> I guess I have once again "lost the bubble" on one of your threads. Let me
>recap my understanding of what has passed here and you can tell me where I
>reached the wrong conclusion.
>
>1) There was a copy of a message to Avweb posted by someone with
>acknowledged expertise advocating the use of an avionics master to protect
>any/all installed avionics in an aircraft during engine start.
>
The keyword here is "protect" . . . the AMS was birthed 30 years
ago. Avionics have evolved through 4-5 generations of technology
advancements but our way of thinking about them has not.
>2) There was a reply from you stating that the avionics master is an
>anachronism that has no place in a homebuilt using modern avionics and
>electrical system layout/design techniques.
>3) Flurry of responses from listers saying, "yeah but if the switch cost so
>little and could potentially save me thousands of dollars in repairs on my
>avionics, why not use it?"
Because, if it's the ONLY power path to all the radios, then
it's far more likely to be the source of failure for all radios
than to be the guardian angel for any radio.
If one wants an AMS, by all means have one but do good systems
design and ASSUME that all wiring and components are subject to
failure. Any such failure kills all radios. Radios, (and a few
other "essential" items should be supplied via independent power
paths. See <http://www.mosci.com/buildersden/nuckolls_philosophy.html>
or Sport Aviation, Feb 93, p.80 for a more detailed discussion.
>4) Ongoing debate amongst you and various listers stating that manufacturers
>should be forced by the laws of supply and demand economics to provide
>avionics that are impervious to any power supply anomalies.
They already are. The thrust of my words was to suggest that if
you don't believe it, ask 'em. If any manufacturer admits to
some form of fragility in their design beyond gross overvoltage,
ask them why they don't conform to the industry standard testing
regime DO-160. I'm working on an article about DO-160 which I
will publish on our website and elsewhere.
> I guess its #4 above that prompted my post. I was under the impression from
>your postings that there is still avionics gear on the market that is
>subject to problems during engine start.
. . . if there are, I'm unaware of them. I've asked everyone to show me
documentation from any manufacturer that requires pilot intervention
to "protect" their product from ordinary airplane systems operations.
Starting the engine seems pretty ordinary. Further, I'd like to
know about it. I'll write or call them and get the scoop. I did write
to Terra some years ago about a statement in their installation
manuals alluding to an AMS . . . the engineer said, "Oh, we just stuck
that in there because everyone seems to expect it. Sure, we've done
our homework for input power conditioning."
> . . . . . . Your response to my post implies
>that this is not the case. If that is so, then how do you explain the Avweb
>post that started this whole thing? Was that guy just plain wrong?
Not for 1970, but for 1997 I'll have to say, "yes".
> . . . . And what
>was the point of your repeated posts calling for change to be forced on
>avionics manufacturers by the marketplace? I guess I am no less confused
>about what you are trying to say than I have been all along.
The "call to arms" was intended as an inducement to consumers of
expensive, complex equipment to find out for themselves if the ol'
mechanics and pilot's tales are worthy of consideration. If you've
got a concern about it, call the manufacturer. If you don't like
what they tell you, I'd like to hear about it. The Avweb
article was simply a repeat of a 30 year old mantra. Quite frankly,
people who build this stuff don't care much one way or another.
It doesn't HURT their product for you to "mother" it. However, based
on points in the article referenced above, the "mothering" circuitry
has a potential for bringing the whole system down. Further, an
electrical system FMEA shows there are times when you may want to
shut down both normal battery and alternator circuits for best utilization
of a finite quatity of energy in the battery. This ALSO calls for the
second power path to essential goodies.
All technology based products experience quantum jumps in performance and
value happen every few years. Gizmos that run from automotive dc power
systems are included. There's not 2 cents worth of difference between
the dc power system in your airplane and the one on your garden tractor
or car. EVERYBODY in these markets have had access to transient
supressors,
high voltage silicon transistors and high quality capacitors for over 15
years. For a very nominal cost, today's products can thumb their nose
at ANYTHING the dc power system throws at it . . . including overvoltage
to the tune of 20 volts for 1 second . . . plenty of time for an ov system
to react and tame the runaway alternator.
> Please understand that I dont intend this as any type of flame. I have no
>knowledge of avionics whatsoever. The various standards that you have
>referred to in this thread mean nothing to me. I, as do many other listers,
>rely on the expertise of others such as yourself to attempt to make an
>intelligent decision. When expert advice from 2 different sources conflicts,
>what are we supposed to do? I'm simply trying to avoid making a very
>expensive mistake.
I fully understand and no offense taken. I also understand your problem.
As spam-can owners, we have had to take comfort in the reams of regulation
and decades of tradition (I prefer to call it mired in the ruts). Now
you're a one-man airplane factory and you're getting some exposure to
a few of the issues that were once left to unseen gurus. If you're more
comfortable having an AMS, put one in. But the REAL issue here is SYSTEM
reliability which goes far beyond concerns for "spikes" . . . real or
imagined. If I have a mission here, it's to ask people to focus less
on the minutia . . . it's admittedly easier to do and we all used to take
great comfort in spec'ing the bejabbers out of everything bolted to an
airplane. But what good is it to put a $100 mil-spec switch on a landing
light bulb that you KNOW is going to burn out? What we're really
interested
in is comfortable completion of every flight. I work toward that by
assuming
that EVERYTHING is subject to failure and I'll either have (1) a backup
or (2) don't need it. My real heartburn with the classic AMS has nothing
to do with "spikes" . . . it has to do with system reliability in that
it goes against well considered design. There are a number of articles
on our website that speak to these issues.
Bob . . .
AeroElectric Connection
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(o o)
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