6.3 Electrical Bonding, Grounding Resistance & Electromagnetic Shielding

6.3 Electrical Bonding, Grounding Resistance & Electromagnetic Shielding

Approved-Data Control

The figures and hardware examples in this section illustrate principles. For an actual aircraft or component, current approved maintenance data, product instructions, organisation procedures, and applicable law control the material, limit, interval, sequence, tooling, PPE, and acceptance decision.

Modern transport aircraft operate in an intensely hostile electromagnetic environment. An aircraft cruising at 500 knots continually collides with airborne particulates, ice crystals, and rain droplets, generating massive electrostatic surface charges. Simultaneously, the airframe must withstand direct lightning strikes delivering up to 200,000 amperes of current, while sensitive flight-control microprocessors and digital avionics must remain immune to High-Intensity Radiated Fields (HIRF) and internal Electromagnetic Interference (EMI). EASA Part-66 Module 7 requires maintenance engineers to understand the physical principles, resistance thresholds, installation standards, and testing procedures for electrical bonding, grounding, and shielding.

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Purposes of Electrical Bonding & Grounding in Aviation

Although frequently used interchangeably, "grounding" and "bonding" describe two distinct aeronautical functions:

• Grounding (Earthing): The intentional electrical connection of an active electrical circuit to the metallic airframe structure or dedicated ground network. The airframe serves as a common, low-impedance power return path for single-wire DC systems (28 V DC) and 3-phase AC systems (115 V AC neutral return), eliminating the weight of dedicated return conductors.
• Bonding: The mechanical creation of a low-resistance electrical connection between adjacent metallic or conductive composite structural parts, control surfaces, plumbing, or equipment chassis that are not intended to carry normal operational current.

The Four Primary Functions of Bonding

1. Lightning Strike Protection: Direct lightning strikes inject extreme energy pulses (up to 200 kA during Zone 1 attachments). Low-resistance bonding paths allow this massive current to enter the airframe (e.g., nose radome or wingtip) and traverse external skins and structural hinges to exit points (trailing edge wicks or empennage) without arcing, explosive resistive heating, or fuel vapor ignition.
2. Electrostatic (P-Static) Charge Dissipation: Friction between the airframe and airborne dust, rain, or snow strips electrons, charging the aircraft to hundreds of thousands of volts. Bonding bridges control surfaces, access panels, and fuel pipes into a single equipotential structure, preventing spark discharges that could ignite fuel or disrupt avionics.
3. Power Return Path Integrity: Ensuring that current returning through structure encounters minimal resistance, preventing localized voltage drops and ground loops that distort sensitive sensor signals.
4. HIRF & EMI Shielding: Bonding creates an equipotential Faraday cage around passenger compartments and electronics bays, shielding fly-by-wire computers against external radar, radio transmitters, and internal motor switching noise.

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Bonding Jumpers: Materials, Types & Installations

Bonding jumpers bridge mechanical hinges, flexible shock mounts, and structural joints:

• Braided Jumpers: Flat, woven straps manufactured from high-purity tinned copper or aluminium alloy wires. Woven braids provide exceptional mechanical flexibility and high surface area, minimizing high-frequency inductive reactance ($X_L = 2\pi f L$) during fast-rise lightning strikes. Used across moving control surfaces (ailerons, elevators, rudders), engine mounts, and landing gear pivot joints.
• Solid Metal Jumpers: Stamped aluminium or copper strips used across stationary structural joints and equipment mounting racks.
• Installation Rules: Jumpers must be kept as short as practicable (typically under 4 inches / 100 mm) to minimize inductance. Jumpers must not be looped or bent sharply, must not rub against adjacent structure during full control surface travel, and must never be installed inside fuel tank vapor zones unless specifically certified as CDCCL items.

Bonding Jumper Installation Geometry:
[Structure A] <--- [Short, Flat Braided Jumper] ---> [Structure B (Hinged)]
   (Zero Sharp Bends; Low Inductive Reactance; Sealed Lug Interfaces)

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Dissimilar Metal Galvanic Corrosion Prevention

When two dissimilar metals contact each other in the presence of an electrolyte (moisture, condensation, salt spray), an electrochemical galvanic cell is established. The more active metal (anode) undergoes accelerated corrosion, while the more noble metal (cathode) remains protected.

The Aerospace Galvanic Hierarchy

• Anodic (Most Active / Corrodes Fast): Magnesium, Zinc, Aluminium Alloys (2024, 7075).
• Intermediate: Cadmium plating, Carbon Steel, Tin, Lead.
• Cathodic (Most Noble / Protected): Nickel, Stainless Steel, Titanium, Copper Alloys, Carbon-Fiber Reinforced Polymer (CFRP).

