Back in the olden days, when I operated an AST-300 sim business, I could
control both ceiling and visibility. The mode I liked best was the variable
ceiling, which required entry of a ceiling figure and a depth figure (I'm
working from memory here, so don't hold me to exactness). The combination
delivered a sine wave to the visual screen...if I entered a 100 foot depth
and a 300 foot ceiling, the pilot would see/not see as the cloud base varied
sinusoidally between 100 and 300. I had no way to control what the cloud
base would be when the pilot was at DA or MDA, so the student and I were
both surprised with the result.

Bob Gardner

"Barry" wrote in message
. ..
At 200 feet all I can see is some light through the fog so I go down
to 100 above TDZE. At 100 feet I can see the chevrons or maybe the
runway numbers. If vis is 1/8 and I can see the runway numbers, its
hard to understand why the FAA prohibits landing.

The only requirement for part 91 is that
you can see the rabbit through the fog at 200

I want to reply to both of these posts by Robert. 91.175(c)(2) says that
to continue the approach below DH, you must have the required flight
visibility (1/2 mile in this case). I agree that this is observed (not
reported) flight visibility. If you have the required viz and the
approach lights are "distinctly visible and identifiable", then you can
continue the descent (but not below 100 feet unless you see the red
terminating bars or red side row bars, or one of the items listed in
91.175(c)(3)). But seeing the lights DOES NOT relieve you of the
visibility requirement, and I'd say that seeing some light through the fog
doesn't count as "distinctly visible and identifiable".

Note that at 200 feet on a 3 degree glideslope, you are about 3000 feet,
or just over 1/2 sm, from the threshold. So if the viz is right at 1/2
mile, you should be able to see the approach lights almost, but not quite,
to the threshold. Within a few seconds, the threshold should be in sight.
At 100 feet, you're only about 1000 feet from the threshold. So obviously
if you don't see the threshold until 100 feet, slant visibility is well
below 1/2 mile. It's true that forward and slant visibility are not
exactly the same, but it the slant viz is less than 1/2 mile, it's almost
certain that the forward viz is less than 1/2 mile at least somewhere
along your path.

Concerning landing out of an approach like this, I think that most GA
pilots, like myself, have very little chance to practice. We don't have
simulators like the airline pilots do, and it's hard in most places to get
this type of practice in actual. The only time I did an actual approach
all the way down to minimum (reported viz was 3/8) I found that it was not
so easy to transition to landing. I'm sure that with practice it would
become much easier, as you describe, but I don't find the FAA requirement
unreasonable. It doesn't leave much margin for error.

Barry