Aircraft Wiring Protection and Bundling

Wiring Protection

• When replacing wiring on an aircraft, the maintenance manual must be checked to see if the OAM has approved of any substitutions.
• If there are no approved substitutions, the OAM should be contacted for an acceptable replacement.
• Wiring must be replaced with equivalent wire when defects are found, such as chafing, brittle insulation, weather cracking, electrolyte exposure, overheating, saturation from fluids, crushing, frayed shields, or breaks in sleeves.
• Splices should not occur at less than 10-ft intervals unless specifically authorized.
• When replacing wiring or coaxial cables, identify them properly at both equipment power source ends.

Wire Bundling

• Wire bundles consist of multiple wires fastened together traveling in the same direction.
• Bundling wires into a loom and anchoring them at suitable points is helpful.
• Good practice limits wire bundles to 75 wires or 5 cm (2 inches) in diameter, whichever is smaller.
• Wires should lie parallel to each other to minimise insulation abrasion.

Bend Radii

• The minimum bend radius for wire groups must be at least 10x the outside diameter of the largest wire, except at terminal strips.
• If the wire is supported, the radius can be 3x the diameter of the wire.
• Use insulating tubing if radius requirements can't be met.
• Thermocouple wire radius should be 20x the diameter.
• RF cables must be bent at a radius of no less than 6x the outside diameter of the cable.

Slack

• Wiring should have enough slack to avoid tension, especially for wires connected to movable equipment.
• There should be enough slack at terminal lugs for two re-terminations.
• Wire groups should not deflect more than 1.2 cm (0.5 inch) between support points.
• Slack is needed to allow replacement of terminals, prevent strain on wires, and enable equipment shifting.
• A loop in the wiring provides adequate slack for equipment removal.

Cable Loom Support

• Bind and support wire bundles to prevent chafing, mechanical strain, arcing, interference, and excessive movement.
• Secure wires to bulkheads, structural members, junction boxes and panels.
• Proper support facilitates re-assembly and allows individual wire replacement.
• Wire groups are laced or tied for installation, maintenance, and inspection.
• Lacing involves continuous cord loops, while tying uses individual ties at regular intervals.
• A wire group is two or more wires tied or laced together, while a wire bundle is two or more wire groups tied or laced together.

General Precautions

• When lacing or tying, ensure bundles are tight enough to prevent slipping but not so tight as to damage insulation.
• Caution with coaxial cable due to its soft dielectric insulation.
• Do not use round cord for coaxial cable or ties inside a conduit.
• When tying behind connectors, start ties far enough back to prevent splaying of contacts.

Lacing

• Continuous lacing is only used in panels or junction boxes.
• Bundles larger than 2.5 cm (1 inch) use double-cord lacing, while smaller bundles can use single- or double-cord lacing.

Tying

• When supports for wire groups or bundles are more than 30cm (12 inches) apart, tying is necessary to maintain the shape of the loom.
• Spot tie spacing is dependent on loom diameter.
• Looms 0.6 cm (0.25 inch) or less require ties every 1.2 cm (0.5 inch)
• Looms 1.2 cm (0.5 inch) or less require ties every 4 cm (1.5 inches)
• Looms 2.5 cm (1 inch) or less require ties every 5 cm (2 inches)
• Looms greater than 2.5 cm (1 inch) require ties every 7.5 cm (3 inches)

Making Ties

• Wrap cord around the wire group or bundle.
• Make a clove hitch, followed by a square knot with an extra loop.
• Trim the ends of the cord to 1 cm (3/8 of an inch) minimum.

Self-Clinching Cable Straps

• Self-clinching cable straps are adjustable plastic straps used for tying wire bundles.
• MS3367 straps are plain, while MS3368 straps have a flat surface for identification.
• Do not use these cable straps: Where temperatures exceed 85°C, where failure would permit movement, to tie wire groups or harnesses within bundles, or where coaxial cable is present.
• Use a Military Standard hand tool to install and cut the strap flush to prevent injuries.

