If we're talking about an ILS, then the aircraft is flying a steady 3º slope towards the runway, so the slant visibility will be steady all the way down, n'est Pas?
Check,
You'd think so, but my experience has been different.
I've had the phenomenon of being able to quite clearly see the runway environs on the slope, only to have the percieved vis drop dramatically once actually in the mist. Can't give you a definitive explanation.
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