Avionics News January 2014 - 57

ABOVE: Inverter wiring diagram; OPPOSITE PAGE: Block diagram
Diagrams courtesy of Duncan Aviation

there's no diagram - or it's inaccurate - the time requirement
of the job increases astronomically. Any STC we do generally
includes a block diagram, even for small systems, just to show
what modules are involved without having to scroll through wiring diagrams."
According to Grzebielski, it's important to keep in mind the
purpose of each type of diagram. "We try to keep our block diagrams as simple as possible," he said. "Name the box, and label
the lines for data type/voltage, etc. That's about it; for more detail, refer to the wiring diagram. We do show switches, so you
can follow the switching logic.
"Wiring diagrams, on the other hand, have gotten more complex over the years - they may include references to maintenance
manuals, other drawings, whatever FAA guidance is applicable
to a particular job. Since we're an ODA (organization designation authorization), our drawings may include references to internal processes. If you have any information that might be useful
to someone troubleshooting down the road, it goes on there. Also
parts lists - including vendor and modification status. Recently
we've included alternative parts for things like connectors. Fundamentally, the diagram maps every wire, and everything those
wires connect to - connectors, antennas and modules."
I asked about the situation for smaller shops, which may lack
the resources available to large organizations like Duncan. According to Grzebielski, "If you can at least document what you
did, that's better than nothing, but it's really invaluable to have a
diagram showing the full wiring."
On a practical matter - how do you create diagrams? Some of
us are old enough to remember when this was done on a physical
drafting board, but it's no surprise to learn that computer software is the rule today. Grzebielski noted that at Duncan, "We use

AutoCAD - any two dimensional drawing software will work."
DuRall agreed and said, "You can use something like AutoCAD.
If you have a full Microsoft Office suite, Visio is a good option,
and I've even seen block diagrams done using Microsoft Word
or Excel."
Generating a diagram - particularly a complete wiring diagram - takes time, but Grzebielski said it's worth it. "There's
a payoff when bidding. For someone who has been one of our
customers, we know what information is in the diagrams and can
give a more accurate quote. Our main point to our customers is
that the more you have on paper (or electronically), the better off
you are. Sometimes the customer supplies you with drawings
that are out of date, and that just takes extra time - you have to
run down what mods have been done, and then do an electrical
load analysis to see if the mods are adequately powered."
We asked about standards for diagrams, and were a little surprised to learn this doesn't seem to be a major focus. DuRall
pointed to General Aviation Manufacturers Association specification No. 2, which "has a number of relevant items for diagrams
and symbols." Grzebielski said Duncan has developed its own inhouse specifications, based in part on ASME Y14.1-2005 Decimal Inch Drawing Sheet Size and Format, ASME Y14.2M-1992
Line Conventions and Lettering and ASME Y14.3-2003 Multiview and Sectional View Drawings. ISO 2042-1973 is an international standard for aircraft wiring diagrams that provides some
basic guidance (among other things, noting that aircraft wiring
diagrams should document systems with power off unless there
is a good reason to do otherwise). An engineer with experience
creating avionics drawings at a manufacturer said MIL-STD
100, Standard Practice for Engineering Drawings, is also used
as a basic reference. q
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Avionics News January 2014

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