Calculating VDP

[Skyboy722]

Freightdog,

Sweet Fancy Moses!

 

[bigsky]

I use HAt minus 50 feet( for threshhold crossing) and divide by 300
Hat= 830 minus 50 = 780/ 300 = 2.6
for widebody ac on 3 bar vasi it is normally 325 feet per minute so now approximately 2.4 miles.....
also if you are talking a large transport ac it reacts a little slower so normally lead the descent or pushover by abot .2 nm. so now the 2.4 +.2= 2.6 just like the other fromula.

For timing, dividing by 10 is really set up for an approach speed of 120
400/10 = 40 seconds from end of ry.
AT higher speeds (150) I use .8 and multiply by hat..400X.8 = 32seconds.

 

[JFReservist]

According to the 217 (AF guys & gals)

8.5.4.6. Visual Descent Point (VDP). The VDP is a defined point on the final approach course of a non-precision straight-in approach procedure from which a normal descent (approximately 3°) from the MDA to the runway touchdown point may be commenced, provided visual reference with the runway environment is established. The VDP is normally identified by DME and is computed for the non-precision approach with the lowest MDA on the IAP. A 75 MHz marker may be used on those procedures where DME cannot be implemented. VDPs are not a mandatory part of the procedure, but are intended to provide additional guidance where they are implemented. A visual approach slope indicator (VASI) lighting system is normally available at locations where VDPs are established. Where VASI is installed, the VDP and VASI glide paths are normally coincident. If VASI is not installed, the descent is computed from the MDA to the runway threshold. On multi-facility approaches, the depicted VDP will be for the lowest MDA published. Therefore, on an approach with a higher MDA, the published VDP will not be correct and must be computed by the pilot. No special technique is required to fly a procedure with a VDP; however, to be assured of the proper obstacle clearance, the pilot should not descend below the MDA before reaching the VDP and acquiring the necessary visual reference with the runway environment. The VDP is identified on the profile view of the approach chart by the symbol “V” (Figure 8.16).
8.5.4.6.1. In some cases a published VDP may be absent from an IAP due to an obstacle that penetrates a 20:1 surface. In addition, there was a period of time where the FAA did not place any emphasis on publishing VDPs on IAPs. As a result, many IAPs were designed without published VDPs. The problem is that when a IAP is published without a VDP, there is no way for the pilot to know if it is due to an obstacle penetration, or because the TERPS specialist just did not publish it. If performing a non-precision approach to an unfamiliar field at night (or very low visibility) without a published VDP, and no visual or “normal” electronic glide path guidance to that runway is available, use caution when departing the MDA, as there could potentially be an obstacle penetrating the 20:1 surface. See Chapter 15, Visual Glide Slope Indicators (VGSI) for more information on obstacles in the 20:1 surface.

that being said, gus wears a hat, gs/2 for vvi and a bit of TLAR.

 

[JFReservist]

And more...

14.2.1.2.6.8. In many cases, the minimum visibility required for the approach will not allow you to see the runway environment until you are beyond the VDP. This accentuates the need to compute a VDP and determine a point along the approach when you will no longer attempt to continue for a landing. A common error is to establish a high descent rate once the runway environment is in sight. This can go unnoticed during an approach without visual glide path guidance and may lead to a short and/or hard landing. Caution should also be used to avoid accepting a long touchdown and landing roll.



