7.7.2 Wiring Protection Flashcards
What is the primary function of electrical cables in modern aircraft?
Electrical cables provide the primary source of power, analogue sensor signal voltage, system data, and feedback to the aircraft systems.
How are electrical power supplies and data distributed in an aircraft?
They are distributed to various systems Line Replaceable Units (LRU), sensors, and computers via specially designed cables.
What is a key consideration for aircraft cables?
Reliability is possibly the prime consideration, as the performance and safety of the aircraft depend on the correct functioning of electrical circuits.
What must be minimized to protect aircraft cables?
The risk of mechanical damage and damage from fluids or vapours must be minimized.
What precautions should be taken regarding cable installation?
Cables must be installed and protected from heat sources and should not be sharply bent.
What is required at the ends of conduits carrying electric cables?
Ends must be prepared or bushed with insulating material to reduce the risk of damage.
How should cables connected to apparatus be arranged?
Where possible, they must run downwards from the apparatus or incorporate a download loop before entering.
What is a cable loom?
A loom, wire bundle, or harness is a group of wires tied at regular intervals to form a semi-rigid assembly.
What are the general requirements for wire bundle forming?
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.
What is the minimum radius of bends in wire groups?
The minimum radius must not be less than 10 times the outside diameter of the largest wire or cable.
What must be prevented in cable loom support?
Chafing of cables, mechanical strain, arcing or overheating, and excessive movement in high vibration areas must be prevented.
What is the difference between lacing and tying wire bundles?
Lacing secures a group of wires using continuous cord loops, while tying uses individual ties at regular intervals.
What precautions should be observed when lacing or tying wire bundles?
Bundles must be tied tightly enough to prevent slipping but not so tightly that they deform the insulation.
What is the maximum spacing for ties in high vibration areas?
In engine and pylon areas, the spacing is a maximum of two inches; in other high vibration areas, it is 6-8 inches.
What is the purpose of self-clinching cable straps?
They are used for tying and supporting cable assemblies and wire bundles.
What checks must be performed during loom assembly?
Individual wires must be checked for continuity point to point following the wiring diagram.
What is the recommended slack for wiring installations?
Wiring must have sufficient slack to prevent tension and allow for re-terminations without replacing wires.
What is important for cable support at terminations?
Cables must be secured as near as possible to the terminal fitting to guard against movement.
What are cable clamps used for?
They are used to securely clamp wire bundles to the aircraft structure.
What must be ensured when selecting clamps for wire bundles?
Clamps must have a snug fit without pinching wires and must not allow the bundle to move through the clamp.
What should be used between clamps?
Tying can be used between clamps but must not be considered as a substitute for adequate clamping.
Where must the back of the clamp be fixed?
The back of the clamp, whenever practical, must be fixed against a structural member.
Where should cable clamps be mounted?
Mount cable clamps directly to ‘Z’ members of the structure.
What should be used if the structural member is angled?
Use an angle bracket with two mounting screws if the structural member is angled.
What is the requirement for clamps on wire bundles?
Clamps on wire bundles must be selected so that they have a snug fit without pinching wires.
What must clamps on wire bundles prevent?
Clamps on wire bundles must not allow the bundle to move through the clamp when a slight axial pull is applied.
What is the requirement for clamps on RF cables?
Clamps on RF cables must fit without crushing and must be snug enough to prevent the cable from moving freely through the clamp.
What can be used to achieve a snug fit for cables?
The cable or wire bundle can be wrapped with one or more turns of electrical tape when required to achieve this fit.
What problems can metal clamps cause on co-axial RF cables?
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.
What must be installed if there is less than 3/8 in clearance?
A suitable grommet must be installed.
How should the grommet be cut for installation?
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.
Where should clamps be installed?
Clamps must be installed with their attachment hardware positioned above them wherever practicable.
What can be used to increase the size of a wire bundle?
Filler rods or plugs, a length of heat-shrink, or approved insulation tape can be used.
What is a common method of supporting cables?
A common method of supporting cables is to raise the loop clamp or ‘P’ clip away from the adjacent structure using a stand-off.
What is needed when using plastic or nylon loop clamps?
Washers are normally needed to spread the load around the mounting hole.
What is a common method of preventing chafing?
Protective strapping is useful, with spiral or ‘SPI.WRAP’ being a common method.
What is the maximum cable droop allowed?
Cable-droop of a loom between supports must not be more than 0.5 in (1.27 cm) when light pressure is applied.
How should wiring be routed to protect against battery acids?
Never route any wires below a battery and inspect wires in battery areas frequently.
What is the requirement for wires installed in aircraft bilges?
Wires and cables installed in aircraft bilges must be installed at least 6 in from the aircraft centreline.
What should be done if wire without a protective outer jacket can be soaked?
Use plastic tubing to protect it, extending past the wet area in both directions.
What must the lowest point of the tubing have?
The lowest point of the tubing must have a 1/8 in (0.32 cm) drainage hole.
What is required for wires located in wheel wells?
All wire bundles must be protected by sleeves of flexible tubing, which are securely held at each end.
What should be maintained when routing wiring parallel to plumbing lines?
Maintain as much fixed separation as possible (6 in (15.24 cm) or more).
What should be done if the separation is less than two inches from plumbing lines?
Use a nylon sleeve over the wire bundle to give further protection.
What is the maximum clearance from control cables?
Route the wiring to maintain a minimum clearance of three in from control cables.
What is the maximum bundle diameter for conduit installation?
The bundle diameter must not exceed 80% of the internal diameter of the conduit.
What is heat shrink tubing used for?
Heat shrink tubing is required to insulate wires, offering abrasion resistance and environmental protection.
What are the three main categories of heat shrink tubing?
Thin-wall, medium-wall, and thick-wall tube.
What is the common heat shrink ratio used in aircraft?
Heat shrink tubing used in aircraft is normally a 2:1 shrink ratio.
What materials is heat shrink tubing manufactured from?
Heat shrink tubing is manufactured from thermoplastic materials such as polyolefin, fluoropolymer, PVC, neoprene, silicone elastomer, or Viton.
What safety precautions should be taken when using a heat gun?
Do not operate the hot-air gun near flammable materials and fluids.
What should be ensured when using nitrogen-fueled hot-air guns?
Ensure that the workspace is properly ventilated.
What is the general procedure for heat shrink application?
Select the applicable heat shrink tubing, cut it to length, apply protection to prevent heat damage, heat the tubing, and shrink it in position.
What is self-fusing insulation tape used for?
It can be used for loom build-up for a connector cable clamp or loom protection.
What is discouraged in the use of spiral wrap (Spirap)?
The use of tube, plastic, and spiral wrap as an alternative method to prevent chafing of wiring is discouraged.
What is the purpose of cable inspections?
Cable inspections are necessary to achieve maintained and continued airworthiness.
What must be regularly inspected in wiring?
Every aspect of the wiring needs to be regularly inspected.
What are the items covered within the Installation/Inspections?
Cable/Loom routing, security, breakouts, bend radius, support and stand-offs, cleanliness, lacing and ties, clamps, conduits, and loom assembly.
