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Date: Mon, 14 Mar 2005 11:03:57 -0500
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From: "Gary Casey" <glcasey@adelphia.net>
Subject: Re: Vacuum supply for backup gyros
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<<BTW. How DO you pump a vacuum? It is like my kids when I ask " Watcha
doin?" comes the reply "Nothing."
BR>>

I'm afraid I used the word "vacuum" loosely.  The system actually runs the
instruments under pressure most of the time.  Under some conditions the
supply to the instruments is just above atmospheric and the discharge is
just below.  The regulator favors the lowest pressure available - if the
intake manifold is below atmospheric by enough it will use atmospheric as
the supply and the manifold vacuum as the discharge of the instruments.
Under boost conditions it will use atmospheric as the discharge and
compressor discharge as the supply, making a gradual transition between so
that at no time is power to the instruments lost.  Patent 6070607 describes
it in detail.

At first I was envisioning the system as a primary supply, but with the
advent of glass panels there certainly isn't much future in gyro primary
flight instruments.  And there is a considerable hurdle to overcome before
there could be enough confidence in a new system to use it as a primary
source.  However, as a backup to an electric panel I think it makes all the
sense in the world.  The backup gyros are not dependent on electricity at
all and the source is a non-wear item, unlike a vacuum pump.

Gary Casey