Vol. 4 May 2006
Velocity's Quarterly Newsletter

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Wiring for the Future
Rick Hole

What is black and blue and red all over??

The answer:  your arms after working behind an instrument panel that was not “wired for the future.”

Wiring is one of the most demanding tasks in building an airplane.  You can get-by with minimal forethought and haphazard workmanship, or you can spend your time wisely and do a careful, well-planned job.  Either way there is one sure thing: eventually your panel will need to be serviced or upgraded.  The time and effort you spend before completing your plane will make those future tasks a dream, a chore, or a nightmare. 

Let your imagination stretch for a moment.  Your beautiful Velocity is complete and flying now for several years.  But an annoying glitch in the wiring has begun appearing; something stops working once in a while.  The faults are increasing and you decide it is time to tackle the issue.

Will you have an easy time?  Or will you have frustration?  Recent service jobs here have leaned toward the nightmare.  Once the panel is in place and all the myriad of electrical, mechanical, and pneumatic connections are made it can be nearly impossible to remove the panel for service.  Even a plug-and-play panel may fool you; a recent one had a long pigtail of audio wiring into the keel preventing its removal.

Plan your panel so that everything can be removed for service.  Recently I spent an hour removing an ELT control head secured in the panel with locknuts.  Those locknuts were a real challenge.  It took a pile of tools and skinned knuckles just to remove the part.  I cannot imagine how I could replace those nuts.   Two hard-points, ¼” x 1” replaced them, and now the control head can be removed and replaced in one minute.  This is bad enough when you do your own work,  but paying shop rate for an hour just to replace a $6 battery is painful indeed.  WIRE FOR THE FUTURE!

Circuit breaker panels are another sore spot.  They are a connection block for dozens of wires and tend to end up so they cannot be pulled out from the panel for service.  When service loops (that’s just extra wire left so things can be removed) are too short, service becomes difficult, time consuming, and expensive.  Simple debug tasks then require removing the canard or instruments, adding to the time, expense, and frustration.  It may be tempting to just “live with” the problem rather than to fix it; a potentially dangerous choice.  Your breaker panel will eventually need to be serviced or new connections added, so be sure to leave that loop.  If you wire the breaker panel while it is removed enough to service you will be certain to have an adequate service loop.

Any device which removes from the pilot side of the panel will need a service loop.  You will not likely be able to gain access from the rear of the panel without removing the canard, if then.  Plan to be able to remove the instrument and to disconnect it from the pilot side of the panel.

Devices which remove to the rear also need planning.  You will need room to slide them out.  Know whether you can reach the connections in place or if it must be done after removal and make wires or hoses long enough to accommodate that need.

Does it matter how wires are routed?  After all, as long as the wire reaches both terminations, what difference does routing make?  It makes a great deal of difference.  Point to point wiring ends up with a spider-web of wires which hamper servicing.  Can’t get that instrument loose through all those **&* wires!   Make wires long enough to be routed in neat bundles.  The wires will help support each other and contain all that clutter.

Carefully mark each wire or cable.  AC-43 requires it, and it makes good sense.  Sure, today you know where each wire goes, but I can guarantee you will forget before the first annual.  Once you have two or more loose wires you have way too many chances to err and only one combination is correct.  Labels made from masking tape will fall off.  Inexpensive labeling appliances tend to use sticker material which is little better than masking tape.  You will save many hours of frustrating wire tracing by good labeling practice.  Some heat-shrink tubings have surfaces that will take ink (especially white or red varieties).  An ultra-fine tip black felt marker marks these tubes well, and after shrinking, your neat printing will look even better. 

And remember: your good wiring practices are not just for show; your quality work will give you a quality airplane and save far more work than it takes when you “wire for the future.”