Click here to upgrade to a newer version of Internet Explorer or Microsoft Edge.
Making Instrument Panels More Accessible
By Tony Bingelis (originally published in EAA Sport Aviation, August 1981)
WHEN YOU PASS an open hangar and happen to see a pair of squirmy legs sticking out of a cockpit, you can bet your bottom dollar that they belong to some poor soul who is propped up on sweaty shoulders squinting at - or messing with - the jumble of wires and plumbing behind his instrument panel. This, of course, is an unpleasant way to work and is at best a most undignified position. But this is a situation that could well happen to anyone who builds his own instrument panel in the highly styled, maintenance-impossible manner found in most autos and aircraft.
It seems that only homebuilders, and not all of them, are really interested in simplicity and built-in accessibility when it comes to aircraft instrumentation. And why not? Guess who has to do all the maintenance and repair work on the airplane?
Out of Sight, Out of Mind
If from the very first flight all your instruments function flawlessly (a rare experience in a new installation), you may never have the need to remove or replace anything. But even a single inoperative or poorly functioning gage could mandate its removal and lead to reenactment of that odd scene to which earlier reference was made.
Surely everyone knows that it is much easier to work on something when you can see what your hands are doing . . . and doing it right-side-up helps too!
Inspecting the plumbing, the installation of instruments, switches and circuit breaker connections behind the typical aircraft (or automobile) instrument panel is usually difficult or near impossible unless you first remove most everything . . . wheels, windshield, wings, etc. (a little overkill perhaps?) beforehand. Such conditions, more often than not tend to induce a "I can't see it but it must be O.K. because everything is working" philosophy during those most important annual inspections, in particular.
Everything-may not be O.K. behind the gages. Some wiring might be on the verge of becoming badly frayed or loosened. Static or pressure hoses may be kinked or wearing through. But all that aside, have you ever tried to wire or rewire an ignition switch - working from behind? One such experience is enough.
Wouldn't it be nice to have a plug-in instrument panel or at least one that could be turned around for inspection and maintenance purposes? A plug-in panel would be hard to make but the next best thing to that would surely be a hinged instrument panel. This is not a new idea, of course, but it is one that is seldom utilized by aircraft manufacturers, or homebuilders for that matter . . . but it does merit your consideration.
Making That Hinged Instrument Panel For Shock-Mounted Instruments
It is easy enough to fit any instrument panel with a piano hinge along its bottom edge so that the panel can be tilted open. As a matter of fact, by making one end of the piano hinge wire accessible you could pull the hinge wire out and remove the panel completely - then take it to the workbench where you can work in comfort. (You're right, amigo, you would have to disconnect the plumbing . . . or have some mighty long entrails trailing behind you.)
Small VFR panels are made to order for a hinged panel installation. These panels do not utilize gyro instruments so shock mounting is not necessary. But what if you wanted to hinge a larger, more complex panel?
Popular practice is to shock mount only the few gyro instruments installed in a separate small panel section. This is all right, I suppose, but still leaves you with the problem of accessibility to other instruments and wiring.
If your aircraft is to have a `full panel', it will probably be divided into two separate panel areas separated by a radio stack. But even this shouldn't present any problem, especially since it will not be necessary to shock mount both panel sections. One side (usually the right with no gyro instruments) would be hinged without shock mounts. The other, the gyro panel portion, however, should be hinged on shock mounts. But this too can be done rather easily.
The method I used to mount and install the panels in my Falco project is depicted in Figure 1 and is also illustrated in several of the photos. Since my panel is rather large and heavy, I had to use several standard instrument shock mounts installed both vertically and horizontally.
These rubber shock mounts are standard items made in rectangular and round shapes with brass protrusions molded into each end. The ends are threaded to take 8-32 nuts (you can obtain shock mounts from a local instrument shop or you may order them from one of the larger homebuilder supply sources advertising in SPORT AVIATION). A less cluttered and neater installation is possible when you mount all switches and circuit breakers on sub-panels located almost anywhere convenient. In my case, the most convenient location was on two sub-panels secured beneath the main panels with small bolts and nutplates. Installation of nutplates makes it easy to remove the sub-panels when you need to wire or service the various switches, circuit breakers and instruments.