6.8. Calculating a Visual Descent Point (VDP). The first step to computing a VDP is to divide the Height Above Touchdown (HAT) from your approach procedure by your desired descent gradient. Most pilots use a 3° (300 feet/NM) glidepath for landing. Here’s the formula to use:
HAT / Gradient (normally 300) = VDP in NM from end of runway

6.8.1. Now that you know how far the VDP is from the end of the runway, you may add this distance to the DME at the end of the runway to get a DME for your VDP. Armed with this information, it is easy to compute the distance from the FAF to the VDP. This distance is important in computing the climb gradient necessary for final approach.
6.8.2. Using the FAF altitude, the MDA, and the distance from the FAF to the VDP, you can compute a descent gradient from the FAF to the VDP along with a target VVI to ensure you are meeting the desired descent gradient.
Example: Use the following information to determine the descent gradient from the FAF to the
VDP:
HAT = 420 feet, MDA = 840 feet MSL, DME at the end of the runway = 0.5 DME, FAF =
6 DME, FAF altitude = 2,500 feet MSL, desired landing gradient = 300 feet/NM, Approach
airspeed = 150 KTAS, no wind.
VDP = HAT/Gradient =420/300 = 1.4 NM
VDP DME = DME at end of runway + VDP distance = 0.5 DME + 1.4 DME = 1.9 DME
Descent Distance = FAF DME - VDP DME = 6.0 DME - 1.9 DME = 4.1 DME
Altitude to lose = FAF altitude - MDA = 2500 - 840 = 1,660 feet
Descent Gradient = altitude to lose / distance = 1660 / 4.1 = 405 feet/NM (4° descent gradient)
VVI = Angle (NM/MIN X 100) = 4 (2.5 X 100) = 1,000 feet/MIN
6.8.3. With this information you can depart the FAF maintaining a 4° descent gradient (400 feet/NM).
Your target VVI is 1,000 FEET/MIN. Each mile you should lose 400 feet. At 5 DME, you should be at
2,100 feet, at 4 DME, 1,700 feet, etc . . . Continue this descent gradient until reaching VDP at 840 feet
MSL. Hopefully, at the VDP, you’ll have the runway in sight. Adjust your descent to a 300 feet/NM
gradient and pick up your normal aim point.

 

[freightdogfred]

And more...

14.2.1.2.6.8. In many cases, the minimum visibility required for the approach will not allow you to see the runway environment until you are beyond the VDP. This accentuates the need to compute a VDP and determine a point along the approach when you will no longer attempt to continue for a landing. A common error is to establish a high descent rate once the runway environment is in sight. This can go unnoticed during an approach without visual glide path guidance and may lead to a short and/or hard landing. Caution should also be used to avoid accepting a long touchdown and landing roll.



6.8. Calculating a Visual Descent Point (VDP). The first step to computing a VDP is to divide the Height Above Touchdown (HAT) from your approach procedure by your desired descent gradient. Most pilots use a 3° (300 feet/NM) glidepath for landing. Here’s the formula to use:
HAT / Gradient (normally 300) = VDP in NM from end of runway

6.8.1. Now that you know how far the VDP is from the end of the runway, you may add this distance to the DME at the end of the runway to get a DME for your VDP. Armed with this information, it is easy to compute the distance from the FAF to the VDP. This distance is important in computing the climb gradient necessary for final approach.
6.8.2. Using the FAF altitude, the MDA, and the distance from the FAF to the VDP, you can compute a descent gradient from the FAF to the VDP along with a target VVI to ensure you are meeting the desired descent gradient.
Example: Use the following information to determine the descent gradient from the FAF to the
VDP:
HAT = 420 feet, MDA = 840 feet MSL, DME at the end of the runway = 0.5 DME, FAF =
6 DME, FAF altitude = 2,500 feet MSL, desired landing gradient = 300 feet/NM, Approach
airspeed = 150 KTAS, no wind.
VDP = HAT/Gradient =420/300 = 1.4
VDP DME = DME at end of runway + VDP distance = 0.5 DME + 1.4 DME = 1.9 DME
Descent Distance = FAF DME - VDP DME = 6.0 DME - 1.9 DME = 4.1 DME
Altitude to lose = FAF altitude - MDA = 2500 - 840 = 1,660 feet
Descent Gradient = altitude to lose / distance = 1660 / 4.1 = 405 feet/NM (4° descent gradient)
VVI = Angle (NM/MIN X 100) = 4 (2.5 X 100) = 1,000 feet/MIN
6.8.3. With this information you can depart the FAF maintaining a 4° descent gradient (400 feet/NM).
Your target VVI is 1,000 FEET/MIN. Each mile you should lose 400 feet. At 5 DME, you should be at
2,100 feet, at 4 DME, 1,700 feet, etc . . . Continue this descent gradient until reaching VDP at 840 feet
MSL. Hopefully, at the VDP, you’ll have the runway in sight. Adjust your descent to a 300 feet/NM
gradient and pick up your normal aim point.

