Avionics


Sportair Electrical Systems & Avionics

I signed up for the Sportair Electrical Systems & Avionics course awhile ago, but gave serious consideration to canceling. I have a degree in electrical engineering, and I worked as an electronics technician for Honeywell Aerospace in college, where I assembled and soldered some seriously complicated and large cables for the systems of an amphibious naval ship known as an LPD. Some of these cables had close to 100 uniquely plotted pins with multiple daisy chains and unique features. Maybe a Sportair course on this topic would be a waste of time.

Well, it wasn’t. While I was clearly more experienced than the the other students in the class, I still learned a few things. And while there were a few techniques that I disagreed with (I’ll detail those below), the instructors were extremely knowledgable, well prepared, and helpful. So what did I learn during this course?

To start with we reviewed the systems and requirements of an aircraft. We looked at the basic circuits, tools, switches, and requirements of an aircraft aviation system. I’m not going to rehash the entire course, but I will attempt to share a few of the things that I learned that I didn’t previously know.

First off, when I was building cables for the LPD, we didn’t crimp pins. Period. Every pin we installed was soldered to MILSPEC. But crimping is easy. What I had never touched were some of these tools. It was good to get my hand on each of the tools below. A is a molex crimping tool. B is a proper stripping tool. I’ve used one like this before at Honeywell, but I don’t own one. I highly recommend buying a decent stripper such as this. C is a crimping tool that has multiple heads you can change out. Installed on this head is the standard terminal crimping tool of three sizes. Red (18 – 22 gage) Blue (14 – 16 gage) and Yellow (10 – 12 gage) dots mark the appropriate wire gage size, and the side in which you insert the wire. The wire goes to the dot! Expensive crimpers are available from Aircraft Spruce, but I’d recommend the E-300 series with multiple heads available for $35 at www.aircraft-tool.com. And finally, tool D is simply a set of side-cutters. A good sharp and precise set like this is a must have. Finally in the picture below you can see our first, super basic project which is simply crimping three different types of connectors to two different wire gages.

I’d never crimped a molex pin before, but its super basic. Our second project was to assemble an intercom wiring harness. This consisted of a headphone and a microphone jack to a 25 pin connector. Pretty cut and dry. Here is where I took two minor faults with the instructions. First, soldering technique. The technique involved too much solder. If you have any excess solder in terms of a mound, spike or raised surface, you’ve over soldered the part. Regardless, generally speaking their techniques are sound. Tin, use the proper heat, and remember two rules: heat travels up, and solder will wick towards the heat source. Use these two rules for good solid soldering. Now the technique that I really took issues with, was how they connected the shielding to the ground pin. The technique, shown in this photo below (I’ll add it later), involved pulling the shield to one side and twisting it. You then remove a small portion of insulation on the the ground wire, and wrap the shield around it. Then you solder the two together. Here are the problems. First off, the strength of this technique is essentially the solder, and because its just a messy twist, you use a lot of brittle solder. Furthermore, by removing just a portion of the insulation, there is a substantial likelihood that you will score the wire and create a location for stress to do its magic. A better technique would be to use a separate wire that is twisted to the pigtail. This removes the chance of nicking the wire itself, but still results in a lopsided and un-supported soldering connection.

The proper technique is to use a solder sleeve. First you trim, and then equally unweave the shield back. About 1/4 inch. Carefully, and equally fold this back around over the insulation. This creates a shield collar shown here.

Finally, place the solder sleeve (one with a wire already inserted) with the solder directly over the shield. Use a heat gun to shrink the solder sleeve and melt the solder. The result is a sealed connection that will last a lifetime. This makes for a solid, sealed, and continuous connection that will not break from vibration. This is how I was taught to make these connections when working for Honeywell assembling cables for a naval LPD. Am I splitting hairs? Possibly. Using a solder sleeve is faster, easier, and creates a stronger longer lasting connection. Where is the downside?

Here’s a tidbit I had never heard of. Uniwrap silicon tape. The stuff is seriously cool. It can be used to tape, buffer, and protect many things in the aircraft. Its virtually indestructible and impervious to many of the corrosive liquids we use. Its available from Aircraft Spurce here:

Another tidbit. I had not previously seen the circle on a schematic that referred to the shielding. Maybe I saw this when I worked at Honeywell, but if I did, it has completely left my brain.

A lot of useful tricks for the starter, alternator, and relays were discussed. I’m really glad they covered this section, as I’m sure I will avoid a few common mistakes now.

All in all, much of the course was review, but its still nice to have a refresher. Add it a couple of learning points, and it was worthwhile. Whether it was worth the money… well that’s another question.