[Marc_H]

VISUAL DESCENT POINT

The process of computing a VDP requires that you determine from the approach
chart the height above touchdown. Example: IAH 15L MDA(H) 460’ (362’).
We can see from this example that when level at the MDA of 460’ you are at a
height of 362 feet above the runway. We know that on a normal 3° glidepath we
descend approximately 300’/nm. Dividing 362 by 300 gives a VDP of
approximately 1.2 miles from the end of the runway.

Distance from the end of the runway can be determined from DME, if available,
or on the CDU if the runway waypoint (RWxx) has been selected.

Note: A VDP based on time can be determined if DME or FMC position is
unavailable. Take the HAT and change it into seconds. Example:
HAT 360 feet. Take the 360 and take off the last digit making it 36. It
should take approximately 36 seconds to descend from the HAT to the
TDZ. Simply take the total time from the FAF to the runway and
subtract the 36 seconds. Example: if the time from the FAF to the
runway is 2:36 subtract the :36 which gives a VDP of 2:00 after the
FAF.

Quelle: Continental Airlines FCOM