All that cipherin and you're 1/10 of a mile different....

 

[Jonny Sacko]

I'm sorry fella's but the winner must go to:


"If you can see it, you can make it"


Too funny, I still bust out everytime I read that. Nice job.

 

[JFReservist]

Yeah, I know...

 

[Amish RakeFight]

Amish in your example VDP is 2 miles from MAP is only true if the MAP is at the end of the runway. VDPs have nothing to do with MAP they are a calculation for reaching the Threshold at 50' HAT. I am sure you meant that but I just wanted to clarify. All your other poop was spot on.

Yep. Thanks for clarifying my poop. My assumption was that the MAP would be the threshold on a non-precision as is many times the case. As you stated, it may be displaced form the threshold and the calculated numbers may have to be reworked slightly (plus or minus the figure).

 

[nosartoofar]

More Math

Multipy the Height Above Touchdown (HAT) for the approach minimums to be used by 3, then divide by 1000 (or just move the decimal place over three places). To limit mathmatical gymnastics, round up the HAT in 50 ft intervals (HAT 327 to 350, 365 to 400, etc.) Add or subtract the distance to the DME MAP to obtain a 3 degree GS.VOR/DME Approach with the FAF over the VOR and MAP @ 4.5 DME. HAT (always AGL)is 300 ft. 300 X 3 = 900 or 0.9 DME - Start descent @ 3.6 DME for a 3 degree GS.

 

[flyboyike]

Just fly a CANPA and be done with it. Never liked dive'n'drive.

 

[BSkin]

Just fly a CANPA and be done with it. Never liked dive'n'drive.

It ain't bad when you're divin' and she's driven'!

 

[BSkin]

Sorry, too easy!

 

[mamba20]

Alright, thanks guys. This may be a stupid question but what is CANPA? I feel like keel hauling my CFII!

 

[Amish RakeFight]

Alright, thanks guys. This may be a stupid question but what is CANPA? I feel like keel hauling my CFII!

Contstant Angle Non-Precision Approach

 

[Way2Broke]

CANPA?

Where do you get the information from? A MFD? I've never flown glass.

 

[pgcfii2002]

Okay.....next question.

How do you get rid of an STD?

 

[Americam]

Hate to be "that guy," but the VDP will always be published on the chart. If no VDP is published, a PDP (Planned Descent Point) is what you calculate in leau of a VDP.

They work exactly the same, with the following distinction:

VDP = published
PDP = calculated by pilot

If no VDP is published, I always calculate a PDP: Just a good habit to get into.

 

[A Squared]

1.3 miles from the end of the runway, works fer just about all of them.

Try it you'll see.

Proabaly wouldn't work all that well here:

http://www.naco.faa.gov/d-tpp/0705/06053NDA.PDF

or here:

http://www.naco.faa.gov/d-tpp/0705/06213LDADH.PDF


or here:

http://www.naco.faa.gov/d-tpp/0705/02335N24.PDF

 

[pkober]

Hate to be "that guy," but the VDP will always be published on the chart. If no VDP is published, a PDP (Planned Descent Point) is what you calculate in leau of a VDP.

They work exactly the same, with the following distinction:

VDP = published
PDP = calculated by pilot

If no VDP is published, I always calculate a PDP: Just a good habit to get into.

Not true. A VDP will only be published if there is an obstacle or the TERPster wanted to put one on the chart. He doesn't have to. The current push from the FAA is that VDP will be published, but there are still plenty of charts without them.

