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From: mat@amdahl.UUCP (Mike Taylor)
Newsgroups: net.aviation
Subject: Re: Re: Pressure vs Altitude
Message-ID: <1459@amdahl.UUCP>
Date: Mon, 29-Apr-85 00:30:56 EDT
Article-I.D.: amdahl.1459
Posted: Mon Apr 29 00:30:56 1985
Date-Received: Tue, 30-Apr-85 03:59:32 EDT
References: <458@cepu.UUCP> <800002@pbear.UUCP>
Organization: Amdahl Corp, Sunnyvale CA
Lines: 34

> 
> 
> 
> 	Most altitude encoding trasnponders that I know of use a seperate
> bellows to drive the transponder. When you upgrade your transponder to
> include altitude encoding information, you DO NOT replace the altimiter you
> have already. A black box is added that has a connection to the static air
> vent. It is fixed at 29.95 inches (I think) and the ATC adjusts for local
> barometric changes to come up with your real altitude.
> 
> 	Therefor your mode C information is still reliable. However if both
> fail then you got troubles, but I would never break the MP glass since I
> would have no indication of engine performance. The MP guage can predict
> engine failure in advance of it actually happening.
> 
> 						Peter Barada
> 						ima!pbear!peterb
> 						ihnp4!inmet!pbear!peterb

What you say is true for many older airplanes that predate altitude encoding
requirements.  However, most newer airplanes use encoding altimeters which
provide electrical input to the transponder.  This is a cheaper arrangement
when you're starting from scratch.  However, I really don't care to argue
the point - I am just trying to pass along an obscure safety tip that might
help somebody someday. Accurate altitude information is essential for safe
IFR, in my opinion. On the other hand, timed climbs and descents can be used
to make altitude changes. I've done enough of them to know that they require
much practice. Personally, I'm not inclined to fly IFR without a backup 
altimeter because I don't want to guess at DH on a nasty day.

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

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