Waypoint A.6 covers the practical use of your diagnostic tools: Measurement and Troubleshooting Using Meters. According to the syllabus, we focus on Voltage, Current, Resistance, Continuity, Insulation, and Loop Impedance.

First, Voltage, Current, and Resistance. Ensure your meter is set correctly. Measuring voltage while the leads are plugged into the “Amps” port will blow the internal fuse—or worse, damage the aircraft circuit. Always start with the highest range and work down.

Continuity checks tell us if a wire is broken. But be careful: a single strand of wire touching a terminal will show continuity (beep), but it won’t carry the current required to drive a pump. A continuity check is not a load test. For power circuits, you may need to verify voltage drop under load.

Insulation testing, often called “Megging,” checks for shorts to ground or between wires. This uses high voltage (often 500V or more). Warning: Never use a Megger on a circuit connected to electronic LRUs or computers. You will fry the internal logic chips. You must disconnect both ends of the harness before insulation testing. If you are unsure what is on the other end, do not Meg it.

Short to ground is a common fault where insulation rubs through against the airframe. When troubleshooting this, isolate the segment. If the fuse pops immediately, you have a hard short. If it’s intermittent, look for areas of vibration or movement (like wheel wells).

Finally, Loop Impedance. This is critical for bonding checks. We measure the resistance between a component case and the airframe structure. This value must be very low (typically milliohms) to ensure lightning strikes and static build-up can safely dissipate. Standard multimeters are not accurate enough for this; you need a Milliohm meter (bonding tester). High resistance here means a risk of fire or EMI issues.

Know your meter. Trust your readings, but verify your setup