What I cited is indeed for the approach designer. But, some of it good
"background" info for pilots.

Padraig wrote:
It would be nice to have a flight engineer figuring this out if I need
to make an approach.

Problem is, we sometimes need to determine where to let down from MDA
in relation to some known point. If we use the threshold (since we
generally have information on distance, i.e. MAP on RNAV approach), we
can use that as a VDP. If you use the 3 degree descent rate trick
(groundspeed x 10 / 2 = FPM), it's always going to err on the side of
being higher than you actually want to be since 300/ft per nm (what we
used for our VDP) is actually shallower, 2.8 degrees. So we'll arrive
at the threshold above our plan.

I guess the other thing, from a practical standpoint, is that we're
landing the aircraft visually. So we're not going to use our
predetermined descent rate/VDP point when we're flying the a/c over
the threshold at the TCH. We'll use the visual indications in the TDZ
(or VGSI) to get us on the ground.

In any case, the information you provide is good to know. Probably
more for design of an approach in the office, not practical
application in the cockpit. I would have to declare an emergency if I
had to do all of that math and fly at the same time!

PW

On Mar 18, 10:58 am, Sam Spade wrote:

Padraig wrote:

Bob, sorry, substitute TDZ with MAP. My point is that generally a VDP
is measured from the end of the runway (beginning of TDZ) which is
often where the MAP is (on RNAV approaches).

Not exactly correct:

(1) For runways served by a VGSI (regardless of coincidence with final
VDA) , using the VGSI TCH, establish the distance from RWT coordinates
to a point where the lowest published VGSI glidepath angle reaches the
appropriate MDA.

(2) For runways NOT served by a VGSI, using an appropriate TCH from Vol.
3, chapter 2, table 2-3, establish the distance from RWT coordinates to
a point where the greater of a three degree or the final segment VDA
reaches the appropriate MDA.

My point is that a VDP has a threshold crossing height (TCH) as do all
vertically guided IAPs.

As to the MAP being at the approach end of the runway (AER), that is
correct only for NPAs whether LNAV, VOR, NDB, or LOC/LDA. The NPA MAP
will be prior to the AER when obstacles in the missed approach segment
preclude siting it at the AER.

Vertically guided approaches (with a DA/DH) always have their MAP at DA/DH.