Calculating VDP

[mastermags]

I had a sheet somewhere at one point on how to calculate a VDP both from a localizer time and from a DME distance. I believe it was something to the effect of localizer time divided by 10 subtracted from the overall localizer time. I don't remember how to calculate the VDP from DME. If anyone could help refresh my memory or maybe give some sort of handout that you use, that would be great.

 

[jawright]

DISTANCE METHOD:

HAT/300= VDP in NM from threshold

TIMING METHOD:

10% of HAT=seconds to subtract from time to MAP

 

[TonyC]

I had a sheet somewhere at one point on how to calculate a VDP both from a localizer time and from a DME distance. I believe it was something to the effect of localizer time divided by 10 subtracted from the overall localizer time. I don't remember how to calculate the VDP from DME. If anyone could help refresh my memory or maybe give some sort of handout that you use, that would be great.

Try this: http://www.clear-and-a-million.com/viewtopic.php?t=641


and this: http://forums.airlinepilotcentral.com/showpost.php?p=9921&postcount=2


(Sorry, I just didn't feel like retyping or copying and pasting. Don't be afraid of the other forums! )



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[Purdue_Pilot]

This depends on what glide slope angle you wish to descend at. The most well known is to divide HAT/300 and add the distance from MAP to threshold. However, most people have no idea where this comes from. The 300 comes from a 300 ft/mile descent. Doing the math, this comes out to a 3.25 degree glide slope assuming your descent rate is 300 ft/NM. hopefully you know approximately what descent rate in ft/min equates to approximately 3.25 degrees.

Now if you wish to descend at a 3 degree glide slope angle, you would want a 277 ft/NM descent. This VDP would be HAT/277 + distance between MAP and threshold.

3.25 degrees is a little easier mental math while barreling down a nonprecision approach at night in turbulence!

 

[TonyC]

This depends on what glide slope angle you wish to descend at. The most well known is to divide HAT/300 and add the distance from MAP to threshold. However, most people have no idea where this comes from. The 300 comes from a 300 ft/mile descent. Doing the math, this comes out to a 3.25 degree glide slope assuming your descent rate is 300 ft/NM. hopefully you know approximately what descent rate in ft/min equates to approximately 3.25 degrees.

I'd like to see the math - - is that taught at Purdue?

-- Using the simple 60-to-1 rule, a 3° descent gradient gives 300 feet descent per 1 Nautical mile traveled.

((6000 ft / 60) * 3 = 300 ft)

-- Application of the 60-to-1 rule, allowing for the actual length of a nautical mile, yields 304 feet loss per nautical mile.

((6076.11549 ft / 60) * 3 = 303.805744 ft)

-- Trigonometry says that a 3 degree glidepath will lose 318 feet vertically for every nautical mile travelled horizontally.

(Descent = TAN 3° x 6076.11549 ft = 318.43572 ft)


Each of these yields a number of 300 or greater. You're saying that the number should be less than 300 (for a 3 degree descent gradient). I'm looking forward to learning something new. How'd you come up with 277?








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