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Aviation Cable in Aircraft Fuel Systems: Essential Practices for Safety & Reliability

Aviation cable is the critical nervous system within an aircraft’s fuel system, carrying vital electrical signals for gauging, pump control, and safety warnings. Using the wrong cable or improper installation can lead to malfunctions, inaccurate readings, or even fire hazards. Here’s what you need to know to ensure these systems operate safely and reliably:

1. Selecting the Right Cable is Paramount

• ​Fuel Resistance is Non-Negotiable: Cables must be specifically designed and certified for use in fuel environments. Look for types like ​MIL-DTL-27500 Tefzel-insulated cable or equivalents meeting ​SAE AS22759 standards. These use insulation (like ETFE) and jacketing resistant to aviation fuels (Jet A, Avgas), oils, and fluids.
• ​Temperature Rating: Cables must withstand the full operating temperature range encountered within the fuel tank or along fuel lines, including potential high temperatures near engines or in hot climates.
• ​Voltage Rating: Ensure the cable’s voltage rating exceeds the system’s maximum operating voltage.
• ​Shielding: Shielded cable is often required to protect sensitive fuel quantity signals from electromagnetic interference (EMI) that could cause erratic gauge readings. Braided shields are common.

2. Correct Installation Prevents Failures

• ​Bending Radius: Strictly adhere to the manufacturer’s specified minimum bending radius (often 10 times the cable diameter). Sharp bends damage insulation and conductors.
• ​Chafe Protection: Where cables pass through bulkheads, conduits, or near structural elements, use adequate grommets, loom, or chafe guards to prevent abrasion over time.
• ​Secure Routing: Clamp cables securely at regular intervals using cushioned clamps compatible with the fuel environment. Prevent sagging or vibration that can lead to fatigue.
• ​Separation: Keep fuel system wiring physically separated from high-voltage ignition wiring or other potential sources of EMI whenever possible. Follow aircraft-specific wiring diagrams meticulously.
• ​Grounding/Bonding: Ensure proper grounding of cable shields as specified in the aircraft maintenance manual (AMM) to drain static charges and maintain shield effectiveness. Use proper bonding straps if required.

3. Inspection & Maintenance are Critical

• ​Regular Visual Checks: During routine maintenance, inspect accessible sections of fuel system wiring for:
  • ​Abrasion or Chafing: Look for worn insulation or exposed conductors.
  • ​Cracking or Hardening: Insulation can become brittle over time, especially if exposed to excessive heat or incompatible fluids.
  • ​Kinks or Sharp Bends: Damage from improper handling.
  • ​Swelling or Softening: Indicates chemical degradation from fuel exposure (wrong cable type).
  • ​Corroded Terminations: Check connections at probes, sensors, and connectors.
• ​Continuity and Insulation Resistance (IR) Testing: Periodically perform electrical tests:
  • ​Continuity: Verify unbroken electrical paths.
  • ​Insulation Resistance (Megger Test): Measure resistance between conductors and between conductors and ground/shield. Low IR indicates moisture ingress or insulation breakdown. Follow AMM procedures and safety precautions strictly (fuel tanks must be purged!).

4. Replacement Best Practices

• ​Use Approved Parts Only: Replace with cable meeting the exact same specification (MIL-DTL-27500, SAE AS22759, etc.) and part number as originally installed or as approved by the aircraft manufacturer/regulatory authority.
• ​Match Length and Routing: Replace with cable of the same length and follow the exact original routing path.
• ​Proper Termination: Use the correct connectors, terminals, and tools. Ensure splices (if unavoidable and permitted) are done with approved methods (solder sleeves, crimps) and adequately protected.
• ​Sealing: Ensure all connectors and penetrations into fuel tanks are sealed according to specifications to prevent fuel leaks or vapor ingress.

Key Maintenance Takeaways:

1. ​Never Substitute: Standard aircraft wire is NOT suitable for immersion in fuel. Always use fuel-resistant cable.
2. ​Handle with Care: Avoid kinking, crushing, or stretching during installation or maintenance.
3. ​Inspect Religiously: Make wiring inspection a core part of fuel system checks.
4. ​Test Periodically: Conduct electrical tests to catch hidden degradation.
5. ​Document Work: Record all inspections, tests, and replacements meticulously in the aircraft logs.

By strictly adhering to these selection, installation, inspection, and replacement guidelines, you ensure the aviation cable within your aircraft’s fuel system performs reliably, providing accurate data and contributing significantly to the overall safety of the aircraft. Always consult the specific Aircraft Maintenance Manual (AMM) for detailed procedures and requirements.