Calculating A VDP

[pilot4500]

I've heard that there is a formula for calculating a Visual Descent Point for an approach where one is not published. I can't find it it any of my flight manuals though. I looked in the Jeppsen Instrument/Commercial book, as well as the Instrument Flying Handbook. I couldn't find it.

Could any of you help me out with the formula for calculating a VDP?

 

[Captain_Bob]

There are two methods... Distance and Timing:

Distance (DME): For normal 3 degree slope...

Divide the MDA HAT by 300... The result will be the distance to the end of the runway for the VDP.

Example: MDA is 450 ft. HAT so... 450/300 = 1.5 NM from the end of the runway.

Timing:

Take the first two digits of the MDA HAT and think of them as seconds: So in our example 450 ft. would be translated as 45 seconds. Subtract this number from the total timing required to the end of the runway. IE: Look at your jepps and see how much time it would take from the FAF to the end of the runway based on your approach speed... then subtract 45 seconds from it.

It may look like this: MDA is 450 ft. HAT and if time to runway is 3 minutes... then 3 minutes minus 45 seconds = 2:15. So you'd begin your Descent at 2 minutes and 15 seconds from the time you passed over the FAF.

Bear in mind... the MAP may not be co-located with then end of the runway... if that's the case... subtract or add as necessary.

Kind of confusing at first... but it get's easier when you start calculating a few.

Hope that helps!

Bob

 

[ZapBrannigan]

On a DME based approach it's VDP = HAA (at MDA) / 300 (ft/nm) = miles from MAP.

On a time based approach take your HAA (at MDA), drop the last zero, subtract from seconds on approach.

i/e If HAA is 600 ft. Drop 0 you get 60. Then adjust your time. 3:30 - 60 seconds = 2:30

Just rules of thumb. Remember you are not permitted to leave MDA until you have at least the approach lights in sight (91.175). What the VDP DOES do for you is tell you, "Hey man -- if you pass me, you're probably not going to be able to continue a stabilized approach to landing so you better get ready for a miss."

Personally, if I pass the VDP (calculated or otherwise) and don't have adequate visual to continue I go miss. -- Climb only. No turns until you reach the MAP unless ATC says otherwise.

 

[pilot4500]

Thanks a lot for the info guys. It does seem a bit confusing at first, but I will practice calculating some VDPs. I'm sure it will get easier.

Those VDPs can be very useful. I have noticed on quite a few non precision approaches that if you break out right at the MAP, it is impossible to make a stabalized approach and landing.

 

[Mr.Maximus]

I must be gettin old. I thought it was enter "Visual Rwy XX - enlarge the rwy heading course & execute". From there - follow the snowflake on your nice little 3 degree aritifical glideslope. And remember to hit APPR & VNAV.

I guess if that's not an option - Capt Bob has it nailed. Been a while.



Max

 

[Rocketman99]

Basically, I'm gonna say what Bob said, but I'm going to add one little thing. To help you remember it, you can use the mnemonic they teach in AF UPT: "GuS wears a HAT."

VDP = Height Above Touchdown / GlideSlope.

Assuming you go with a standard 3 degree glide slope it becomes the HAT / 300, as has been mentioned before.

 

[USAFplt]

yeah in the af they taught us Gus wears a HAT..works perfect..easy to remember!

 

[falconvalley]

It's scary how little time you realize you have to spot the approach lights after calculating a VDP sometimes at mins.