Re: Displaced Landing Thresholds

From:         brianm4463@aol.com (BrianM4463)
Organization: America Online, Inc. (1-800-827-6364)
Date:         29 Dec 95 22:22:22 
References:   1
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I may be able to answer a few of your queries.  First of
all, it's a misconception that you land on "the numbers",
or that the GS guides you there.  A glideslope normally
aims for a point at least 1000 feet beyond the threshold,
whether displaced or simply defined by runway end, in
an area called a touchdown zone, usually marked by two
large, wide, white bars.  Next time you fly, notice the
skid marks on the runway; chances are there are very, very
few on the numbers themselves.  Most will be in an area
starting 500' from the threshold extending another 1500'
or so down the runway.

Second, there are many reasons for displaced thresholds.
One as you mentioned, is noise sensitivity.  Other reasons
include surface deterioration, interference with the ILS,
proximity to obstacles, and so on.  Occasionally, thresholds
are temporarily displaced for construction or other short-term
reasons.  Very often, the area behind the displaced threshold
is used for taxi and takeoffs, such as in San Diego.

I don't have an answer about penalties for landing short of
a displaced threshold, sorry.

Finally, I don't know of any reason why a glide slope would be
repositioned back towards what you call "the numbers" once
the threshold has been redefined.  Again, your use of "numbers"
is mistaken - when a threshold is displaced, the numbers that
define the runway direction move along with it and the ILS.

Hope this helps you.  Take care.

B