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From: marcum@sun.uucp (Alan Marcum)
Newsgroups: net.aviation
Subject: Re: Out to dry on the glideslope
Message-ID: <3293@sun.uucp>
Date: Wed, 26-Feb-86 11:15:42 EST
Article-I.D.: sun.3293
Posted: Wed Feb 26 11:15:42 1986
Date-Received: Fri, 28-Feb-86 22:15:10 EST
References: <2473@pixar.pixar> <474@gcc-milo.ARPA> <2493@pixar.pixar>
Organization: Sun Microsystems, Inc.
Lines: 36

> In article <474@gcc-milo.ARPA>, brad@gcc-milo.ARPA (Brad Parker) writes:
> 						    At any rate, in many of
> the approaches we have flown during training we have intercepted the GS
> *before* reaching the localizer.  This is actually quite handy since you
> can already have the rate of descent established by the time you have that
> other little needle to center.

If you begin descent on an ILS prior to getting "established on the
localizer," you may fly through a granite (or steel or cement) cloud.
An approach clearance could read, in part,

	"Fleetfoot 1234A is 5 miles from the outer marker.  Turn left
	heading 330; maintain at or above 3000 until established on
	the localizer.  Cleared ILS 30L approach."

Now, if you're above 3000, and the glideslope needle starts coming in,
fine, start the descent.  Flying around (SF Bay Area), the clearance
usually is something like "maintain 3000 until established" -- I'm
already as low as I should be, anyway.  So, the question becomes, "When
am I established?"  I learned that you're established when the localizer
needle comes off the peg during intercept.

I don't happen to have my FAR/AIM handy here, nor other descriptions of
the ILS transmitter system.  If I recall correctly, there are indeed lobes
on the GS -- "false gs" -- but spurious localizer signals are detected by
the receiver, and the flag on the omni head goes up.

From my own experiences during training, a fals GS will indeed foul things
up.  I found one while learning about the ILS tracking abilities of the
autopilot.  The autopilot captured the localizer and a false GS.  Both my
instructor and I noticed we were a tad (!!) high at the outer marker,
and we decided to let the A/P continue to fly the approach (we were in VMC)
for demonstration, until things got a bit out-of-hand.
-- 
Alan M. Marcum				Sun Microsystems, Technical Consulting
...!{dual,ihnp4}!sun!nescorna!marcum	Mountain View, California