Nuts & Bolts
Building Basics INSTALLING A RADIO OR TWO IS A new experience for most first-time homebuilders. While position lights and shielded ignition wiring is your undergraduate electronics, hooking up a stack of black boxes puts you straight into your doctorate. If you don’t envision the $60,000 glass cockpit, you can probably do some, if not all, of the installation work yourself. A single nav/comm radio isn’t a complicated installation. It’s just different. The typical nav/comm radio has two multi-pin “dsub” outlets on the back of the case, similar to the bulky cable connectors on the backs of computers. Aircraft radios today use a myriad of low-powered microprocessor-controlled circuits supplying a heap of computing power, so it’s no surprise they use this kind of connector. Herein lie two problems: First, there can be literally hundreds of connections involved in installing a stack of avionics. For example, GPS receivers often use a 37-pin dsub connector, and comm radios may use 15- or 26-pin dsub connectors. While not every pin has to be hooked to something, knowing which ones is important. Figuring that out will take some quality time with the unit’s installation manual and pinout chart. Why so many connections? In an advanced IFR panel, comm radios may connect to a multifunction display (MFD), audio panel, intercom, and other devices. Navigation radios can interact with moving maps, annunciator panels, autopilots, engine monitors, storm scopes, and even traffic-avoidance equipment. Which brings up the second problem. If each connection isn’t proper96
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Voice Box Installing your aircraft’s radio GREG LASLO ly made—with the right kinds of connections, wires, and routing— those circuits won’t work. If other connections within the aircraft’s electrical system aren’t shielded, such as the P-lead to the magnetos, the wires to the alternator, and the power to the aircraft strobes, the radios will pick up extra interference.
You’re not installing a new AM radio into that Ford Pinto you drove through college. “The signal to a GPS receiver is so weak that we used to call it ‘noise,’” says Gary Morris of Kings Avionics in Olathe, Kansas. If there’s interference entering a circuit, the receiver will misread the signal, resulting in an inoperative instrument or—even worse—incorrect displays. In other words, you’re not installing a new AM radio into that Ford Pinto you drove through college.
Testing 1, 2, 3 The process starts with getting power and a ground to the unit. The aircraft’s main electrical bus typically includes an avionics master switch, the radios’ main power source. From there, each side of each radio typically gets its own circuit and fuse or breaker on the panel—a nav/comm will have power to both the navigation and communications radios.
The manufacturer determines the fuse or breaker’s amperage, and it will be indicated on the wiring diagram. So is the gauge of wire connecting the radio to the power supply. Antenna placement is important, too. Your chief concerns are keeping the antennas from interfering with one another while using as short a coaxial antenna cable as possible. Your nav/localizer/glideslope radios can use the same antenna, split with a triplexer, but you’ll need a separate comm antenna for each radio. Install antennas according to manufacturer instructions. Be especially aware of minimum distances between nav/comm antennas and those for other devices—in some cases, distances of several feet are required to minimize interference. Improperly routing cable can also cause problems. In most cases, you don’t want to run antenna cables side-by-side along the bottom of the fuselage. In any antenna application, use 50-ohm coaxial cable. The fewer splices—because you incorrectly measured or split the cable with triplexers—the better your signal reception. Likewise, a good fit is important. Leaving a single service loop to make a connection is acceptable, but don’t coil antenna cable. Chapter 3 of FAA Advisory Circular 43.13-2A, “Acceptable Methods, Techniques, and Practices” offers advice on antenna installation. You’ll need a wiring harness to connect your radio. Airframe manufacturers and avionics shops use large easels to stretch hundreds of predetermined lengths of wire into a harness, but a simple installation shouldn’t require that elaborate of
Building Basics Working Together
that they’re probably right. But you have several options for sharing the work with a shop. A number of shops build a custom panel that you install. Or you can have a local avionics shop do some of the work for you. The final option is to have an avionics shop handle the job outright. With a full-panel installation, you’re often getting a lot more than a stack of radios. You’re getting flight and engine instruments, a custom layout, and a painted panel. When the big box arrives, mount it according to the enclosed instructions, connect all the wires, and you’re set. If it gets power, it should operate properly. You can also get your local avionics shop to make wiring harnesses. With a plug-and-play harness, you’ll know each circuit is wired to manufacturer specs, which means it’s
The saying goes that there are two kinds of homebuilders: Good mechanical builders, and good electricians. Rare is the builder who can do both well. They are very different skills (even the FAA considers “avionics technician” a different discipline than “A&P mechanic”). In addition, by the time the builder is ready to install an avionics stack, the airplane is almost done. Emotionally, that’s not exactly the time to start from scratch with a new subject. In many cases, Garmin, Bendix/King, and other avionics manufacturers won’t sell their sophisticated avionics to individuals because proper installation requires expertise, tools, and abilities most people don’t have. A glimpse of the wiring diagram for a panel incorporating a moving map display, DME, and GPS/COMM will convince you
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more likely to work the first time. A custom harness will cost around $1,500—more than doing it yourself, but maybe not that much more. Remember, you’ll need special crimping tools for each multipin and butt-splicing connector, plus 20-, 22-, and 24-gauge wiring in a variety of shielded configurations, and all sorts of other odd materials. You’ll have a lot of extra materials left over. Worse yet, you’ve got to troubleshoot the installation if it doesn’t work. If the thought of doing your own radio installation is keeping you from finishing your project, your local shop will be more than happy to do the work for you. Tell them what you want, leave the airplane, and they’ll call you when it’s done. You can spend the extra time scheduling your FAA inspection or reacquainting yourself with those people you live with. Gary suggests working closely with the avionics shop throughout the building process. The first visit should occur before construction begins, so the builder and shop can discuss antenna placement, cable routing, and electrical needs. Later on, the builder and installer can discuss panel layout and electrical system specifics, then design the harnesses and connectors. Finally, once the radio is installed, the shop can do the avionics certifications for you. Tackling a simple radio installation doesn’t have to be a lot of work, but ensuring the proper operation of a full stack can turn into a nightmare project if you’re not comfortable in the world of amps, ohms, and electrons. The installation manuals for many popular radios are available online, so see if this is a project you’re up to. If you choose to give it a try, an EAA Sport Air Workshop on aircraft electrical systems will give you many of the skills you’ll need. With a little patience, it won’t be long till you’re cleared to land for real. 98
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