Wiring installation practices

Preventing damage before it starts

Aircraft wiring is expected to operate for decades while exposed to vibration, heat, moisture, pressure changes, maintenance activity and constant aircraft movement. A properly installed wire bundle may never attract attention. A poorly installed one can lead to intermittent faults, electrical arcing, smoke events, equipment failures or even fire. For that reason, wiring installation practices focus on one goal above all else: preventing damage before it starts.

The practices described in AC 43.13-1B are designed to eliminate these hazards before they become failures. Most wiring problems are not caused by the wire itself. They are caused by:

• Chafing against structure
• Excessive vibration
• Fluid contamination
• Improper support
• Poor repairs
• Tight bends
• Loose connections
• Incorrect routing

Why aircraft wiring fails

Aircraft wiring operates in a harsh environment. Over time, insulation can deteriorate due to:

• Heat
• Vibration
• Repeated flexing
• Hydraulic fluid exposure
• Fuel contamination
• Cleaning chemicals
• Moisture intrusion
• UV exposure in some areas

What failed wiring causes

A wire that rubs against structure may eventually wear through its insulation. Many electrical troubleshooting problems ultimately trace back to installation issues rather than component failures. The result can be:

• Intermittent faults
• Nuisance breaker trips
• Equipment malfunctions
• Electrical arcing
• Smoke in the cockpit or cabin
• Fire hazards

Wire selection and voltage drop

Selecting the correct wire size is critical. The conductor must be large enough to carry the required current safely and to limit voltage drop to acceptable levels. Undersized wire may overheat, create excessive voltage loss, reduce equipment performance and shorten wiring life. Two factors determine wire size:

• Current-carrying capacity: the wire must safely carry its expected electrical load without exceeding temperature limits.
• Voltage drop: every wire has some resistance, so voltage is lost along the conductor as current flows. The longer the wire and the higher the current, the greater the voltage drop.

Typical allowable voltage drops

AC 43.13-1B provides guidance for voltage-drop limitations. Keeping voltage drop within limits ensures equipment receives the power it was designed to use. Typical allowable continuous voltage drops are approximately:

• 14 V DC system: 0.5 V
• 28 V DC system: 1.0 V
• 115 V AC system: 4.0 V

Wiring routing principles

Good routing prevents future problems. A routing path that looks convenient today may create a maintenance problem years later. Whenever possible, wiring should be installed where it is protected from:

• Physical damage
• Excessive heat
• Fluid leaks
• Moving equipment
• Maintenance traffic

Avoid chafing areas

Wiring should not contact structure, control cables, hydraulic tubing, fuel lines, oxygen lines or moving mechanisms. Even slight rubbing over thousands of flight hours can wear through insulation. Protection may include:

• Clamps
• Grommets
• Conduit
• Sleeving
• Edge protection

Avoid high-heat areas

Excessive temperature accelerates insulation deterioration. Where heat exposure is unavoidable, approved heat-resistant protection should be used. Routing should avoid:

• Exhaust components
• Bleed-air ducts
• Engine hot sections
• High-temperature plumbing

Wire bundle support

Unsupported wiring vibrates continually. Over time, vibration leads to broken conductors, chafed insulation, loose terminals and connector failures.

AC 43.13-1B recommends supporting wire bundles at intervals of no more than 24 inches, unless they are installed within conduit, ducting, troughs or other approved support systems. Proper clamp spacing distributes loads evenly and prevents sagging.

Clamp selection

Only approved clamps should be used, such as cushioned Adel clamps, bundle clamps and raceway supports. Overtightening can be nearly as harmful as inadequate support. Clamps should:

• Hold bundles securely
• Avoid crushing insulation
• Prevent movement
• Protect against vibration

Separation from fluid lines

Electrical wiring and fluid systems should be separated whenever practical. Fluid leaks and electrical wiring are a poor combination.

• Recommended: where practical, maintain at least 6 inches of separation from fuel lines, oil lines, hydraulic lines and oxygen systems.
• Absolute minimum: where installation constraints exist, 2 inches may be acceptable.
• When positively clamped: if the wiring is securely clamped and protected, 1/2 inch may be permitted.

Route above fluid lines

Whenever practical, route wiring above fluid lines and avoid routing it below them. This helps prevent leaks from dripping directly onto electrical wiring and connectors.

