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Path: utzoo!linus!philabs!cmcl2!seismo!hao!hplabs!amdahl!mat
From: mat@amdahl.UUCP (Mike Taylor)
Newsgroups: net.aviation
Subject: Re: Pressure vs Altitude
Message-ID: <1369@amdahl.UUCP>
Date: Sun, 7-Apr-85 21:37:41 EST
Article-I.D.: amdahl.1369
Posted: Sun Apr  7 21:37:41 1985
Date-Received: Wed, 10-Apr-85 04:12:10 EST
References: <1164@druxn.UUCP> <1330@amdahl.UUCP> <463@terak.UUCP>
Organization: Amdahl Corp, Sunnyvale CA
Lines: 35

> > In case of altimeter failure without backup, break glass on MP indicator
> > and use it as a substitute altimeter, with the above relationship.
> 
> Huh?  Although the Manifold Pressure gauge can indeed provide some
> gross altitude info, I don't see how breaking the glass is going to
> help.  What you need to do is to get the manifold vacuum to zero,
> for example by shutting down the engine and stopping the prop.  With
> a non-turbo engine, firewalling the throttle will also bring the
> vacuum to near zero (but set your prop to fine pitch first :-)
> 
                                           
Shutting down the engine seems an extreme solution to the problem.
Mode C is derived from an encoding altimeter (usually). If the altimeter
failed (the original problem) then your mode C won't be working.
While VFR, eyeball is an obvious solution (unless VFR on top, of
course).  Breaking the glass on the altimeter allows cabin air to
enter the body of the MP gauge, providing a static source which replaces
the line to the intake manifold. The MP gauge contains an exhausted
bellows and a spring. This will react and measure the cabin pressure
instead of the manifold pressure. The manifold pressure will only equal
the static pressure when the engine is stopped. The flow through the
ductwork and manifold will cause a loss of 1 to 2 inches at least,
equivalent to a 1,000 to 2,000 foot error. That's why you break the
glass. It's a good idea to always have two altimeters, or one
altimeter and an independent encoder ... a so-called "blind encoder".

With all due respect, I suggest that breaking the glass is a much
better solution than stopping the engine if you don't have backup -
my original premise.


-- 
Mike Taylor                        ...!{ihnp4,hplabs,amd,sun}!amdahl!mat

[ This may not reflect my opinion, let alone anyone else's.  ]