Vdp

[CaptO'Brien]

I am just wondering if anybody can give me some information on VDP calculations.

I have looked into how to do the calculations, but I don't really understand how to apply them to an actual approach. Dont know if this sounds stupid but i would appreciate somebody explaining it to me.

If my calculated VDP is 3 miles what does that mean?

Thanks.

 

[BOHICAgain]

I am just wondering if anybody can give me some information on VDP calculations.

I have looked into how to do the calculations, but I don't really understand how to apply them to an actual approach. Dont know if this sounds stupid but i would appreciate somebody explaining it to me.

If my calculated VDP is 3 miles what does that mean?

Thanks.

No problem. 3 miles is a bit much, most are like 1.5-2 miles away from the runway. If the MDA is 700 feet and u get there with 4 miles to go and u have the runway env. in sight it would be to soon to descend so the VDP is used to wait till u get to the mileage you calculated to begin a normal 3 degree descent to the runway. It is also used as the point of no land. If you reach your vdp while at your MDA and see nothing you know u cannot make a normal 3 degree descent to landing so you should have made up your mind to go around at the MAP.

 

[CaptO'Brien]

Oh ok..So basically just a way of calculating the point that will get you a 3 degree descent path from teh MDA onto the TDZ and anything past that will cause you to have a steeper approach....Makes alot more sense. Thanks!

 

[regionalcap]

Just a side note. You calculate a PDP not a VDP. The VDP is already charted. If you do not see a VDP on the chart, then you calculate a PDP (Planned Descent Point).

 

[SSDD]

If you have a 3 mile VDP, it means your MDA is 900'.

Technically the VDP is published on the approach, you don't calculate it. But you can figure out a point that accomplishes the same thing.

Basically take your height above touchdown and divide by 3, or actually 300, and that will give you your distance to start the descend for a 3 degree glideslope.

So MDA is 300' above touchdown, your point of descent is one mile. 450' its 1.5 miles. 500' is 1.66, or 1.7 miles.

 

[Amish RakeFight]

Do a search... lots about VDP/PDP's

You calculate a PDP - VDP's are published on the approach chart.

DISTANCE:

HAT / 300 = PDP (miles)

ex. HAT = 600'/300 = 2 miles.
PDP is 2 miles prior to MAP




TIME:
10% of HAT = PDP (time)

ex. HAT = 600' x .10 = 60 seconds.
PDP is 60 seconds less than the published time for category.

 

[bailout]

Also, its not necessarily the mileage from the MDA. If you are 6 miles out or so and higher than the MDA, as in an approach that has a mid point step down, you can figure a PDP from that point as well. Or in case of a FAF fix, like a VOR thats 4/5 miles away, or whatever, you can figure a PDP from that point.

 

[Amish RakeFight]

VDP/PDP Descent

Oh yeah, since we're on the subject of VDP/PDP's - If you have the required visuals for a landing prior to VDP/PDP, you shouldn't descend from MDA before reaching this point. Think obstacle clearance.