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How to Test Aviation Cable for Conductivity: A Critical Maintenance Procedure

Ensuring the electrical conductivity of aviation cables is not just a maintenance task; it’s a fundamental requirement for flight safety and operational reliability. Faulty or degraded cables can lead to intermittent signals, system failures, or even catastrophic events. This guide details the essential steps and best practices for accurately testing the conductivity of aviation cables.

Why Conductivity Testing is Paramount in Aviation

Aviation cables are the nervous system of an aircraft, carrying critical signals for flight controls, navigation, communication, sensors, and power distribution. Reduced conductivity, often caused by corrosion, physical damage (kinking, crushing), loose connections, or internal wire strand breakage, increases electrical resistance. This can result in:

• ​Erratic System Behavior: Faulty sensor readings (e.g., fuel quantity, engine parameters).
• ​System Failures: Loss of communication, navigation, or control signals.
• ​Voltage Drops: Insufficient power reaching essential components.
• ​Overheating: Increased resistance can generate heat at connection points or damaged sections, posing a fire risk.

Regular conductivity testing is mandated by maintenance schedules and is crucial during troubleshooting and after any repair or modification involving wiring.

Essential Tools for Accurate Testing

1. ​Digital Multimeter (DMM): A basic tool for initial checks and continuity testing. Choose one with good resolution.
2. ​Digital Low-Resistance Ohmmeter (DLRO) / Microhmmeter: ​This is the industry-standard tool for precise conductivity testing. Unlike standard DMMs, DLROs use a 4-wire (Kelvin) measurement technique and can apply a significant test current (often several Amps). This allows them to accurately measure very low resistances (microhms or milliohms) typical of aircraft cables and connections, overcoming the lead resistance errors common with 2-wire DMMs.
3. ​High-Quality Test Leads & Probes: Ensure leads are in good condition. Kelvin clips (four separate probes: two for current injection, two for voltage sensing) are essential for DLRO measurements.
4. ​Cleaning Supplies: Isopropyl alcohol and lint-free wipes for cleaning connection points.
5. ​Aircraft Wiring Diagrams (AWM): Essential for identifying cable types, routing, connection points, and allowable resistance values.
6. ​Maintenance Manuals: Provide specific procedures, test points, and acceptance criteria for the aircraft type.

Step-by-Step Guide to Testing Aviation Cable Conductivity

1. Preparation is Key:

• ​Safety First: Ensure the aircraft is in a safe state – typically, the relevant system circuit breaker pulled and/or the aircraft electrically powered down (“cold and dark”). Follow all Lock Out / Tag Out (LOTO) procedures. Verify no residual voltage is present using a DMM.
• ​Identify the Cable: Locate the specific cable segment to be tested using the Aircraft Wiring Manual (AWM). Identify accessible test points (connector pins, terminal lugs, splice points).
• ​Gather Documentation: Have the AWM and relevant maintenance manual section readily available.
• ​Prepare Test Points: ​Crucially, disconnect the cable at both ends. This isolates the cable segment from the aircraft’s systems, preventing parallel current paths that would give false low resistance readings. Clean the contact surfaces at the test points thoroughly with isopropyl alcohol and lint-free wipes to remove oxidation, grease, or contaminants that could add resistance.
• ​Prepare Equipment: Set up your DLRO according to the manufacturer’s instructions. Ensure it’s calibrated. Connect Kelvin clips if using.

2. Performing the Measurement (Using a DLRO):

• ​Connect Kelvin Clips: Attach the DLRO’s Kelvin clips firmly and directly to the metal conductors at the prepared test points on the isolated cable segment. Ensure:
  • The ​Current (I+ and I-) leads/clips are placed outside the ​Voltage Sense (V+ and V-) leads/clips on the same conductor end.
  • There is good metal-to-metal contact. Avoid clipping onto insulation or corroded areas.
• ​Apply Test Current: Initiate the measurement on the DLRO. The instrument will inject a known test current through the cable via the outer (I+ and I-) leads and precisely measure the voltage drop across the cable segment using the inner (V+ and V-) leads. This 4-wire method eliminates the resistance of the test leads themselves from the measurement.
• ​Record the Reading: Note the stable resistance value displayed by the DLRO. Record the value, the date, the test equipment used, the specific cable identifier, and the test points.

3. Interpretation and Acceptance:

• ​Compare to Specifications: Consult the aircraft maintenance manual or component maintenance manual (CMM) for the specific cable type and length. These documents provide the maximum allowable resistance per unit length (e.g., milliohms per foot/meter) or a maximum total resistance for a given run.
• ​Evaluate: If the measured resistance is significantly higher than the specified limit (considering the cable length), the cable segment has degraded conductivity and likely needs replacement. Even resistance values approaching the limit warrant investigation and possible replacement, especially for critical systems.
• ​Continuity Check (DMM): While a DLRO measures resistance, a DMM set to the continuity (beeper) function is useful for a quick “go/no-go” check to confirm the conductor isn’t completely open. However, it cannot reliably detect partial degradation or high resistance that a DLRO can find.

Critical Considerations for Aviation Cable Testing

• ​Temperature: Electrical resistance increases with temperature. If testing in an environment significantly different from standard conditions (usually 20°C/68°F), temperature compensation might be necessary. Refer to equipment manuals.
• ​Cable Length: Resistance is proportional to length. Always consider the length of the segment being tested when comparing to per-unit-length specifications.
• ​Connection Quality: The measured resistance includes the resistance of the connection points (lugs, pins). Ensure connections are clean and tight. A significant portion of the measured resistance might originate from a poor connection rather than the cable itself.
• ​Parallel Paths: ​Always disconnect both ends. Failure to do so is the most common error, leading to falsely low resistance readings as current finds alternative paths through the aircraft’s structure or other connected components.
• ​Follow Approved Data: Strictly adhere to the procedures and limits outlined in the aircraft manufacturer’s maintenance manuals and relevant standards (e.g., SAE AS4373, MIL-DTL-5015).

Conclusion

Accurately testing the conductivity of aviation cables using proper techniques and calibrated equipment, especially a DLRO with Kelvin connections, is a non-negotiable aspect of aircraft maintenance. By isolating the cable, ensuring clean connections, and comparing measured resistance against stringent specifications, technicians can identify degraded cables before they compromise system performance or flight safety. Consistent adherence to these procedures ensures the integrity of the aircraft’s electrical system, directly contributing to safe and reliable operations. Always prioritize using the correct tools and following approved maintenance documentation.