Aircraft Wiring

Top 10 wiring mistakes.

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There are several common mistakes that builders make, and the tips below are meant to help you get the most from your electrical system. Before we dive in, here are some process items that will help:

  • Plan your electrical system before you start wiring.
  • Ensure the electrical system and the avionics meet your needs.
  • Record changes during construc- tion and ongoing maintenance.

With that said, here are the “Top 10” wiring mistakes made during installation:

10. Not Sure How the Alternator Works

It can be confusing to understand the differences among the various types of alternators, voltage regulators, overvoltage protection modules, and low-voltage settings. Information is available from various resources including the Aircraft Wiring Guide.

9. Not Using Proper Wiring Techniques

Today’s aircraft depend heavily on the electrical system, so correct wiring techniques are important for safety and ongoing reliability. If you’re not sure how to crimp a wire, get some terminals and wire, then practice with them. They’re relatively inexpensive. Carefully read the instructions that come with the crimp tool. It’s easy to be lulled into thinking that a crimper is pretty simple to use—and it is, once you know the right technique. If you’re not comfortable soldering, find a friend who can help you practice. Learn how to work with shielded wire and coax wire. Coax can be tricky and cause problems down the road if not done correctly. Experimental aviation is about learning—use this opportunity to do so. Here are some good resources:

  • FAA Advisory Circular AC 43.13, Acceptable Methods, Techniques, and Practices—Aircraft Inspection and Repair available from faa.gov
  • EAA Hints for Homebuilders videos on the EAA web site
  • Attend forums and seminars at AirVenture each year.
  • Attend the regional EAA

SportAir Workshops. They offer an electrical workshop.

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8. Not Making Good Crimp Connections

Each crimp connection is a possible failure point. Follow the basics and you should be OK:

  • Size the crimp terminal for the wire size. Better-quality terminals have a stamp that shows the recommended wire size. Insulated terminals are also color coded based on wire size: Yellow=10/12 AWG, Blue=14/16, Red= 18/20/22.
  • Use the correct crimp tool. Many of the better crimp tools require the terminal to be installed one way only. Crimping in the wrong direction may look OK, but the integrity of the crimp is compro- mised. Further, use the right crimp tool for that specific terminal.
  • Use a good-quality terminal. For insulated terminals, use nylon (semi-transparent) insulation, not vinyl (opaque) insulation.
  • Give each crimp a good tug. Verify it is properly crimped and visually inspect the crimp. Surprisingly, even terminals that pass an informal pull test can fail in the future, so go back and tug on the connections if a certain circuit is problematic in the future.
  • Use the right terminal. Crimp terminals are designed for use with a specific connector housing. For example, there are standard and high-density D-sub pins. The high-density pins will fit in the standard connector housing, but not quite exactly right. They also require different crimp tools.

7. Falling Into the “Single Point of Failure” Trap

For some reason, there appears to be lore about making sure you don’t have any single points of failure in your wiring system or even the whole aircraft. This statement is simply too broad and imprecise to act as a guideline for planning your aircraft systems.

There are many single points of failure in aircraft today because it is impractical or unnecessary to add backups. Conversely, backups should be used where absolutely necessary. There is always a trade-off that usually adds complexity, weight, or reduces overall system reliability. Redundancy should be chosen carefully.

As part of your electrical system planning (and other systems too) you should consider what happens if any single component fails. Does it cause a catastrophic failure, or is it simply an inconvenience? What is the likelihood of failure? A lot of decisions depend on the intended mission of your aircraft. A failure of the EFIS, for example, has very different consequences for an IFR aircraft versus a pancake-run airplane. Build the plane that’s right for you.

“Simplicate and add lightness.”
—Ed Heinemann

6. Ignoring How Contactors Work and Differences Between Them

Contactors are used to switch high-current loads on the aircraft. Don’t mix up the starter and master contactors. Think of them as a big relay. A coil, when energized, creates a magnetic field and pulls the mechanical switch closed. The coil only draws a little bit of current, but allows you to switch very-high-current loads like the starter. Figure 1 is a diagram showing a generic contactor.

Figure 1: The coil in a contactor only draws a small amount of current, but it allows you to switch high-current loads like the starter.

5. Not Using the Right Tools

Figure 2: Double-crimp die and ratcheting crimp tool.

Owning or borrowing the right tool for the job makes a big difference! It’s common to think a Molex crimp tool, for example, can crimp the pins for any Molex connector (or even an AMP or other connector). Molex, AMP, and others make hundreds of different types of connectors, and each connector has a specific crimp tool. Be sure you know which crimp tool goes with each type of connector.

I recommend using a double-crimp die for insulated crimp terminals. One crimp crimps the electrical connection and the other crimps the insulation. Each crimp is slightly different, and the terminal must be inserted only one way.

It is important to use a ratcheting crimper. The ratcheting crimper ensures you squeeze down all the way to get the proper compression on the terminal—it does not release until closed all the way. This helps ensure a gas-tight seal on the wire.

Finally, the big cables (0 through 8 gauge) typically use non-insulated ring terminals. You can insulate the big terminals with heat shrink tubing after crimping. Do not crimp these with pliers or Vise-Grips. There are special tools available to properly crimp these cables. The crimper indent on the terminal is formed on the side opposite the brazed seam.

Good wire strippers are important, too. Bad ones can compromise the wire itself with nicking or cutting.

4. Not Using the Correct Size Wire and Circuit Breaker

Circuit breakers (and fuses) protect the wiring, not the device. If the breaker is too large, the wire or insulation may overheat and fail. If too small, the breaker may trip inadvertently.

It can’t hurt to use wire that is too big, but it can be bad to use wire that is too small. Wire that is too small:

  • Gets hot and may lead to insulation failure.
  • Causes excessive voltage to drop along the length of the wire.

If you’re not sure, err on the side of using a larger wire. The weight impact is negligible (18 gauge Tefzel wire weighs 0.00789 lbs per foot, for example). Most kit aircraft companies and avionics companies provide recommendations for sizing wires and breakers. If not, you can use an ammeter (typically under $50 at Radio Shack, etc.) to measure the current draw of each electrical device and then determine the size yourself. Wire an ammeter in series to measure current, as shown in Figure 3.

Figure 3: Connect the ammeter in series between the battery and the load.

3. Mixing Up the Power and Ground Leads

Connecting wires to the battery backwards can cause avionics and other equipment to fail (accompanied by popping sounds and smoke). This can be easy to do if you install the battery backwards or don’t label the wires. To minimize this risk in the future, use color-coded heat shrink (black for negative and red for positive) on the wire terminals. If possible, shorten the wires so that they cannot be connected if the battery is installed backwards.

Tip: Disconnect the battery ground cable first, then the positive cable. When reconnecting, connect the positive cable first, then the ground cable. Doing so ensures you won’t spark the positive connection to the airframe (and burn a hole in the firewall).

The battery(ies) in the aircraft should not be connected until the wiring is installed and each circuit is individually tested. Do not run wires while the battery is connected. Of course, this is not a hard and fast rule, but simply a way to help avoid those dumb, unintentional mistakes that we all make.

2. Ignoring Your Grounding Strategy

Sloppy grounds lead to noise and unreliable operation. Grounds are detailed in the Aircraft Wiring Guide.

1. Ignoring Your Spouse or Significant Other While Building the Airplane

Nuff said!


Read the Book

Hopefully this article will help you avoid common wiring mistakes. It is an excerpt from my new book entitled Aircraft Wiring Guide. For more information, or to order a copy, visit www.aircraftwiringguide.com.

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