Vdp

[flyboyzz1]

How do you figure out a VDP? Can anyone tell me which aviation books discuss this the best.

 

[Otto77]

the saying Gus wears a hat. HAT (height above touchdown) divided by GlideSlope angle.

 

[Doc Holiday]

This was a double post, but to answer you question about books, Mental Math For Pilots addresses this issue.

 

[Doc Holiday]

You cannot calculate a VDP, it is published on the approach plate and evaluated for obstacle clearance.

You can, however, calculate a planned descent point which will put you on an approximate 3 degree glide path to the runway. Here are a couple of examples of how to do this:

Using time - take 10% of the HAT and subtract that many seconds from the time to MAP.

HAT = 500' Time to MAP = 3:00

Your planned descent point would be 2:10

Using distance - Divide the HAT by 3 and plan to start the descent for landing that far from the runway.

HAT = 600' Start your descent 2nm from the end of the runway.

As a note, when using this method verify what the DME is at the threshold of the runway (typically the MAP). This can be seen on the profile view. For example, the MAP on a LOC approach is generally not 0.0 DME. It will more likely be somewhere between .5 and 2.0 DME. If the MAP is 1.2 DME and the HAT is 600', you would add 2nm to the MAP DME of 1.2 and plan to begin your letdown at 3.2 DME. If you did not correct for this difference and just started down at 2.0 DME, you would be well above the desired descent profile.

Figuring out your own "VDP" is an excellent tool for getting the airplane to a position where a normal descent to landing can be made. Just realize that figuring it on your own does NOT necessarily provide terrain clearance.

Hope this helps.

 

[Otto77]

Using distance - Divide the HAT by 3 and plan to start the descent for landing that far from the runway.

HAT = 600' Start your descent 2nm from the end of the runway.

As a note, when using this method verify what the DME is at the threshold of the runway (typically the MAP). This can be seen on the profile view. For example, the MAP on a LOC approach is generally not 0.0 DME. It will more likely be somewhere between .5 and 2.0 DME. If the MAP is 1.2 DME and the HAT is 600', you would add 2nm to the MAP DME of 1.2 and plan to begin your letdown at 3.2 DME. If you did not correct for this difference and just started down at 2.0 DME, you would be well above the desired descent profile.

Figuring out your own "VDP" is an excellent tool for getting the airplane to a position where a normal descent to landing can be made. Just realize that figuring it on your own does NOT necessarily provide terrain clearance.

Hope this helps.

I'm glad Doc put this in detail..its a great description! Like I metioned above..this is the best method and the way they teach us in the AF..easy to do. Just remember my phrase above!

 

[FL420]

http://forums.flightinfo.com/showthread.php?t=77673&highlight=PDP