On Mar 21, 10:57 am, "Barry" wrote:
Nitpicking here, 91.175(c)(3) does not mention the "rabbit" aka RAIL
or Sequenced Flashers as an acceptable visual reference. You can
descend to 100' using the *approach lights* as a reference...
91.175(c) was changed to make "the runway environment" explicit.
RAILs or SFs are not on the list.

I disagree, 91.175 (c)(3)(i) says "the approach light system" which certainly
includes the sequenced flashers - see the AIM 2-1-1 which states "Some systems
include sequenced flashing lights".

So you don't have the "system" in sight if you only have the RAIL/
SF, but not the approach lights themselves, and the next clause refers
to descending with the approach lights in sight.

If "approach light system" is to be parsed as lights plus RAIL, you
need both. If you parse it as the lights, you need the lights. It
doesn't say "any part of the approach light system".

From a practical point of view, in my experience, having *only* the
RAIL as a visual reference is more of a distraction than a help.


One important point that people often miss is that in addition to seeing the
lights, the pilot must have the required flight visibility in order to
continue the descent below DH. Since 100 feet above TDZE on glideslope is
only about 1000 feet from the threshold, if you can't see the runway well
before then, you probably don't have landing minimums, and are not authorized
to go below DH.

When you get to a 200' DH, you are 4000' from the fixed distance
marker, 3500' from the beginning of the TDZ and 3000' feet from the
threshold. So with a minimum 2400/1800 foot visibility, you won't be
able to see the threshold yet. That's why you are allowed to descend
to 100AGL with only the approach lights as a reference - you should be
able to see *them* if you have the required visibility.
(Parenthetically, if you can't, and can only see the RAIL, I find it
not credible that you do have the required visibility) At 200AGL (DH),
you get to decide whether to continue the approach on the basis of the
visibility at that instant and the presence of at least one the visual
references listed in 91.175. If at some time thereafter, eg at 100AGL
you still can't see the threshold, or for that matter the beginning of
the TDZ which is now 1500 feet away (the fixed distance marker is
still 2000 feet away, so with a 1800RVR minimum, you maybe can't see
it yet) *then* you have to miss the approach.

So you haven't thought through the logic here. It's true you'll need
to be able to see the runway before 100AGL, but you won't necessarily
be able to see it before DH. So you can't say you shouldn't have gone
below DH on the basis of not being able to see the runway at DH! You
also can't say retroactively that you shouldn't have gone below DH on
the basis of what you didn't see later. The required visibility is a
requirement that applies continuously below DH. In addition, there
are *specific* required visual references that apply at DH and
100AGL.