Re: Displaced Thresholds

From:         wells.b@portseattle.org
Organization: InterServ News Service
Date:         29 Dec 95 22:22:22 
References:   1
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I think you'll find that most displaced thresholds exist so that an
approach slope clears an obstacle, particularly one that the airport can't
remove or lower.  They may also be also used to provide appropriate runway
safety area dimensions.  Unless there is a sizeable displacement, there's
little noise benefit.  A 1,000 ft displacement of a 3-degree glide slope
is only 52-53 ft change in altitude.  The glide slope is aligned with the
displaced threshold; locating aligning back towards the pavement end would
result in a approach that didn't meet clearance margins.  Turbojet ops
normally set up the approach on the glide slope & once the descent rate
& alignment are set up & stable, I doubt there's much last-moment tweaking.