Limitations of autoland systems

From: (Peter Coe)
Date:         16 Jan 95 21:58:18 
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A couple of weeks back we were talking about what was special about
the Tristar on its debut.  I mentioned that it was something to do
with it's on delivery autoland ability, and I related a story about
a landing in pretty bad conditions.

Well I have something to add to this.  I flew home to England for
Christmas, and again did an autoland.  Heathrow on 12/23 was in a
really bad state.  I had a window seat, and had at most a second's
warning of touchdown.  The captain came through the cabin later
and said that he only saw the runway 7 seconds before touchdown.
The landing though was the easy part.  After leaving the runway,
we moved to a parallel taxiway (very very slowly)and then parked
for 2 hours.  The problem was of course that the airport was
completely congested.

So after all that the question is what are the limitations when the
weather gets this bad.  The flight crew on my return flight left
Heathrow on 12/23, and they were delayed for more than 3 hours for
the same reason as us.  According to them, in normal low visibility
conditions, Heathrow puts a 2 minute separation on aircraft, on this
day it was 3 minutes.  In addition, many aircraft had to stay at the
gates because they just did not have the technology to be able to
safely fly in that weather.

The conditions were really bad.  I had a great opportunity to view the
runway, and the fog varied in thickness, so that sometimes I could
quite clearly see aircraft on the runway, and sometimes couldn't even
see the outer engine of the 747.  But I have a hard time translating
these impressions into what ATC call acceptable.  I would be really
grateful if any of the professional aviation people here could give
more details on the categories of autoland, and the limitations that
they have.

-- Peter Coe
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