Wire Bundles

• Must be securely clamped to aircraft structure using cable clamps.
• Clamps and primary support devices must be compatible with their environment.
• Clamps should be spaced no more than 60 cm (24 inches) apart.
• Clamps lined with non-metallic material should support the wire bundle; tying is not a substitute for adequate clamping.
• The back of the clamp should rest against a structural member.
• Cable clamps can be mounted directly to Z members or use an angle bracket for angled members.
• Select clamps for a snug fit without pinching wires.
• Prevent the bundle from moving through, but allow restricted movement for RF cables.
• If there is less than 1 cm (3/8 of an inch) clearance, install a grommet at the bulkhead cutout.
• Mount clamps with hardware above them to prevent rotation.

Protection Against

• Chafing
• High temperatures
• Contact with personnel and cargo

General Precautions

• Conduit must be used if structure is not adequate.
• Do not route wires below a battery, inspect battery area wires, and replace discoloured wires.
• Do not subject wires to fluids.
• Install wires in aircraft bilges at least 6 in. from the centreline.

Tubing

• Use plastic tubing to protect wire without a jacket, extending past wet areas and draining with a 1/8-in. hole.

Wire Protection in Wheel Wells

• Flexible tubing sleeves must be used to protect wire bundles in wheel wells.

Separation From Plumbing Lines

• When routing parallel to combustible fluid or oxygen lines, maintain at least 6 in. separation.
• Route wires above plumbing lines.
• Clamp both wire bundle and plumbing line if 6 in. separation is not possible.
• Use a nylon sleeve if separation is less than 3 cm (2 inches) but more than 1.2 cm (0.5 inch).
• Maintain a minimum clearance of 7.5 cm (3 inches) from control cables or install mechanical guards.

Wire Protection in Conduit

• Bundle diameter must not exceed 80% of the conduit's internal diameter.
• No ties or splices are allowed inside a conduit.

Heat-Shrink Tubing

• Heat shrink tubing shrinks when heated and is used for insulation and protection, with 2:1 shrink ratio being common in aircraft.
• It is made from thermoplastic materials like polyolefin or fluoropolymer.

Hot Air Gun

• Select a hot air gun according to AMMs.
• Use extreme caution when operating a hot air gun to avoid explosions and burn injuries.
• Ensure adequate ventilation when using nitrogen with a heat gun.
• Use electrical power from a ground power unit (GPU).

Hot Air Generators

• Use hot air generators with reflectors according to instructions.
• Use equipment with the correct hot air temperature for the shrinkable sleeve.
• Use a continuously adjustable hot air temperature and contact protection.
• Ensure the heating equipment has a nozzle attachment.

Selecting Heat-Shrink Tubing

• Choose tubing that can be easily moved and held in position.
• Overlap the wire or cable insulation by 0.5 cm to 1.2 cm (0.2 to 0.5 inch).

Cutting Heat-Shrink Tubing

• Cut tubing with sufficient length, adding a maximum 10% for longitudinal shrinkage.
• Ensure cutting edges are smooth.
• Use split Teflon tubing or fish paper to prevent heat damage. The end of the protection touches the end of the heat-shrink tubing
• The protection extends a minimum 2.5 cm (1 inch) from the end of the heat-shrink tubing.

Applying Heat-Shrink

• Allow the heat gun to run for 5 minutes to allow the temperature to stabilise before use.
• Hold the gun 7.4 cm (3 inches) minimum from the tubing.
• Point the gun at the centre of the tubing for 5 to 10 seconds.
• Do not apply the heat for more than 20 seconds at any one time.
• If the heat-shrink tubing does not fully shrink: -Wait for 5 minutes minimum to allow the cable to cool. -Shrink the heat-shrink tubing into position again.

Solder Sleeves

• Solder sleeves attach a ground wire to the braid of a shielded cable.
• When heated, the solder melts and flows, connecting the ground lead to the shield.

Tape

• Stretched tape has a self-fusing property.
• It may be used for loom build-up, connector cable clamp or for loom protection.

Spirap Tubing

• Spirap tubing is spiral wrapping used to protect wiring from abrasion, but is discouraged.
• Spirap is easily applied and removed, and is secured with cable ties.