Avionics News January 2014 - 56

industry

ESSENTIAL DOCUMENTATION:

BLOCK & WIRING
DIAGRAMS
S T O R Y

B Y

J O H N

or the past two years, I've been writing a series of articles focused on real-world avionics issues. Twice in
that time, wiring and block diagrams have come up as
essential documents - indeed, Robert DuRall, senior manager
of customer support at Cobham Commercial and Integrated Systems, said an accurate wiring diagram is the most important issue
an avionics shop can address when making major changes to an
aircraft.
Anyone who goes through a formal basic electronics course
learns to read simple, circuit diagrams. During advanced training, you will see block diagrams, in which the complex circuitry
for modules is abstracted into boxes. Wiring diagrams are a bit
more specialized, and amount to a special form of block diagram
in which each and every wire connecting any module to another
is specifically labeled. They're a vital aid and huge timesaver
when troubleshooting and/or modifying avionics; the only alternative can be the extremely time-consuming process of tracing
wires individually.
While avionics vendors typically supply wiring and/or block
diagrams as part of a supplemental type certificate package, DuRall noted that "many new avionics are installed using approved
model list STCs. Drawings provided by the manufacturer are
usually generic and do not cover every conceivable interface. If
the installing shop does not provide the owner with some level of
wiring diagram that shows how things are installed or connected,
continued maintenance after the installation becomes very difficult; especially if the installation was performed five to 10 years
ago and the installer is out of business or the staff moved to other
companies."
Besides the complete wiring diagram, DuRall sees a role for
simplified block diagrams to illustrate data flow in complex
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avionics news

january

2014

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installations. "Block diagrams are designed to aid technicians
in understanding the flow of information from one module to
another, providing a top-level view of how systems relate," he
said. "Once the technician understands data flow, looking at the
wiring diagram can provide a better sense of how to diagnose
problems."
DuRall is not alone in thinking that diagrams are essential.
Last year, Grady Dees, Universal Avionics director of technical
sales, recommended drawing a diagram whenever new equipment is added - showing any new avionics modules and everything they're connected to, with each connector and port clearly
identified and labeled. "That puts the design on paper so you can
see what's being done and avoid mistakes," he said. "It makes
life easier down the road, and you can put the block diagram in
the flight manual supplement when you're done."
Dees suggested an additional step during complex installations. "Once you have a full survey of new and old avionics and
a block diagram, we generate a spreadsheet-based port allocation
map, which identifies every pin on every port in the system," he
said. "Our displays have six ARINC-429 transmitters and receivers; so in this spreadsheet you assign each port, which becomes a
design guide. Now you're ready for detailed design and integration. Done this way, you'll have very few surprises."
That leaves the question of how to create a diagram - while
we all learn to read them, designing a new wiring diagram from
scratch can be complicated. We also spoke with Nick Grzebielski, Duncan Aviation systems engineer, who has considerable
experience as both a user and creator of block and wiring diagrams. "The first thing I ask for when we bid a job is the wiring diagram," he said. "Without that, it's really difficult to do
complex installations. Dealing with higher-end business jets, if

Avionics News January 2014

Table of Contents for the Digital Edition of Avionics News January 2014