Re: APUs on A340s

Date:         06 Aug 97 01:49:31 
From:         k_ish <kenish@ix.netcom..com>
Organization: Netcom
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Joseph Edward Nemec wrote:
> I was wondering if someone could answer a question. I have flown on an
> A340 several times now, and there has always been a problem with
> something. Once, the reading lights didn't work properly; they would
> turn on, but not turn off. A second time, none of the toilets worked,
> and the flight was badly delayed. A third time, I got on the airplane,
> and all the cabin lights kept going off and on. Some lights that looked
> like emergency lights came on a few times.
>
> What's the deal? Does the A340 have problems with its auxiliary power
> units? Someone told me they did.

Perhaps they do have APU problems, but the "light show" you saw was a
different system.  Widebody aircraft systems such as cabin lights, PA,
audio/video entertainment, attendant call, reading lamps, NSFSB (no
smoking/fasten seatbelt) lights, etc. are all controlled by a digital
multiplex system running through the cabin. This is called the PSS/PES
(Passenger Entertainment System/Passenger Service System).  When you
press the reading light button, for example, on the armrest controller
(called a Passenger Control Unit, PCU), a digital command is transmitted
to a computer, usually one per cabin zone.  It tranmits the command to
another unit in the overhead compartment that turns on your reading
light.  This system may be complex, but it saves weight versus a
"hardwire" system, and can be reconfigured as seating arrangements and
classes of service change.

If this system goes nuts and starts sending out garbage data, all sorts
of interesting cabin lighting glitches can occur.  It sounds like this
is what you saw.

Ken Ishiguro