A piece of e-mail conversation I thought I would share:
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>More thoughts on reliability. I happen to like the dual-bus architecture
>but I want to play devil's advocate for a moment. Is the main-buss/essential
>-buss really an advantage? Would it not be possible to achieve the same
>thing by providing a bypass for the master contactor and just manually shut
>off the nonessential stuff?
Why add to pilot work-loads? And, would you want to rate the "bypass" system
so that it could carry full system loads? You'd have to assume that it may
be operated at some time before the pilot forgets to turn something off.
The MB/EB system is absolute. Figure out in advance what your essential
loads are and keep 'em small! Have at most two switches to throw. Make it
impossible for the EB alternate feed to backfeed the main bus (that's what
the diode is for).
>You would reduce your parts count and simplify the construction.
At the expense of making the pilot do systems analysis and checklist
driven procedures just when he SHOULD be concentrating on flying
the airplane.
>OK, one possible problem is a short on the main buss. . . . .
Pick any airplane, grab a toolbox and fabricate a short to the
main bus using only materials that currently exist and are in
close enough proximity to main bus or feed wiring to pose a
real threat . . . If you can find an airplane where this
is possible, it's been very badly designed. I've not found one
yet where I can meet the challenge.
>But aren't we protected by having *all* devices protected by switches
>and fuses? And what is to prevent a short from occurring on the
>essential buss?
Individual circuits ARE protected and one must assume that any
one may fall prey to a fault that takes that particular system
down. Essential busses should enjoy the same freedom from
extraneous faults to ground. The fuse-block bus structures
I favor are totally enclosed except for a feed-point stud
on the end which is easy to cover with a boot. . . .
>I was also thinking more about the polyswitch single-board
>switch-buss. Your points are well taken about mechanical
>integrity and security. I know that there is no built-in
>rigidity for the wiring but what about adding an outboard
>tie-bar to take up the mechanical load for the wiring bundle
>thus letting the terminals supply only electrical connections?
Can't visualize what you're describing . . . but it
can't be a lower parts count nor be any more vibration
resistant than fuse-blocks and switches.
>Also, having the switches and protection right at the bus
>reduces the number of wires by almost a factor of 2.
Perhaps, but this architecture demands that the protection
be right behind the switches and makes the switches hard
to change. It also uses electrical contacts of switches
for mechanical support of the etched circuit board . . .
a stress the switches were not designed for. It makes
switches hard to replace and uses premium, behind the panel
volumes to contain equipment that is better off under the
seat!
>I know you say that the avionics master is outmoded these days but
>the major reason people want them is convenience. I hate turning swtiches
>on and off. I want to turn on all the radios with their individual
>switches and then turn the whole shooting match on or off with a single
>switch.
How many radios do you plan to have? The well-stacked panel of
old is no more . . . A nav/com, xpnder and GPS about fills out
the requirments for 95% of the airplanes being built. The Beech
A36 I used to fly had LOTS of switches but most of the airplanes
I rent these days don't even get some of the radios turned on
for the flight . . .
>Heck, if it weren't for the potential problem of transients from the
>starter motor causing problems with the solid-state devices, I would
>just switch them with the master switch.
That's okay with me . . starters never did put out radio-killing
transients. Low voltage during cranking used to kill a radio or two
and over-voltage from alternators has killed a LOT of radios but
I've searched and searched and never found a starter circuit that
could develop transients that would hurt a properly designed radio.
There's lots of solid state stuff that gets tied to the bus with
NO means for shutting it off . . . audio amps, turn coordinators,
voltmeters, electric clocks, fuel flow measurement systems, air
data converters . . . yet the myth persists that a RADIO is somehow
a "spike magnet." Old pilot's tales die hard . . . . .
>The avionics master is a potential point of failure. A wire is
>much more reliable than any switch or breaker.
A wire with one diode in it isn't much less reliable . . that's what
I use between main bus and essential bus and it's backed up anyhow.
>OK, let's consider the possible problems of transients on the bus
>raising havoc with the avionics. How about MOVs for absorbing bus
>transients?
Right on . . . . . . let's design a transient free system instead
of guarding against transients that may or may not be there . . .
>What kinds of transients does a starter motor produce? How much energy
>are we talking here, i.e. voltage, current, and duration given that
>the battery is going to absorb a bunch of that?
Now you're talking like an engineer. If there's something to be
protected from, let's quantify it and either (1) protect against
it or (2) eliminate it.
>Most radios . . . .
No . . . ALL radios
>have some sort of input filtering usually consisting of a moderately-
>sized (10-100 uf) electrolytic cap. If I know the voltage changes
>and rise times I can figure out the inrush current for the radios.
The DO-160C spike test is EASILY passed with a good quality,
10uF capacitor across the line. . . .
>That is one area where I thought that Control Vision had a good idea:
>the avionics master was switched off when the starter was engaged. . . .
It's never a bad idea to have everything off that can be off before
engine cranking . . . it's simply good cockpit hygene.
>The cost is a relay (point of failure, I know). Thought: when I
>release the starter switch and the starter contactor opens, does
>the avionics relay close before the starter contactor opens?
>If so, my radios see the tail end of the transient(s) anyway.
Got no idea . . . it's a mute point anyhow. Before you install
any radio in an airplane, you should call the manufacturer and
ask them if they've tested to DO-160 for all bus voltage conditions.
If the answer is NO, then they've NOT DONE THEIR HOMEWORK. In the
case of battery powered hand-helds and cigar-lighter adapter cords,
they are under no obligation to consider DO-160 so some simple
pre-filtering and transient clamping is in order. However, if anyone
claims to be airplane panel-mount qualified and won't sign up
to DO-160 should not get the pleasure of your business. I've
been building products to that spec for over 25 years!!!! It
ain't hard.
>Oh well, just some mental ramblings. I enjoyed our conversation.
>It is a pleasure to talk with someone who thinks about how things
>really work.
Thank you . . . I enjoyed it too. If we're to avoid being
dragged under the protective arm of the FAA, all of us
had better spend more time learning about how things work.
The "experts" are no longer resident at BPC&M nor the FAA.
The real experts are working away in basements and garages
all over the world. Communication is the key to building
airplanes that are better in every way than anything which
can be purchased . . . long live the internet!
Bob . . .
AeroElectric Connection
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