Grouping of the fuses and switches on sub-panels also serves to simplify the wiring of the airplane. For example, a single heavy gage of copper wire or copper strip may be soldered across the ground terminals of all of the grouped circuit breakers instead of making separate ground connections for each unit. The same can be done with switches if they are grouped.
Some Instrument Panel Design and Installation Considerations
A sophisticated aircraft such as the Falco deserves extensive instrumentation and provision for a radio `stack' . . . even though I may initially install only a hand-me-down old wireless set picked out of some Good Will's goodie rack. (For my overseas readers . . . 'Good Will' is a U. S. charitable source for used clothing, pots and pans, etc.)
If you elect, as I did, to provide space for panel mounted radios, you will need a depth of approximately 14 inches behind the panel. Although most radios are but 12 inches or less in length, additional space is needed for their hook-up and removal. The implication is that if you must have a fuel tank installed behind the instrument panel you will, undoubtedly, have to build an indentation (recess) into the tank to accommodate the radio(s). Unfortunately, when you are forced to do this you cut into the available fuel capacity. Not only that, a lesser fuel load up front will also influence your weight and balance situation . . . especially if you have a large aft-mounted fuselage tank. At any rate, a hinged panel would also afford easy access to the radio racks as well as to the instruments.
The plumbing connections to the various instruments will probably be completed while the panel is swung open on its hinges. If so, the length of the tubing and wiring used will automatically be sufficient to allow the panel to be re-opened at any time without difficulty. Otherwise, if you are making your initial installation with the panel in its upright position do not forget to allow extra length and some slack in the wiring and plumbing.
Since the panel is hung on rubber shock mounts you will have to electrically connect the swing-open panel section with the stationary part of the panel. Use a short metal strap or a piece of stranded wire for this purpose (Figure 3). When installing the individual instruments be sure each mounting screw is tightened firmly but without excessive torquing. Many instruments are made with plastic cases and may be easily damaged. Exercise similar caution when tightening the plumbing and the wiring connections behind the instruments.
Speaking of mounting screws, one difficulty I've always experienced in making an instrument panel was in getting the screw holes for securing the instruments properly located. Invariably, I would have to resort to the corrective influence of a small round file. It seems I always had to elongate one or more holes before all of the 8-32 brass screws would go in. Then as if that was not embarrassing enough, invariably at least one instrument would not be level . . . a bit cockeyed, you might say. Well, my friends, no more problems like that for me!
I devised a simple little drilling fixture made with a couple of those useless instrument hole cut-outs to give me an instant capability for drilling correctly located instrument attachment holes. All I needed, in addition to the two scrap cut-out pieces (one from a 21/4" hole and the other from a regular 31/3" hole), was a small piece of aluminum about 3.5 inches square (about.50" or.060" thick, or whatever you have) and a short 1/4" bolt and nut. No need to describe how to assemble it when you can take a peek at Figure 2.
Be careful when you initially lay out and drill the fixture and your hole drilling jig will serve you well. The best part of the whole (oops) thing is that the device can be made in less time than it could take to measure, layout and drill the holes for any single instrument the old way.
Use the device after all the holes have been cut in your panel. Simply insert the proper sized side of the fixture into one of the instrument openings. Then, using a square, align the edge of the fixture and square it with the bottom of the instrument panel. Clamp the fixture in place, and drill instrument screw holes with a .169" drill bit. Remove the fixture and move on to the next instrument opening.
Some gyro instruments may be damaged if screws longer than 1/2" are used for securing them to the panel. This does not apply, of course, to instruments with attachment holes which are not internally threaded. These can take any length screw but do require the use of a lock washer with the nuts. Read all the labels on each instrument before undertaking its installation.
Current practice is not to use a thread seal or lubricant when connecting instruments. Any connection that you think would require a thread seal or lubricant can be installed using Teflon tape wrapped around the male fitting only. Many discount and hardware stores now carry this tape.
Each instrument opening should be covered or capped until the instrument is to be connected. This precaution is especially important if you are using one of your good instruments to check each of the instrument panel openings for trial fits during the instrument panels fabrication.
Need More Information and Ideas?
Other references regarding basic instrumentation and installations:
1. SPORT AVIATION, March and April 1975.
2. The Sportplane Builder By Tony Bingelis, Section 9, "Instrumentation."