vdp

[unlvjj]

Can some one please tell me why some approaches have VDP's and others do not.

 

[gnvav8r]

ILS don't have 'em 'cause you miss at a DH, which just happens to be very close to the VDP (if you calculated it). If you break out, you are essentially at VDP and can safely continue your descent

Some VOR approaches do not have 'em, especially if there is no DME readout. How would you know if you are at VDP? You miss at the VOR which may be midfield.

Ditto for NDB approaches. No distance measurements available for you to look at in cockpit.

On the other hand, some VOR approaches (e.g. VOR 29 @ KGNV) do have a VDP, 'cause they have some waypoints published, and you get a GPS readout of just where you are.

Pete

 

[bertengineer]

THe VDP on a Non-Precission approach is there to keep you at that altitude for obstacle clearance and a point at which a 3 degree glide slope would get you to the runway. Once past the VDP you have the possibility of pushing over on the nose too much and diving for the deck. If you do not have the runway in sight by the VDP you might want to re-think your landing and go missed at the MAP.

For the NP approaches that do not have a VDP it is useful to do a PDP...pilot decissson point

HAT/300 for distance (300 for 3 degree G/S)
HAT /10 percent for time this is to be subtracted from the timing to give you a better estimate of the PDP.