A technique I was taugh in USAF UPT in the 90's.

"Gus wears a HAT"

Gus = GS

HAT / GS = distance from runway. Sometimes use 3.0 or some other number.

If I repeated some one, sorry.

 

[Amish RakeFight]

I think you're misreading his post. What he's saying is that the VDP will be published if calculated by the FAA. VDP's are published and PDP's are calculated by the pilot. He's clarifying the fact that pilots tend to use the terms interchangeably.

Clearly, we don't see VDP's appearing on every NP plate, so therefore we can assume that if a VDP exists, it will be published.

I think that is what the poster was refering to, not that a VDP is published on every NP plate.

 

[pkober]

I meant to say that a VDP won't be published if their is an obstacle that would penetrate (great word) the 20 to 1 slope.

The danger is that you don't know why their is not a VDP. Is it because of an obstacle or a lazy TERPster? I guess you can find out the hard way, at night in bad weather, that it was because of a tower.

 

[buckets]

I have NEVER gone around and NEVER will! Just make it work the first time! Fuel is expensive! Don't go-arounds scare passengers? I'd be scared that I'd probably have to do a visual pattern and I don't do those either. I hope some of you are laughing.

 

[freightdogfred]

Last edited by A Squared : 05-23-2007 at 19:33. Reason: edited to clarify what I meant.

Dude, I'm sorry the only job available to you is in that Douglasaurus firetrap..
Those approaches you linked are visuals, if you ain't got the field by that FAF you ain't gonna make it, no matter what VDP you conjure up.

 

[A Squared]

Dude, I'm sorry the only job available to you is in that Douglasaurus firetrap..

Hey, now there's something fresh and new: If you don't have anything intellegent to say, toss out some completely unrelated insults. Wow, nobody on Flightinfo has ever thought of that before. An original thinker and a class act to boot, a real double threat kind of intellect.

Those approaches you linked are visuals,

Well, actually, no, they are not, they're just examples of approaches with high hat's, some of the many approaches where your "Just use 1.3, dude" strategy isn't going to work really well, or at all. I picked a couple with fairly extreme MDA's, thinking you could follow the logic, but I guess I overestimated your capabilities rather badly. I suppose that's my fault for not recognizing that someone who interrupts a discussion of calculating PDPs with "just use 1.3, dude" probably isn't the sharpest crayon in the box. My apologies.

In deference to your limitations, I'll post a couple more approaches, these a little less extreme, in which your "just use 1.3, dude" advice won't work very well.

http://www.naco.faa.gov/d-tpp/0706/01253G14.PDF

hat is 739', which would give a PDP 2.3 NM from the threshold, You'd be about a mile off with "just use 1.3, dude"

http://www.naco.faa.gov/d-tpp/0706/10059R7.PDF

on this one, hat is 861, which would give a 2.7 nm PDP, over twice the distance you'd get with "just use 1.3, dewd"

Here's another:

http://www.naco.faa.gov/d-tpp/0706/09297R13.PDF

Hat on this one is 1005'. PDP would be 3.2, which is a long way off from "just use 1.3, dewd".


These are just a couple that I grabbed from the back of my jepps. I could list dozens here in Alaska with hat's high enough that "just use 1.3, dude", wouldn't work. It's not just Alaska, either. There's hundreds of similar examples in the west, northwest, and the hilly parts of the southeast and northeast. The fact is, unless your flying is restricted to FLorida, or Kansas, or some other really flat state, "just use 1.3, dewd" isn't going to work a significant percentage of the time.

 

[ferlo]

Hit the VNAV button and get back to the newspaper.

 

[freightdogfred]

Hey, now there's something fresh and new: If you don't have anything intellegent to say, toss out some completely unrelated insults. Wow, nobody on Flightinfo has ever thought of that before. An original thinker and a class act to boot, a real double threat kind of intellect.




