By Bob MacGillivray 

Recently I came across an approved drawing from an established aircraft manufacturer that advocated a method of attaching and crimping electrical connectors that is clearly not in accordance with accepted or published ‘good aeronautical practice’. This prompted me to put in my two bobs’ worth regarding some simple aspects of electrical wiring in our aircraft.

We must always remember that the electrical systems in our aircraft are an integral part of the aircraft and not only must operate as intended but must not, in themselves, constitute a safety hazard. The drawing I mentioned above could adversely impact both operational reliability and safety  - more on this later.

In designing, manufacturing or repairing electrical wiring, we should always use aircraft specification wire. Never use cheap automotive wire in aircraft electrical systems as its quality can vary greatly. More importantly, the insulation used is not fire rated and in the event of a fire, or simply wire overheating at high current flows, the insulation may soften excessively, or even burn - giving off  noxious fumes in the process. Aviation specification wire is specifically designed to avoid these issues. Many different wire specifications are available for specific types of applications but for the purposes of this article the most commonly used wire for general airframe wiring is the American specification MIL-W-22759/16. This wire utilises tin plated copper as the conductor, ETFE (Tefzel) insulation, and is available in most standard wire gauges.

Except in the odd application, general airframe wiring will use MIL-W-22759/16 wire from the heavy 10 AWG to light weight 22 AWG. The actual current carrying capacity of the various gauges varies with several factors such as whether or not the wires are bundled together, the individual wire length, and the acceptable voltage drop. As an indication however, 10 AWG wire may handle up to 55 amps while 22 AWG will only safely carry three or four amps.

Now, of course, the wire used is only part of the problem: the connector on either end is the other. In this article I will only consider the installation of simple ‘ring’ type or similar crimped connectors - other connectors are more specialised. Firstly, it is very important to use the correct type of crimp connectors and crimping tools. I cannot stress enough, only use high quality connectors such as, for example, AMP pre-insulated, Diamond Grip (PIDG) connectors. These connectors are available in three standard colour coded sizes to suit 10 down to 22 AWG wire sizes. RED connectors are designed to safely and properly crimp on to 16 to 22 AWG wires, BLUE, 14 to 16 AWG, and YELLOW, 10 to 12 AWG. AMP connectors are readily available from various aviation supply sources in Australia such as Edmo. Lower cost connectors, such as those that may be purchased from the discount automotive parts suppliers, may not follow any specified standard as to colour coding, wire size compatibility or even mechanical strength. I have seen some of these automotive type connectors that are clearly non-standard. One red connector I have seen would accept a 12 AWG wire! Clearly, no matter how well crimped, this connector would easily pull off a small wire such as a 22 AWG wire.

This now leads to the third part of the equation, the crimping tool. Hand tools for applying crimped connectors cost from about $10 to maybe $400. The differences in them lie in the quality of the end result. Expensive crimp tools have precision dies which are brought closed against hard stops thus insuring the proper upset of the metals in the connector/wire interface. These tools also feature ratchets in their mechanisms that insure that the tool is operated through a full crimp cycle thus preventing inadvertent under-crimping. Additionally they crimp, in the correct proportions, both the inner portion of the connector that engages the wire conductor and, at the same time, crimp the outer section of the connector that is designed to clamp the insulated portion. Unfortunately cheap tools, such as those that may be purchased from the discount automotive parts suppliers, just do not cut it. Unlike the more expensive ratchet types, the crimp quality varies with the operator technique. A ‘hard’ crimp is required for the wire conductor bit and a separate ‘softer’ crimp for the insulator. Quality and consistency from one crimp to another cannot be assured this way.

The bottom line is - if the correct colour-coded connector is used on the wire size for which it is rated, and a proper ratchet type crimping tool is used, the wire will be secured properly mechanically and will also meet the electrical resistive specifications for the joint.

So where was I leading to in my opening paragraph? The drawing I am referring to stated that for smaller wire sizes the conductor must be folded back over the insulator before being inserted into the connector and crimped. This is a totally unacceptable procedure for several reasons. Firstly, it is unnecessary if the correct wire/connector combination is chosen and the crimp is done using the correct crimping tool. Secondly, at higher temperatures such as may occur in hotter environments or when carrying current up to the maximum allowable for the wire size, the insulator will soften thereby reducing the clamping force as the bent back conductor sinks into the soft insulation. The inevitable result is not only a significant reduction in the mechanical strength of the crimp but also an increase in the electrical resistance. Need I say more!