Aerospace Galvanic Series (Anodic to Cathodic):
[Magnesium / Aluminium] ===> [Cadmium / Tin] ===> [Nickel / Stainless Steel] ===> [Copper / CFRP]
       (Anode: Corrodes)                                             (Cathode: Protected)

Practical Mitigation Rules in Bonding

1. Aluminium to Copper: Bare copper must NEVER contact bare aluminium directly; the aluminium will rapidly corrode into white aluminium oxide powder. Copper bonding lugs must be electro-tinned or nickel-plated, and installed with sacrificial cadmium-plated steel or aluminium washers.
2. Carbon-Fiber Composites (CFRP): Carbon fiber is electrically conductive and electrochemically extremely noble (similar to titanium). Direct attachment of aluminium bonding lugs to CFRP results in rapid aluminium destruction. Fasteners in CFRP must be titanium, A286 stainless steel, or Monel, and bonding points must incorporate co-cured bronze or nickel mesh.
3. Perimeter Fillet Sealing: After assembly and torquing, technicians must apply an approved polysulfide sealant (e.g., PR-1422 or PR-1440) completely around the outer perimeter of the bonding terminal. The sealant acts as an environmental barrier, excluding moisture while leaving the center fastener accessible for milliohmmeter testing.

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Surface Preparation & Installation Standards

Paint, anodized coatings, and structural primers are dielectric insulators. A secure bonding connection demands rigorous surface preparation:

1. Paint and Anodize Removal: The bonding contact area must be cleaned down to bare, shiny metal using non-metallic abrasive pads (e.g., Scotch-Brite maroon/grey) or approved chemical paint strippers. Carbon steel wire brushes and coarse emery cloth are strictly prohibited; they embed microscopic iron particles into aluminium, triggering severe pitting corrosion.
2. Chemical Conversion Coating (Alodine): Cleaned bare aluminium oxidizes within minutes, forming an insulating oxide skin. Immediately after cleaning, bare aluminium must be treated with a conductive chemical conversion coating conforming to MIL-DTL-5541 (such as Alodine 1200 or 1132). Alodine forms a microscopic chromate film that provides long-term corrosion protection while preserving ultra-low electrical contact resistance.
3. Hardware Assembly Stack-Up: Hardware must be stacked in the exact sequence specified by the AMM:
   • Structure -> Bonding Lug -> Flat Washer -> Lock Washer -> Nut/Bolt.
   • Washers must never be placed between the bonding lug and the bare airframe structure; the lug must make direct face-to-face contact with the prepared structural surface.
4. Fastener Torquing: Fasteners must be tightened to specific engineering torque limits using calibrated torque wrenches to ensure adequate contact pressure without stripping threads or warping the lug.

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Bonding Resistance Regulatory Thresholds

Bonding resistance cannot be measured with standard two-wire digital multimeters. Standard test leads introduce 0.1 to 0.5 Ω of lead and contact resistance—far too high to verify milliohm thresholds. Technicians must use a calibrated four-wire Kelvin bridge milliohmmeter (e.g., Megger BT51 or Avtron bonding meter), which separates current injection from voltage measurement to measure true resistance down to micro-ohms.

Illustrative Bonding Values — Approved Aircraft Limits Control

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Static Discharge Wicks: Operation & Maintenance

As an aircraft flies through precipitation, ice crystals, or dust, friction strips electrons, building electrostatic potentials up to 500,000 V on the airframe. If unmanaged, this charge discharges intermittently from sharp wingtips or antenna masts as energetic electrical arcs. These micro-sparks produce broad-spectrum radio frequency noise (P-static) that completely blinds VHF communications, HF radios, VOR, and ILS receivers.

Corona Discharge Mechanism

Static discharge wicks—installed along the sharp trailing edges of wings, ailerons, elevators, and vertical stabilizers—dissipate static charge continuously and quietly:

• Wicks feature micro-fine discharge needles or carbon tufts.
• The extremely sharp geometry concentrates the electrostatic field gradient.
• The intense electric field ionizes surrounding air molecules, producing a continuous, harmless, silent corona discharge that drains excess electrons into the slipstream without generating radio-frequency sparks.

Construction and Resistance Verification

• Internal Construction: A static wick consists of a high-resistance carbon-impregnated fiber core (typically 6 to 100 MΩ resistance) enclosed within a protective fiberglass sleeve, terminating in stainless steel discharge pins. The high resistance acts as an RF choke, dampening high-frequency oscillations and preventing the wick itself from acting as an active radiating antenna.
• Maintenance & Resistance Testing:
  • Technicians visually inspect wicks for physical erosion, lightning strike burn-off (melted tips), missing needles, and secure base mounting.
  • Electrical testing is conducted using a calibrated high-voltage megohmmeter (typically 500 V DC test potential). Resistance measured from the discharge tip to the aircraft structural mounting base must fall within manufacturer limits—typically between 6 MΩ and 100 MΩ.
  • Both a dead short (0 Ω) and an open circuit (infinite resistance) represent unserviceable conditions requiring immediate wick replacement.