Well, actually, no, they are not, they're just examples of approaches with high hat's, some of the many approaches where your "Just use 1.3, dude" strategy isn't going to work really well, or at all. I picked a couple with fairly extreme MDA's, thinking you could follow the logic, but I guess I overestimated your capabilities rather badly. I suppose that's my fault for not recognizing that someone who interrupts a discussion of calculating PDPs with "just use 1.3, dude" probably isn't the sharpest crayon in the box. My apologies.

In deference to your limitations, I'll post a couple more approaches, these a little less extreme, in which your "just use 1.3, dude" advice won't work very well.

http://www.naco.faa.gov/d-tpp/0706/01253G14.PDF

hat is 739', which would give a PDP 2.3 NM from the threshold, You'd be about a mile off with "just use 1.3, dude"

http://www.naco.faa.gov/d-tpp/0706/10059R7.PDF

on this one, hat is 861, which would give a 2.7 nm PDP, over twice the distance you'd get with "just use 1.3, dewd"

Here's another:

http://www.naco.faa.gov/d-tpp/0706/09297R13.PDF

Hat on this one is 1005'. PDP would be 3.2, which is a long way off from "just use 1.3, dewd".


These are just a couple that I grabbed from the back of my jepps. I could list dozens here in Alaska with hat's high enough that "just use 1.3, dude", wouldn't work. It's not just Alaska, either. There's hundreds of similar examples in the west, northwest, and the hilly parts of the southeast and northeast. The fact is, unless your flying is restricted to FLorida, or Kansas, or some other really flat state, "just use 1.3, dewd" isn't going to work a significant percentage of the time.

Well, get back in that museum piece you fly, and go shoot some of them. All your approaches have a common thread-They're in Alaska, with mountainous terrain, My original post said it works for most of them, and most of them got HAT's around 400 ft. You spent your evening digging out extreme cases to prove me wrong, so be it. As far as not being "the sharpest crayon in the box", I've flown NDB approaches from Medellin to Mumbai. Do you even know where these cities are?

There's a reason they call them "Non-Precision".

Enjoy your next trip.

 

[A Squared]

All your approaches have a common thread-They're in Alaska, with mountainous terrain, My original post said it works for most of them, and most of them got HAT's around 400 ft.

Uhhh, yeah, they're in Alaska because that's what was handy in my Jepp binder. You could pick up a handful of approach plates for Vermont, or Idaho, or Washington, or North Carolina or a dozen other areas and find plenty more approaches with similarly high hats. In actuality, your original post said "just about all of them" which is a a pretty inane statement.

As far as not being "the sharpest crayon in the box", I've flown NDB approaches from Medellin to Mumbai.

It's a further testament to your level of intelligence that you think this proves something. I know quite a few guys flying international freight, some are fairly smart cookies, however the stupidest pilot I know though has been to those places, or if not, he'd certainly tell you he had, because in addition to being really, really dumb, he's also delusional and a pathalogical liar as well as a career alcoholic. So, if you think that having been to Mumbai and Medellin proves that you're intelligent, you're probably not the sharpest crayon in the box.

 

[freightdogfred]

So, if you think that having been to Mumbai and Medellin proves that you're intelligent, you're probably not the sharpest crayon in the box.

What it proves is I've flown these types of approaches all around the world.

What it proves is yore sorry a$$ is stuck in the Douglas dinosaur cause yer unemployable elsewheres.

Have a nice life.

 

[A Squared]

While your concern for my career is touching, it's a bit misplaced. The topic was calclating PDPs, and how you've been talking out your a$s on the subject. Try to focus, I know it's difficult

 

[Say Again Over]

Relax guys, why are two highly experienced pilots hashing it out over the subject of non-precision approaches, most of us reading this all have a current instrument rating and understand both points of view, geez!

Even though most of us don't know each other here, we could at least have a little bit of respect for each other, ok, rant over. :beer: