nuts & bolts

building basics Connection by Crimping Par t science, par t ar t, all critical GEORGE R . WILHELMSEN

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f you are looking for a subject that is probably one of the least enthralling, it is a story about making good electrical connections. The majority of electrical connections are made with crimp-style connectors. There are still some folks who use solder and heat shrink, but they are few and quite far between. As a result, this article will focus on how to make good crimps. If you talk with people who have been doing them for a while, they’ll tell you it is an interesting mix of science and art. The science comes from the approach that is needed to provide a consistent level of quality in all crimps executed for your aircraft project. Believe it or not, there are right ways and wrong ways, right tools and wrong tools, the right wire and the wrong wire—all of these elements when correct come together to work splendidly. The art part of this equation is in knowing how much to strip on a consistent basis to obtain quality results without having to pull out the ruler and mark the wire each time. Consider this: electrical connections are the lifeblood of your aircraft electrical system. This is one place where your personal commitment to quality absolutely, positively must be maintained at the highest level. All it takes is one or two bad connections, and you will either spend months trying to chase down the errant connector or, worse yet, find yourself with a cockpit full of smoke or a fire sometime in your aircraft’s career. Now that we have your attention, we’ll get into the crimps.

The Right Tools For starters, to make good crimp connections, you need to start with the right tools. In this case, the right tools are non-return crimpers and wire strippers designed for aircraft wire. If you bought one of those “universal strippers” at the EAA AirVenture Fly Market, you can pretty much pitch it, since they tend to stretch and elongate the insulation as well as nick the wires. Nicking wires is covered quite well in FAA Advisory Circular AC 43.13-1b, with the limits set at levels that some builders might think are overly conservative. To the FAA’s credit, the limits on missing and damaged strands are designed to assure several things. First, the limits assure that the wire will be able to carry the maximum, worst-case rated current without overheating. If you lose too many strands, when the wire is loaded up by a fault, that “electrical choke point” can cause rapid overheating, which can lead to an in-flight fire. Second, the wire also has to maintain its structural strength, so cutting strands takes away from that strength and is therefore limited. The limits for allowable nicked or broken strands for wires are provided in Chapter 11 of AC 43.13, in Table 1113. Aluminum wire is the most restrictive, with no broken or nicked strands allowed. The majority of wire in aircraft falls in the range of 10 to 24 gauge. For 24 to 14 gauge, you are allowed two nicked and no broken strands. If you are using 12 to 10 gauge, you get four nicked and no bro-

All it takes is one or two bad connections, and you will either spend months trying to chase down the errant connector or, worse yet, find yourself with a cockpit full of smoke or a fire sometime in your aircraft’s career.

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ken strands. Those are pretty stringent requirements. In order to get there, the AC provides the advice that the cutting tools used for stripping need to be sharp, and when using special stripping tools, to adjust the tool to avoid nicking, cutting, or otherwise damaging the strands. The last part of the AC includes the point that you should strip no more insulation than is necessary. Now that we know what the rules are for the wires and strippers, we’ll move on to the crimpers. In this case, there is only one good way to go—with a ratcheting, nonreturn-type crimper. The most popular and expensive of these is the Amp T-Head Crimper, which is used by the airline mechanics and even the nuclear industry to assure quality crimps. There are less expensive ratcheting, nonreturn tools available from a variety of firms. The key here is to buy a crimper that will work with your connectors. Based on experience, I recommend using Amp brand connectors for a number of reasons. The best reason is most crimpers are designed to work with Amp connectors, and work best with them. The secondary reason is that the connectors have a transparent insulator, which allows you to check that you have inserted the wire to the proper depth to assure you will get a good connection, the first time, every time.

You can see one broken and several nicked strands in this stripped wire. Given the size of the wire, this end could not be used, and would have to be cut off.

The Orange Handle Crimper/Strippers If you look in your toolbox, you’ll probably find a pair of “universal household crimpers.” These look pretty much alike—they have large orange handles and do everything from cutting screws to stripping wire. As aviation crimpers, they are not acceptable, since the jaws are not controlled as to the depth they can crimp a connection, and the force required to make the crimp is variable. This is a problem for several reasons. The strength of a crimp connection comes from the connector and the wire, and even the insulation. If you exert too much force on the connection, you can damage the connector and in doing so reduce its structural strength by a significant margin. Conversely, if you don’t put enough force into the crimp, the wire will not be held properly, which can result in hot spots, intermittent connections, or even the wire pulling free of the connection. From this perspective, it is easy to see why you should spend the money and pick up a non-return crimper. If you are one of several EAA members building a plane, you can consider pooling resources and buying one crimper to share between the builders. In that way, you can cut down on your initial costs and still have access to the quality tool you need to make professional, dependable crimps.

Practice, Practice, Practice Before you go and start on your project, take around an hour and practice making good crimps. This can be as

Here, the crimp on the right side was not centered and the connector is not locked around the wire as it should be.

All you need to do is compare the ends to see the difference here–the good crimp on the left has secured the wire in the center of the connector, while the inferior crimp on the right has not. The crimp on the left offers better strength and strain relief than the crimp on the right, and will be more dependable.

EAA Sport Aviation

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building basics Stripped of its protective plastic sheath, this is a perfect crimp. The wires meet at the divider, and the insulation is centered in the ends, just as it should be.

The crimp on the right was made with an approved non-return crimper, while the crimp on the left was made with a standard “automotive” crimper.

In this case, the wire wasn’t stripped far enough, or wasn’t seated properly. The wire should bottom out on the tab in the center of the splice before it is crimped to assure.

simple as starting out with scrap wire, stripping the insulation, and then installing a crimp connection. The best folks know exactly how far back to strip the wire to match the connector, and you’ll need to gain the necessary skills to do a perfect strip the first time, every time. Remember, nicked or broken strands are a no-no for most of the gauges of aircraft wire that you are working with in the cockpit, so your stripping needs to be pristine and perfect for your wiring to have the right level of quality and reliability. This is an even more compelling reason to practice, since having to cut off the end and start over can result in having to pull replacement wires. 90

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Even if you do leave margin (extra wire) for error to avoid repulling wires, it usually isn’t the best idea to

Remember, nicked or broken strands are a no-no for most of the gauges of aircraft wire that you are working with in the cockpit, so your stripping needs to be pristine and perfect for your wiring to have the right level of quality and reliability.

In the case of ring, spade, or knife terminals, the wire should extend slightly beyond the open end of the connector before it is crimped. This allows you to see if the wire moves during the pull test.

coil up the extra in your cockpit. The best reason to avoid coils is that they tend to create circulating currents and magnetic fields, which can cause havoc with instruments and electronics. Thus, it is best to gain the skills you need before stepping off into the cockpit and engine compartment and making connections that come with rework attached. The real trick or art here is starting the crimp. Basically, you put the connector into the correct die of the tool and start the crimp to just where the tool grabs the splice or connector. By “die” we mean the color-coded dot or mark on the tool, which matches the color connector you are using. Putting a connector in the wrong color die can result in excessive force, which can pinch or damage the wire, or insufficient force, which can allow the wire to pull free. With the connector in the right die and ready to crimp, you insert the wire that is stripped in accordance with the manufacturer’s direction and cycle the crimper fully through the stroke. What should emerge after this effort is a perfectly completed splice, which is crimped both on to the conductor as well as the insulation. EAA Sport Aviation

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building basics While the tools are excellent, there is always the chance that an error will develop in the crimping process. These problems could be as subtle as the connector getting cocked in the crimper and not being properly crimped, to as complex as the crimper being worn or incorrectly adjusted, or even the wrong dies selected, all of which would lead up to an unsatisfactory and nonsecure crimp. The more times you use the crimper and the more difficult the crimping location, the greater the chance that an error will occur. As the potential for error exists, the crimping process is completed with a pull test, to make sure the connector has integrity and is securely attached to the wire. This can be as simple as grasping the conductor and the connector firmly and giving it a good tug, or by using a commercially available fish scale and putting 10 pounds of pull on each connector. The intent of this final check is to assure that the connector is firmly attached and is not likely to come loose in flight.

Bad Crimp? The actions to take on a bad crimp are simple: cut the wire off, restrip if possible, and install a new crimp. An over-

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crimped wire is unrecoverable, since the end has likely lost structural strength. An undercrimped wire is equally challenged, since you should not try to recrimp it—the connectors are designed for a single cycle and may not have deformed properly due to an oversized die the first time around. Given the cost and the potential to introduce a problem, the cleanest way out is a “do-over.” As always, before you start out and get into your project, take a few moments to pick up the various instruction manuals that came with the tool, or that are available on the web, and review them. If you take this action, and get your practice in before you make connections that count, you’ll find that the wiring on your project will go smoothly and with a minimum of problems and, most importantly, will work the first time, every time when you need it! Contributing writer George Wilhelmsen is an Illinois-based pilot. He holds a commercial certificate, ASEL with an instrument rating, and has more than 1,000 hours of flight experience. He has a bachelor’s degree in engineering technology, with a background in DC, analog, and digital controls. He flies a Beech Debonair.