I know you’ve been there: the mechanic replaces the old noisy turn coordinator and on the very next flight you discover your handheld GPS has no power from the aircraft. You are certain the mechanic did something and he/she is certain he/she never touched the supply to the cigarette lighter receptacle.
When troubleshooting electrical problems I find a great percentage of the failures are caused by poor wire splicing and terminal end installation. Everything is fine as long as the problem connector is never disturbed. The minor flexing of the wire bundle by the mechanics forearm may be all it takes to break that last strand of wire clinging to the butt splice on the cigarette lighters’ in line fuse holder. Why would something so simple be such a problem so often?
The butt splice on top is crimped properly on the wire but no crimp on the insulation. The splice on the bottom isn’t crimped correctly anywhere.
Lack of training
I spend a great deal of time educating mechanics on electrical system trouble shooting and wiring issues. Many problems in electrical systems are actually caused by improper wiring repairs. It’s possible a part of the lack of knowledge in this area is the FAA has almost no information on proper wire termination or splicing. Much ado is made in FAA Advisory Circular 43-13.1b change 1 chapter 11 about the correct wire types, wire sizes, how many terminals on a stud, environmental protection, clamping, types of connectors, and inspections. Unfortunately, it says nothing about what to do if you actually find a problem. It’s as if the possibility of an actual wiring repair would never come up.
The pliers on the left are often used to produce very poor crimps. The two other tools will produce good crimps if the operator uses them correctly.
Proper Parts First
I have always preferred the AMP (now Tyco) product line for quality crimp connectors. The Pre-Insulated Diamond Grip (PIDG) terminals, splices, and end caps use a metal wire barrel and a copper sleeve encased in a nylon insulator. I have crimp splices and terminal end connections in engine compartments and nose wheel wells that I installed 20 years ago still in excellent condition. I’m sure other companies make good products as well, but the PIDG line is what I am most familiar with.
Most all manufacturers use color coded insulators to identify the proper wire gage (American Wire Gage – AWG) for the part. Red insulation identifies 18, 20, and 22 AWG. Blue insulation identifies 16 and 14 AWG. Yellow insulation identifies 12 through 10 AWG. There are several others but these are the most common found on our piston Cessnas.
Proper Tools Next
The most common problem I find with crimp terminals is the use of improper tools or improper use of the correct tool. There are many incorrect tools that will “smash” a terminal enough to temporarily hold a wire in place. I’ve seen teeth marks from common pliers, a drift punch, automotive crimpers, and a nicropress tool for flight control cables.
The proper tools are designed specifically for these terminals and will make lasting connections when used. The right tool will make a smooth compression of the wire strands with enough strength to grip the wire with out damaging it. The same tool will compress the aft end of the part around the insulation of the wire to minimize work hardening and eventual failure where the wire exits the terminal.
There are several hand “ratcheting” tools that will crimp the bare wire and the wires’ insulation simultaneously not releasing the part until full compression is achieved. The less expensive non ratcheting hand tools are capable of good quality crimps if the operator knows how to use them. Regardless of the tool used, the finished product should be the same.
A good wire stripper that won’t scratch the wire strands and a good quality crimping tool are a must for long lasting wire fastening.
Proper Crimping Last
Every crimp should be smooth and hold the wire securely. It will also gently grip the insulation of the wire for strain relief and flexing. A properly crimped 18 AWG terminal can easily support more tension load than most of us can apply by hand. See attached figure 7.
The two yellow terminals are properly crimped. The wire in the red terminal has so much bare wire the it is fully exposed. This won’t last long.
The pricier “ratcheting” tools will ensure a good crimp with little effort on the part of the technician. The non ratcheting tool requires the technician use two separate actions. The first action crimps the bare wire and the second crimps the wire insulation. The proper position in the jaws of the tool must be selected to get it right and complete compression of the tool to make the crimp secure.
The figures attached to this article are taken by permission from Tyco Electronics Application Specification 114-2157. They give very good details of what makes a proper crimp. Many thanks to the good folks at Tyco Products for providing this and several other documents in support of this article.
I know this article will reach more aircraft owners with no intention to do any wiring repairs to their airplane than it will mechanics, but it is an important topic to improve the reliability and longevity of aircraft wiring. This is just one more “aging aircraft” issue.
Copyright © Paul New 2008. All rights reserved.
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