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Planning Your Instrument Panel

By Tony Bingelis (originally published in EAA's AeroCrafter - 6th Edition)

Almost any aircraft panel will have room for the few mandatory instruments required by Federal Aviation Regulations, specifically FAR Part 91. Just to show you how little you really need, here is the complete list of the instruments necessary for legal flight:

    1. An Airspeed Indicator
    2. An Altimeter
    3. A Magnetic Direction Indicator (compass)
    4. A Tachometer
    5. Oil Pressure Gauge
    6. Oil Temperature Gauge (air cooled engine)
    7. Fuel Quantity Indicator (need not be an instrument)

So, if you are really interested in keeping your airplane as light and spritely as possible, and intend to use it primarily for daytime VFR sport flying, the minimum instrumentation could fill your needs adequately.

Naturally, finding space for those few gauges is no problem, even in the skimpiest single seater instrument panel.

Let’s face it, though, most of us would never consider such a spartan instrument installation - and therein lies the problem. If the minimum instrumentation is not adequate for your needs, what would be? Well, there is nothing really wrong with wanting to add a few more of your favorite instruments to keep you company, and to help you monitor the attitude and behavior of your aircraft as well as the status of the engine. After all, the more you know about both, the safer and more professional your flying can become. Well, so much for rationalization.

Sometimes we find the number and type of instruments we want to or can install will be governed by factors over which we have little control. For example, the aircraft might not be equipped with either an electrical system or a vacuum system...and then there are always 
financial considerations.

No electrical system? That means you will not need to install either an ammeter or voltmeter. Nor should you contemplate the idea of installing navigation and strobe lights, unless you are willing to provide some sort of battery power source to operate them. Also, the engine instruments that will be needed must be the nonelectrical type gauges.

No vacuum system? Without an engine driven vacuum system on board you should not entertain the idea of installing any gyro instruments, not even a turn/bank indicator, unless, of course, you are willing to provide and external vacuum source. That is, a vacuum source 
capable of powering one or more gyro instruments through venturi affixed to the airplane externally. To drive all three gyro flight instruments would require a large nine inch super venturi stuck to the side of the airplane...and most builders don’t like that idea. On the other hand, you could but probably wouldn’t install a set of those outrageously priced electrical flight instruments in place of the more common vacuum driven gyro gauges.

And there is still another limiting factor- space, or rather the lack of instrument panel space.

The lack of instrument panel space is not ordinarily a problem found in the average two seater aircraft with side-by-side seating, but it definitely is with single seaters and tandem seating two seaters. An instrument panel 24 inches wide is hardly large enough to accommodate a complete set of gyro instruments, radios, switches and other controls.

Many builders immediately think of adding a sub-panel to the bottom of the regular instrument panel. You can’t, however, arbitrarily do that in order to accommodate additional gauges, switches and radios. For one thing, your control stick will probably not clear it ... and if it does, there is the risk that you will occasionally skin you knuckles against that sub-panel.

Cutting off some of the control stick may seem like a clever way to solve that problem. It ain’t. The shorter the control column, the more sensitive the controls can become as a little movement will then yield amazing responses. Many homebuilts are too sensitive to control stick movements anyway.

There is another reason for limiting the depth of a sub-panel. In many installations it may restrict knee room to the extent that entry into the cockpit will become awkward to accomplish and comical to behold.

Like many builders, when it came time to lay out my own RV-4 panel, I found that my "need to have" list of instruments was dwarfed by my "would like to have"’ list. So much so that I, too, had to consider the installation of a sub-panel and make use of other strategic areas to cram in everything I thought I "needed".

Since I was obviously going to cram my panel full on instruments regardless of logic, the exact positioning of each gauge in that small panel was critical and would have to be accurately predetermined before I dared start cutting the instrument holes in it. After all, aluminum panel blanks are quite expensive.

The best way to start, I think, is by making a numbered list of the instruments you intend to install. Arrange each listed instrument according to some semblance of priority. For example, and airspeed indicator would be higher on the list than a color radar Stormscope- and a tachometer would have priority over a stereo. The primary purpose of such a list is to help guarantee that you don’t goof and forget to provide a place for each of the most important gauges before you start cutting out the panel holes. To overlook one could ruin your day....and panel.

Of course, if you can’t find room for all the gauges, you will already know, from your list, that the one or two instruments on the bottom will be the ones to leave out.

