Visual Descent Point

[DAS at 10/250]

Need a good explanation of VDP with references to some official publication. Thanks.

 

[great cornholio]

This site might help

http://www.smartregs.com/data/sa617.htm

Anyways best way I can think to explain it is the VDP is a point on the MDA (non precison approach) where if you have all the things you need to see to leave the MDA then you can start a normal landing. IE if you start down from the MDA at the VDP then it will kinda be like a 3* glideslope from the MDA to the runway. Practial reason to know this is that if you decend after the VDP then you will have to "dive bomb" it toward the ruwnay. There is some formula to figure out the VDP but I forgot it. As far as where to look try the AIM, or the Flight Training Handbook (its not published anymore stopped a few years ago, but ask around someone you know might have it) anyways it has some stuff in it that they left out of the replacement book called the Airplane Flying Handbook.

 

[stillaboo]

'Real' VDP's and how to approximate them

There is some formula to figure out the VDP but I forgot it.

Time:

HAT * .1 = x (or HAT/10, same thing)
MAP - x = Time

example: HAT is 400', MAP is 1:26 after passing VOR.

400 * .1 = 40 seconds
1:26 - :40 = :46 seconds

VDP is 46 seconds after passing VOR, missed is 1:26 seconds

Distance:

HAT/300 = x
DME +/- x = VDP

example: HAT is 400', MAP is 3 DME from VOR/FAF.

400/300 = 1.3
3 - 1.3 = 1.7

VDP is 1.7 DME after passing VOR.

Note: be careful on the distance one. If the DME is measured from the runway (like a localizer approach) then you add the DME to the x (in the example, if the MAP was 3 DME from the runway, the VDP would be 4.3 DME from the field).

Note: the little V's on your approach plates are actual VDP's that are in the databases of the big fancy jets, and are calculated based on speed, aircraft weight, etc,. by the FMS. The #'s you calculate using these formulas are rough calculations designed to aid in flying your approach, but are not actual VDP's. The big fancy jet cannot ignore the VDP calculated in the nav system; the jet MUST go missed if the field isn't in sight (please tell me if I'm wrong on this). The #'s derived from these calculations are just #'s, and can be ignored at the pilots whim.

-Boo!

[editied to reflect the correct math ]

 

[tarp]

Official Pub's

AIM - Pilot-Controller Glossary
AIM - Section 5-4-5(e)
Jeppesen - Instrument Commercial Manual Chapter 7
Jeppesen Chart Glossary and Chart Introduction

 

[Slye]

"Distance:

HAT/300 = x
DME +/- x = VDP

example: HAT is 400', MAP is 3 DME from VOR/FAF.

400/300 = 1.3
3 - 1.3 = 2.6

VDP is 2.6 DME after passing VOR. "

-------------------

Shouldn't the DME from the VOR/FAF to the VDP in this case be 1.7? Just want to make sure I'm not losing it...

 

[stillaboo]

No, I'm losing it. I editied it to show the right #'s.

-Boo!

 

[CoopDog]

Time to see

Scroll down to the bottom of the page here on flightinfo and check out "Rules of thumb." Look at #12 in the descent topic, "Time to see" (basically the same as a VDP)

 

[MooseTrax]

Gus Wears a HAT

We used to teach that you can figure out the VDP for any approach by using this little formula that is remembered as GS wears a HAT:

HAT
----- = VDP Dist. from runway
GS

If height above touchdown is 360 agl and you want a 3 degree glide slope, then:

360
------ = 1.2 nm from the runway.

Regards,

 

[100LL... Again!]

Careful about calculating VDP's based on 10% HAT.

If the MAP is before the runway threshold, the VDP will be too soon.


This is also true if using DME.


If you have no reliable way to determine the threshold, then do-it-yourself VDP's will be of only some use.