question 47

What is heat shrink tubing used for?

Answer 47

Heat shrink tubing is required to insulate wires, offering abrasion resistance and environmental protection.

Question 48

What are the three main categories of heat shrink tubing?

Answer 48

Thin-wall, medium-wall, and thick-wall tube.

Question 49

What is the common heat shrink ratio used in aircraft?

Answer 49

Heat shrink tubing used in aircraft is normally a 2:1 shrink ratio.

Question 50

What materials is heat shrink tubing manufactured from?

Answer 50

Heat shrink tubing is manufactured from thermoplastic materials such as polyolefin, fluoropolymer, PVC, neoprene, silicone elastomer, or Viton.

Question 51

What safety precautions should be taken when using a heat gun?

Answer 51

Do not operate the hot-air gun near flammable materials and fluids.

Question 52

What should be ensured when using nitrogen-fueled hot-air guns?

Answer 52

Ensure that the workspace is properly ventilated.

Question 53

What is the general procedure for heat shrink application?

Answer 53

Select the applicable heat shrink tubing, cut it to length, apply protection to prevent heat damage, heat the tubing, and shrink it in position.

Question 54

What is self-fusing insulation tape used for?

Answer 54

It can be used for loom build-up for a connector cable clamp or loom protection.

Question 55

What is discouraged in the use of spiral wrap (Spirap)?

Answer 55

The use of tube, plastic, and spiral wrap as an alternative method to prevent chafing of wiring is discouraged.

Question 56

What is the purpose of cable inspections?

Answer 56

Cable inspections are necessary to achieve maintained and continued airworthiness.

Question 57

What must be regularly inspected in wiring?

Answer 57

Every aspect of the wiring needs to be regularly inspected.

Question 58

What are the items covered within the Installation/Inspections?

Answer 58

Cable/Loom routing, security, breakouts, bend radius, support and stand-offs, cleanliness, lacing and ties, clamps, conduits, and loom assembly.