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Stan Prevost wrote:

> Hi, y'all -
>
> Some precision IAPs contain a note that the glide slope is unusable below
> XXX feet, some say that glide slope is unusable for coupled approach below
> YYY feet, some say both, where YYY > XXX.
>
> What makes the glide slope unusable down low? What makes it unsuitable for
> autopilot coupling at a higher altitude than it is otherwise unusable?

It is a matter of flight inspection tolerances--roughness and scalloping.
Beyond a certain parameter it is too rough for coupled approaches but still
"okay" for flight director or manual vertical guidance because it is still has
integrety and accuracy as a vertical flight path. When it is marked unusable
the degradation is so significant that it is unreliable. The specs are contain
in the flight inspection handbook.

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