Re: shielding of digital avionics and subnets

From:         Robert Dorsett <rdd@rascal.ics.utexas.edu>
Date:         01 Feb 93 22:30:55 PST
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In <airliners.1993.123@ohare.Chicago.COM> kannan91@iastate.edu wrote:

>Finally since Mr. Drinkard is from Boeing I would also like to
>know how does this philosophy about shielding change with respect
>to the 777 as it will be a fly-by-wire airliner where EMI could
>affect control actuation as well.

I can't speak for Mr. Drinkard or Boeing, but back when the A320 was
being developed, Airbus was claiming a 600N weight savings over a con-
ventional control system.  This was around 1984: by 1988, that figure was
distributed as 200 *pounds*, and in late articles, the savings aren't
mentioned at all.  It is safe to conclude that Airbus neglected to include
the importance of shielding in its weight forecasts.

There have also been articles which indicate that spurious interference,
or even latent static buildup, causes a great number of "unsubstantiated"
component failures and erratic behavior.  I.e.: device fails: crew reports 
it.  Removed from aircraft, bench-tested.  Nothing wrong.  Ergo: damned pilots 
overstating the problem again, gotta take 'em out of the loop. :-)  This 
affects relatively "old-technology" airplanes, such as the 747-200, as well 
as more modern airplanes.

The A320 has been exensively tested, according to a standard "DO" something-
or-another.  The Brits even made Airbus fly the A320 near one of those
gigawatt-range military arrays near the English channel, as part of its
certification conditions (this was shortly after a series of reports
of Apaches suddenly wanting to flip over after doing the same).  From 
published reports, though, I suspect that the probability of "minor glitches"
is *much* higher than single-instance catastrophic failure.  Of course,
it's hard to estimate when a "lot" of minor problems suddenly become a big
one.

What is UTTERLY APPALLING about the current situation is the trend of 
manufacturers to hide circuit breakers from the pilots.  In the A330 and
A340, circuit breakers are below deck.  Yet in the A320, A310, 757, and vir-
tually other modern airplane, pilot-developed CB work-arounds to faults in
the *system* design are common.  It should be interesting to see what the
A330 and A340 dispatch reliability turns out to be.

When the 747-400 was being developed, Boeing initially considered hiding
the circuit breakers in a similar manner, but abandoned the idea.  Does 
anyone know if they've returned to this philosophy in the 777?   I'm quite
disturbed by the prospect of maintenance-reported "non-existent" glitches
getting imbedded in manufacturer human-performance engineering design 
considerations.





--
Robert Dorsett
Internet: rdd@rascal.ics.utexas.edu
UUCP: ...cs.utexas.edu!rascal.ics.utexas.edu!rdd