7.2 Wiring Installation Practices, Safety Wire, and Corrosion

7.2 Wiring Installation Practices, Safety Wire, and Corrosion

Proper physical installation of avionics wiring harnesses and mechanical fasteners is critical to aircraft flight safety. Inadequate wire routing, loose hardware, or unmitigated corrosion can lead to electrical arcing, localized fires, loss of flight control telemetry, or airframe structural failure.

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Wire Harness Installation and Routing Practices

Aircraft wire bundles must be installed to withstand flight loads, engine vibration, temperature extremes, and exposure to aggressive aerospace fluids (such as Skydrol hydraulic fluid, aviation fuel, and de-icing agents).

Routing Clearances and Separation Rules

1. Fluid Line Separation: Wire bundles must be routed above fluid lines (fuel, hydraulic, water, oxygen) whenever possible. A minimum clearance of 6 inches must be maintained from control cables and fluid lines. If 6 inches of physical separation is impossible, a minimum clearance of 2 inches is permitted provided the wire bundle is securely clamped and encased in a protective non-conductive sleeve or conduit.
2. Moving Parts Clearance: Wire bundles crossing or running parallel to flight control cables, trim cables, or mechanical linkages must maintain at least 0.5 inches (12.7 mm) of clearance under maximum physical displacement.
3. Bend Radius Limits: To prevent insulation cracking and conductor strand displacement, the minimum bend radius for a tied wire bundle must not be less than 10 times the outer diameter of the bundle. For high-frequency coaxial cables, the bend radius must not be less than 6 times the cable outer diameter.

Bundling and Clamping Specifications

• Cushioned Loop Clamps (MS21919 / Adel Clamps): Wire bundles must be supported using rubber-cushioned metallic clamps. Uncushioned plastic or plain metal clamps are prohibited because vibration causes them to cut through wire insulation.
• Maximum Support Interval: Wire bundles must be clamped to airframe structure at intervals not exceeding 24 inches (610 mm) along horizontal and vertical runs.
• Harness Slack: Bundles must have sufficient slack (typically 0.5 to 1 inch of deflection between support points) to allow for structural flexing, engine movement, and maintenance disconnection of Line Replaceable Units (LRUs) without placing tension on connector pin crimps.

Drip Loops and Drainage

Where wire harnesses route downward toward an electrical connector, junction box, or LRU backshell, technicians must form a drip loop (a smooth loop extending below the connector level). Condensation, hydraulic fluid, or water running down the harness gathers at the lowest point of the drip loop and drips harmlessly away, preventing liquid entry into the connector potting or backshell assembly.

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Safety Wire (Lockwiring) Standards and Techniques

Safety wire (lockwiring) is a positive locking mechanism designed to prevent threaded mechanical fasteners, electrical connector coupling rings, and fluid fittings from backing off or loosening under intense airframe vibration.

Wire Diameter and Material Selection

Standard aviation safety wire is fabricated from high-tensile 302/304 stainless steel, brass (for emergency break-away items like hand-held fire extinguishers), or Monel.

• 0.020-inch Stainless Steel: Used on small, closely spaced fasteners, electrical connector backshells, and low-torque instruments.
• 0.032-inch Stainless Steel: The primary standard wire size used for structural bolts, engine oil drain plugs, turnbuckles, and heavy electrical connectors.
• 0.040-inch Stainless Steel: Reserved for high-strength applications, large fluid fittings, and heavy engine components.

Twist Density and Application Rules

1. Double-Twist Method: The standard method for lockwiring pairs or groups of fasteners. The wire is twisted tightly between bolt heads at a rate of 6 to 8 twists per inch (using safety wire pliers or hand twisting).
2. Direction of Pull: The safety wire must be routed through the fastener hole such that any tendency of the bolt to loosen applies additional tension to the wire, pulling the fastener into the tightening direction.
3. Pigtail Termination: Upon completing the lockwire lock, a pigtail of 1/4 to 1/2 inch (3 to 6 twists) must be formed and folded neatly inward toward the component to eliminate sharp snag hazards and technician injury risks.

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Corrosion Mechanisms in Avionics and Electrical Systems

Corrosion is the electrochemical deterioration of a metal resulting from its reaction with the surrounding environment. In aircraft electronics, corrosion increases contact resistance, causes intermittent signal dropouts, and destroys structural airframe ground paths.

Key Aircraft Corrosion Types

• Galvanic Corrosion: Occurs when two dissimilar metals are in direct electrical contact in the presence of an electrolyte (moisture, salt fog). The more active metal (anode) corrodes rapidly while protecting the nobler metal (cathode). On aircraft, connecting aluminum skin directly to stainless steel hardware without protective cadmium plating or zinc-chromate primer initiates rapid galvanic pitting.
• Pitting Corrosion: A highly localized form of aggressive corrosion that creates micro-cavities or deep pits in aluminum alloy structures. It often forms under dirt deposits or standing moisture in avionics bays.
• Fretting Corrosion: Occurs at contact interfaces subjected to slight oscillatory slip or continuous micro-vibration under pressure. In avionics pin-and-socket connectors, fretting rubs away gold or tin plating, producing fine metallic oxide debris ('black powder') that causes open circuits.

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Anti-Corrosion Treatments and Maintenance Procedures

1. Chemical Conversion Coatings: Aluminum airframe structures and equipment shelves are treated with chromate conversion coatings (e.g., Alodine 1200S or MIL-DTL-5541) to passivate the metal surface and establish a corrosion-resistant conductive base.
2. Corrosion Inhibiting Compounds (CICs): Ultra-thin hydrophobic films (such as LPS 3 or Boeshield T-9) are applied over wire harness breakouts and structural ground studs to displace moisture and block oxygen ingress.
3. Sacrificial Anodizing and Plating: Cadmium plating on steel hardware acts as a sacrificial anode, corroding preferentially to protect the underlying structural steel.

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Avionics Traps & Common Inspection Pitfalls

• Over-Twisting Safety Wire: Applying more than 8 twists per inch work-hardens stainless steel safety wire, introducing internal micro-cracks that cause the wire to snap under normal flight vibration.
• Routing Bundles Below Fluid Lines: Routing wire harnesses directly beneath hydraulic lines without a physical drip pan or protective sleeve risks soaking the harness in Skydrol fluid, degrading wire insulation.
• Abrasive Cleaning of Plated Contacts: Cleaning oxidized gold-plated or silver-plated connector pins using abrasive wire brushes or emery cloth strips away the micro-inch precious metal layer, exposing the underlying brass/copper to immediate fretting corrosion.

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Shear Wire vs Safety Wire (NCATT Safety Devices)

Safety wire (lock wire) secures fasteners against vibration loosening and is typically stainless steel in diameters from about 0.020 in to 0.041 in. Use 0.020-inch wire on electrical connectors and small screws so collar holes are not broken out; 0.032-inch wire is the general-purpose size for many equipment fasteners.

Shear wire is different: it is usually soft 0.020-inch copper or brass wire used on emergency equipment so a deliberate pull can break the wire (fire extinguisher handles, emergency exit mechanisms, and similar breakaway devices). Never substitute hard stainless safety wire where shear wire is specified—the emergency device may not break free when needed.

Avionics Trap: Using 0.032-inch stainless on a connector collar can fracture the drilled ears; using stainless where shear wire is required can disable an emergency handle.