Static Wick Operating Principle:
[Airframe Negative Potential] ===> [6-100 MΩ Carbon Core] ===> [Sharp Needle Tip] ===> (Silent Corona Discharge into Slipstream)
                                            |                                      (Zero Broadband RF Noise)
                                            +--- RF Damping Choke

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Coaxial Cable Construction, Impedance & Handling

Radio frequency (RF) signals for VHF, GPS, TCAS, weather radar, and satellite communication travel through precision coaxial cables.

Concentric Construction Layers

1. Center Conductor: Solid or stranded silver-plated copper (or copper-clad steel for mechanical strength).
2. Dielectric Insulator: Precision solid PTFE, foamed polyethylene, or tape-wrapped fluoropolymer that holds the center conductor precisely centered.
3. Outer Shield: High-density woven braid of silver-plated copper wire (single or double braid) providing the signal return path and 100% Faraday shielding against EMI and lightning transients.
4. Protective Outer Jacket: Extruded FEP, ETFE, or polyurethane protecting against abrasion, UV, and hydraulic fluids.

Characteristic Impedance ($Z_0$)

Aviation avionics RF systems operate strictly at 50 Ω standard characteristic impedance (e.g., RG-58, RG-142, RG-400, and low-loss foam cables like Times Microwave LMR series). Impedance is determined entirely by the geometry and dielectric constant:

$$Z_0 \approx \frac{138}{\sqrt{\varepsilon_r}} \log_{10}\left(\frac{D}{d}\right)$$

where $D$ is the inner diameter of the outer shield, $d$ is the outer diameter of the center conductor, and $\varepsilon_r$ is the relative dielectric constant.

Handling Precautions & Dielectric Crushing

• Minimum Bend Radius: The minimum bend radius for RF coaxial cable is 6 to 10 times the outside cable diameter (6x to 10x OD).
• Dielectric Crushing Hazard: If a coaxial cable is bent too sharply or compressed by an over-torqued cable clamp, the soft dielectric is permanently crushed, displacing the center conductor off-center. This alters the $D/d$ ratio, creating a localized impedance discontinuity. This discontinuity reflects transmitted RF energy back toward the transmitter, creating a high Voltage Standing Wave Ratio (VSWR). High VSWR results in severe signal attenuation, receiver sensitivity loss, and can destroy the final power amplifier transistors in weather radar or transponder units.
• RF Connectors: Connectors (BNC, TNC, N-Type, SMA) must have their outer shield braid crimped or clamped 360° around the backshell to ensure continuous RF shielding.

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Realistic Maintenance Scenario & Common Exam Traps

Realistic Maintenance Scenario

Following a reported lightning strike on a Boeing 737, the certifying engineer inspects the empennage. The technician identifies an arc attachment point on the left elevator trailing edge where a static wick base had partially melted.

Using a four-wire Kelvin bridge milliohmmeter, the engineer tests the elevator hinge bonding jumpers to the horizontal stabilizer structure: the jumper resistance reads 0.008 Ω while the applicable task for this hypothetical installation states a 0.003 Ω limit. The technician removes the jumper, strips the attachment pads down to bare metal using Scotch-Brite, treats the bare aluminium with Alodine 1200, installs a new braided tinned-copper jumper, and torques the cadmium-plated fasteners. Re-testing confirms a resistance of 0.0018 Ω (pass). After sealing the terminal perimeters with polysulfide sealant, the engineer installs a replacement 100 MΩ static wick and verifies its base bonding.

Common Exam Traps

• Trap 1: Attempting to measure lightning bonding with a standard multimeter. Standard two-wire multimeters are generally unsuitable for very low bond-resistance measurement because lead resistance alone is typically 0.2 Ω. A four-wire Kelvin milliohmmeter is mandatory.
• Trap 2: Believing a static wick should have zero resistance. Technicians often incorrectly assume that a good static wick should read 0 Ω. In reality, a static wick must have a high internal resistance (6 to 100 MΩ) to suppress RF noise. A 0 Ω reading indicates a defective, unserviceable wick.
• Trap 3: Using carbon steel wire brushes to clean aluminium bonding surfaces. Steel bristles leave ferrous particles embedded in aluminium that trigger rapid galvanic pitting corrosion. Only non-metallic abrasive pads (e.g., Scotch-Brite) may be used.