The next preparatory step entails making a few small scribbled sketches to get a rough idea of which instrument arrangements look promising. I try to keep in mind the fact that a good instrument panel layout will be one that is standardized to a degree. That is, the flight instruments will be centrally located in the upper portion of the panel, and the engine and other miscellaneous gauges along the bottom or sides of the panel. Some minor instruments may be even further removed to adjacent locations.

I always try to group all switches and circuit breakers together in the same general area in order to simplify the wiring job. I also try to make these items reasonably accessible from behind or below for maintenance and trouble shooting.

To save space and simplify the wiring in my current project, circuit breaker switches will be installed and all separate fuses and/or circuit breakers will be eliminated.

Now, with these preliminaries out of the way, the next step is to make a full sized cardboard duplicate of the instrument panel, a dummy panel, let’s call it. This, you will find, is always worth the time expended unless only a few instruments are involved, and you are willing to risk a random buckshot-like pattern for an instrument arrangement.

You should always verify whether or not there is anything behind the panel that would interfere with the placement of any of the instruments anywhere in the panel. For example, RV-4 builders know that they cannot locate any instrument closer than 5/8" from the edges of the panel. Mark and outline areas such as these right on the dummy cardboard panel (as a reminder) if they will affect the installation of some instruments in your panel.

Drawing in each instrument location directly on that full size dummy panel is not the easiest way to design or layout an instrument panel. It can result in a lot of false starts and you may even have to draw several different layouts, and perhaps spend a lot of time making erasures as you continuously change your mind.

For that reason, I suggest you take time to cut out separate "playing cards" or cardboard patterns for each instrument. Each card should be labeled and numbered. Use the instrument layout dimensions for the cards. That is, make one properly dimensioned card for each of the large 3 1/8" instruments, and also one for each of the smaller 2- 1/4" instruments. If you plan to install some automotive gauges (oil, pressure, oil temperature, fuel quantity, etc.) be warned that these gauges fit a 2-1/16" diameter hole, and not the standard 2-1/4" one required for the small aircraft gauges.

The reason I recommend cutting out the individual instrument patterns square is so that they will be easier to arrange, space and align on your dummy cardboard panel.

Before you start laying out the instrument cards on the dummy panel, mark a space for your radio if you plan to locate your radio(s) in the instrument panel. This means you should have selected the radio you will be installing because you must know the dimensions for the panel cutout. This is especially critical for those of us with small instruments panels that don’t have space for a conventional radio "stack".

Radio widths have been standardized to the degree that most any make will fit into a 6-3/8" wide opening, however, their heights do vary somewhat from brand to brand. In addition, there are radios that are designed to fit into a regular 3-1/8" instrument hole. That alternative should help matters for some builders. You really should know beforehand which radio, or radios, you will install and make your panel cutouts to fit them.

Here’s the good part...designing your own instrument panel layout. Begin by laying all of those individual cards (templates) that you should have already cut out and labeled onto the full size dummy instrument panel and shuffle them around until you arrive at an arrangement that you like. Then stick each instrument card to the dummy panel with a small dab of library paste or with small pieces of masking tape. This should keep the templates from slipping out of position. NOTE: Be sure to space your large (3-1/8") instruments no closer than on 3-1/2" centers. This will provide a 1/4" space between instrument faces. The smaller 2-1/16" and 2-1/4" instruments should be spaced no closer than 1/4" apart, either.

Check your instrument list to be sure you have included all the gauges, and your layout for a logical arrangement. And then you can stand back for a critical look (you may have to deal yourself two or three layouts before you are satisfied). If it still looks good, place the dummy layout pattern on the aluminum instrument panel blank and tape it around the edges enough that it won’t slip out of alignment. Next, with an automatic center punch, make punch marks to establish the center of each instrument and to locate its screw attachment holes. At this point you can really appreciate how quickly and accurately you will be able to lay out and align the square instrument templates. No need to trace each instrument circle onto the aluminum blank although most builders will feel more comfortable in doing so.

Some builders will even prefer to lay out the instrument locations directly onto the aluminum blank, taking the measurements from the dummy master layout, but it will take much longer to do it that way.

After all that is done, cutting out the instrument holes with a circle cutter will be anticlimactic. One word of wisdom, though, do not cut the instrument holes so precisely that the instruments are a press fit because, after the panel is painted, the instrument won’t slip in and you will have to file the hole larger, ruining your paint job.

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