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How to Test Aviation Cable Inductance: A Practical Guide

Accurately measuring the inductance of aviation cables is critical for ensuring the proper functioning and safety of aircraft electrical systems, particularly in high-frequency applications like communication buses, sensors, and power distribution. Inductance impacts signal integrity, voltage drop, and potential EMI. This guide outlines industry-standard methods for obtaining reliable inductance measurements on aviation cable assemblies.

​Core Method: Using an LCR Meter​

The most accurate and practical method employs a dedicated LCR (Inductance, Capacitance, Resistance) Meter.

​Step-by-Step Testing Procedure:​​

1. ​Preparation:​​
   • ​Safety First:​​ Ensure the aircraft system is completely powered down (dead system) and properly locked out/tagged out (LOTO). Disconnect the cable assembly from any connected equipment.
   • ​Identify Conductors:​​ Clearly identify the specific conductor pair (or conductor/shield pair) you need to measure within the cable assembly. Aviation cables often contain multiple conductors.
   • ​Clean Connection Points:​​ Ensure the ends of the conductors to be tested are clean, free from corrosion, oxidation, or contaminants. If necessary, lightly abrade connection points for optimal contact.
   • ​Prepare LCR Meter:​​ Turn on the LCR meter and allow it to warm up according to the manufacturer’s instructions. Ensure it’s calibrated.
2. ​Set Test Frequency (Crucial!):​​
   • ​Consult Specifications:​​ Refer to the aircraft wiring manual (AWM), relevant SAE AS standards (e.g., AS50881), or component specifications for the test frequency requirement specific to the cable’s application or qualification standard (e.g., DO-160 Section 20, MIL-STD-461 for EMI susceptibility). ​This is vital.​​
   • ​Common Frequencies:​​ Frequencies often range from 50 Hz/60 Hz (for power applications) to kHz (e.g., 1 kHz) or MHz (e.g., 1 MHz, 10 MHz) for signal integrity analysis.
   • ​Set Meter:​​ Configure the LCR meter to the specified test frequency. Incorrect frequency selection will yield invalid results.
3. ​Select Measurement Function:​​
   • Set the LCR meter to measure Inductance (L). Choose the appropriate measurement range or use auto-ranging if available.
4. ​Select Equivalent Circuit Model (Key Accuracy Factor):​​
   • LCR meters often allow selection between series (Ls) or parallel (Lp) equivalent circuit models. Choose the one specified in the relevant standard or documentation.
   • ​General Guidance:​​ For lower impedances (lower frequencies, thicker cables), the ​Series Resistance Model (Ls)​​ is typically more accurate. For higher impedances (higher frequencies), the ​Parallel Model (Lp)​​ might be better. When in doubt and if not specified, Ls is a common starting point.
5. ​Connect to the Cable:​​
   • ​Two-Terminal vs. Four-Terminal Kelvin:​​ Use the meter’s ​Four-Terminal Kelvin (4-wire) clips​ if available. This method separates the current force (HCur, LCur) and voltage sense (HPot, LPot) paths, effectively eliminating the influence of contact resistance and lead resistance at the test points, providing the highest accuracy, especially for low impedances.
   • ​Minimize Lead Length:​​ Keep the distance between the meter clips and the actual cable connection points ​extremely short. Long leads add significant parasitic inductance to the measurement.
   • ​Secure Connection:​​ Attach the HCur (or +) clip firmly to one end of the conductor under test (e.g., Pin 1). Attach the LCur (or -) clip firmly to the other end of the same conductor (e.g., Pin 2), forming a loop. Attach the HPot and LPot clips at the exact same points as HCur and LCur, respectively, but to their dedicated sockets on the meter. For the parallel/shield model, connect LCur/LPot to the shield.
6. ​Stabilize and Record:​​
   • Ensure the cable is positioned consistently – keep it straight, avoid sharp bends or coils near the connection points. Parasitic inductance is heavily influenced by cable geometry and proximity to other conductors or ground planes. While full aircraft installation conditions aren’t replicated, a straight-line configuration is a standard test setup.
   • Allow the measurement reading on the LCR meter display to stabilize.
   • ​Record the Inductance Value:​​ Note the value displayed, including the unit (typically nanohenries, nH, microhenries, µH, or millihenries, mH).
   • ​Record Test Parameters:​​ Crucially, document the ​Test Frequency​ (e.g., 1 kHz), ​Equivalent Circuit Model​ (e.g., Ls), and the ​Cable Pair/Spec​ measured.
7. ​Verification (Recommended):​​
   • Perform a “Short/Open/Zero” calibration on the meter at the test fixture ends if possible, following the instrument manual. This minimizes fixture and lead artifacts.
   • Take multiple readings to ensure consistency.
   • Reverse the clip connections on the cable pair and take another measurement. Results should be very close; significant variation indicates poor contact.

​Additional Considerations for Aviation Cable:​​

• ​Accuracy:​​ High-precision LCR meters (typically 0.05% – 0.5% accuracy) are essential for aviation-grade testing.
• ​Cable Assembly vs. Raw Cable:​​ This guide focuses on cable assemblies (wires terminated to connectors), which is typically what needs testing for installation qualification or troubleshooting. Measuring raw cable requires specialized fixtures to control geometry (e.g., separating conductors to a specified spacing).
• ​Temperature:​​ Inductance can be slightly temperature-dependent. Where critical, measurements may need to be performed in a controlled environment.
• ​Standards Compliance:​​ Testing should align with the requirements outlined in relevant aviation standards like DO-160 (Environmental Conditions and Test Procedures for Airborne Equipment), MIL-STD-202 (Electronic Component Testing), MIL-STD-461 (EMI Requirements), or specific aircraft manufacturer wiring standards (SAE AS50881, etc.). Always prioritize the specifications applicable to the cable and its system.

​Conclusion:​​

Precise inductance measurement of aviation cables is achievable using a properly calibrated LCR meter configured with the correct test frequency and equivalent circuit model. Meticulous attention to connection quality using Kelvin clips, minimizing lead length, maintaining a stable cable geometry, and accurately documenting all test parameters are paramount for obtaining reliable and actionable results critical to aircraft safety and system integrity. Always adhere to specific aircraft maintenance manuals and relevant industry standards.