Wiring checks are a vital part of aircraft maintenance. Electrical faults can often be elusive and tricky to pinpoint, especially when they cause intermittent problems that do not appear consistently.
Whether you are investigating an issue in a flight control computer or diagnosing unexpected behaviour in a hydraulic power control unit, thorough wiring checks help ensure everything is working safely and reliably as well as highlight faulty components.
To make this process easier to understand, it can help to think of a simple everyday analogy: imagine a mouse connected to a laptop. The mouse represents the power control unit (PCU) and the laptop acts as the flight control computer.
In this relationship the laptop provides power to the mouse to operate and the mouse provides feedback discrete data to the laptop to tell it what is happening. Just like the mouse needs a reliable connection and power from the laptop to work, aircraft systems depend on proper wiring to function correctly.
How to perform a voltage check on aircraft wiring
The first test you should perform when troubleshooting aircraft wiring is a voltage check. This is the simplest way to confirm that power is actually reaching the component you are testing. If there’s no power to the component, chances are the issue lies elsewhere. To carry out this test, you need to set your multimeter correctly to measure voltage.
Most aircraft components operate on DC voltage, so your multimeter will often be set to measure DC volts. If your multimeter has manual range settings, select the range appropriate for the expected voltage level.
Many modern multimeters are auto-ranging, meaning they will automatically detect and display the correct voltage range. However, it is important to ensure the meter is on the right mode before testing.
Note: While most aircraft systems use DC power, some components require AC voltage. These include AC motor-driven pumps, transformer-rectifier units, and certain galley equipment for example, ovens. For these components, you will need to switch your multimeter to AC voltage mode before testing.
When placing your test leads on the relevant terminals or wiring, watch the display for the expected voltage value. The reading should be steady and stable, indicating that power is present and flowing without interruption. If the voltage is missing or fluctuating, it suggests a power supply problem or a break in the wiring.
Think of this test like checking whether a laptop’s USB port is delivering power to a mouse. If the power isn’t steady or isn’t there at all, the mouse won’t function properly.
Performing this voltage check early in your troubleshooting process saves time by quickly identifying whether the component has power. It also helps narrow down the location of any faults, ensuring that the systems you test meet their basic power requirements before moving on to more detailed checks.
Below are images showing how to set up a multimeter for both DC and AC voltage measurements. The first image illustrates the multimeter configured for DC voltage, while the second shows the setup for AC voltage testing.
Aircraft wiring continuity testing: Step-by-step guide
If the voltage is present and correct, but the system still isn’t functioning as expected, the next step is to perform continuity tests on the wiring. Continuity testing verifies that the wires themselves are intact and that there are no breaks, open circuits, or unintended shorts between conductors.
Voltage and discrete data will be sent via different wires. So, although we have confirmed that the voltage is reaching the mouse, this doesn’t necessarily mean the discrete data being sent to the laptop from the mouse is reaching it.
To conduct a continuity test, set your multimeter to continuity mode. This mode is usually indicated by a symbol such as a speaker or a diode. When you place the test leads at either end of a wire and the circuit is complete, the multimeter emits a beep.
This audible signal makes it easy to identify whether the wire is continuous. You can also place the multimeter onto ohm’s selection if you are looking for specific resistance levels. Bleed loops are an example of a system that you would be looking for a very specific resistance level.
Checking continuity is crucial because even if voltage is present at one end, a break or damage in the wire could prevent signals or power from reaching their destination. Think again of the mouse and laptop: the voltage check confirmed power is coming from the laptop, but continuity testing is like making sure the cable connecting the mouse is not broken internally.
If the wire is damaged, the mouse won’t work even though the laptop is supplying power. It’s important to test each wire individually when possible, as a break or short can be hidden in any part of the harness.
How to carry out aircraft wiring insulation resistance tests (Megger testing)
When to use a megger for aircraft wiring insulation tests
Once you have confirmed wiring continuity, it is also important to assess the quality of the insulation around the wires. Good insulation prevents current leakage, short circuits, and interference between wires, all of which can cause unpredictable system behaviour or damage.
This is where a megger, or insulation resistance tester, becomes essential. A megger applies a high DC voltage, typically between 500 and 1000 volts, between conductors or between a conductor and earth (ground).
It then measures the resistance of the insulation. High resistance indicates that the insulation is intact and performing well. Low resistance can indicate moisture ingress, damaged insulation, or worn cables, which require repair or replacement.
Using the mouse and laptop analogy, insulation testing is like making sure the mouse’s cable coating remains intact and nothing inside is exposed or leaking electricity.
Aircraft components you should never test with a megger
Because megger testing involves applying high voltage, it must be performed with caution. High voltages can be definitely hurt the technician and can also easily damage sensitive electronic equipment if used improperly.
Some aircraft components, such as bleed air loops or other calibrated sensors, should never be tested with a megger. These components can be very sensitive to high voltage, and applying the typical 500-volt test can destroy them, leading to expensive repairs and grounding delays. It is essential to consult the aircraft maintenance manuals and wiring diagrams before using a megger on any system.
Best practices for aircraft wiring fault troubleshooting
Approaching wiring faults methodically is key to efficient and safe troubleshooting. Begin with the simplest, least invasive tests, voltage and continuity, which can quickly identify the most common wiring issues.
Only move on to insulation resistance testing with a megger when it is safe and necessary to do so. This approach minimises the risk of damaging sensitive avionics or wiring harnesses, while maximising the chance of locating the fault efficiently.
Documenting each step and measurement during your checks can also be helpful for future reference and to support any repair decisions. If you are working in a line environment versus a base environment, keep in mind the different operational pressures and resources available; our post on line maintenance vs base maintenance explores those differences and can help you plan your troubleshooting workflow on the day.
For technicians looking to deepen their qualifications and understanding of electrical aircraft systems, see our guide on gaining a B1.E Part-66 licence.
Key takeaways for safe and effective aircraft wiring checks
Wiring checks are a cornerstone of maintaining reliable and safe aircraft electrical systems. Knowing how to properly perform voltage and continuity tests, understanding when and how to safely conduct insulation resistance testing, and recognising the limitations of each method will empower technicians to diagnose and repair wiring problems confidently.
Using the simple analogy of a mouse and laptop can help make these technical procedures more approachable, highlighting the fundamental importance of power delivery, wiring continuity, and insulation integrity in complex aircraft systems.
If you want real-world case studies showing how a small connector or a hydraulic fault can ground an aircraft and how teams recovered, check our AOG Diaries: AOG Diaries #5 – fuel tank inerting and AOG Diaries #2 – hydraulic manifold leak.