Bend radius requirements

Excessively sharp bends damage both conductors and insulation. Gentle bends increase wire life and reduce mechanical stress. Bending can:

• Stretch insulation
• Damage shielding
• Break conductor strands
• Create hidden internal damage

Minimum bend radius

• Wire bundles: a minimum bend radius of 10 × the outside diameter of the largest wire within the bundle.
• Supported individual conductors: where properly supported, such as near terminal strips, 3 × the wire diameter may be acceptable.

Drip loops

A drip loop is a downward bend intentionally installed in wiring. Its purpose is simple: prevent fluids from traveling directly into connectors or electrical equipment. If moisture or fluid runs along the wire, it collects at the low point and drips off, so the connector stays protected. Drip loops are especially important in:

• Exterior installations
• Wheel wells
• Avionics compartments
• Moisture-prone areas

Protection from personnel damage

Aircraft wiring should never become part of the aircraft structure. It is not intended to be used as a handhold, a step, a tie-down point or equipment support. Bundles should be routed where people cannot accidentally:

• Pull them
• Step on them
• Hang tools from them
• Damage them during maintenance

Wiring separation and segregation

Not all wires should be routed together. Certain systems require separation to prevent electrical interference, signal corruption and fault propagation. Separation requirements are often specified by the aircraft manufacturer. Examples include:

• Power wiring separated from sensitive data wiring
• Ignition wiring separated from avionics circuits
• Flight-control wiring segregated from non-essential wiring

Splices

Every splice introduces additional resistance, weight, inspection points and failure opportunities. For that reason, the best splice is often the splice that was never needed. Where splices are necessary:

• Use approved splice methods.
• Follow manufacturer instructions.
• Keep splice numbers to a minimum.

Stagger splices

Multiple splices should be staggered along the bundle. A staggered bundle remains flexible and easier to support. Staggering prevents:

• Large bundle bulges
• Mechanical weak spots
• Stress concentration

Distance from connectors

As a general rule, there should be no splices within 12 inches of connectors, terminal blocks or other termination devices, unless specifically permitted by approved data or applicable guidance. Placing splices near terminals creates crowded, difficult-to-maintain installations.

Environmentally sealed splices

In areas exposed to moisture or contamination, preferred splice types include environmentally sealed splices, heat-shrink sealed splices and approved self-insulated splices. They protect against water intrusion, corrosion, fluid contamination and vibration damage. They are particularly important in:

• Wheel wells
• External structures
• Unpressurized areas
• High-moisture environments

Connector installation practices

Connectors are common failure points. Many intermittent electrical faults originate at improperly installed connectors rather than the wire itself. Proper installation includes:

• Correct pin insertion
• Proper crimping
• Correct torque
• Strain relief installation
• Environmental sealing where required

Inspection areas for technicians

Early detection prevents small issues from becoming major electrical failures. During inspections, pay close attention to:

• Chafing signs
• Flattened insulation
• Burn marks
• Loose clamps
• Missing grommets
• Corrosion
• Fluid contamination
• Broken lacing or ties
• Unsupported bundles
• Sharp bend points

Common wiring defects

• Chafing: the most common wiring defect.
• Broken clamp cushions: allow wiring movement and vibration.
• Overtightened ties: may damage insulation and conductors.
• Improper bend radius: creates stress and hidden conductor damage.
• Fluid contamination: accelerates insulation deterioration.
• Unsupported bundles: lead to vibration-related failures.

Key takeaways

• Most wiring failures begin with chafing, vibration or contamination.
• Choose wire size based on both current capacity and voltage-drop limits.
• Support wire bundles at intervals of no more than 24 inches unless otherwise protected.
• Maintain 6 inches separation from fluid lines where practical, 2 inches minimum, and 1/2 inch only when positively secured.
• Route wiring above fluid lines whenever possible.
• Use a minimum bend radius of 10 times the diameter of the largest wire in a bundle.
• Install drip loops to keep fluids away from connectors.
• Stagger splices within bundles.
• Avoid splices within 12 inches of connectors unless specifically authorized.
• Use environmentally sealed splices in moisture-prone areas.
• Never use wiring as a structural support or handhold.

The bottom line

Aircraft wiring reliability is determined as much by installation quality as by the wire itself. Proper routing, support, separation, protection and repair practices prevent the chafing, vibration damage, fluid contamination and overheating that cause many electrical failures. By following the guidance in AC 43.13-1B and the aircraft manufacturer's approved data, technicians ensure that wiring remains safe, reliable and protected throughout the aircraft's service life.