VDP

luvflyin said:
I’ll say it again. The VDP is not a certain height above the runway. How the VGSI relates to it is that will determine at what the distance from the runway the VDP will be.
Click to expand...

I think we're discussing semantics here. No, the VDP is not a certain height above the runway, if by that you mean "it's always 500 feet above the runway" or something like that. But in the sense that I mean it, and I believe @midlifeflyer does too, it IS a certain height above the runway, that height determined by the MDA, and is therefore different for each procedure. But in the LUK case the VDP is at 799 feet above the touchdown zone. Therefore 799 is a "certain height". MDA, after all, is a direct input into the VDP formula.
 
RussR said:
I did misspeak, but the VDA is not relevant here, the VGSI angle is (assuming there is a VGSI), and I'm not sure how the terminology is pertinent in this case, they are functionally synonyms here.
Click to expand...
I’d say the VDA is very relevant. This whole thing started out with where the big V, the VDP was located on the chart relative to the bold black line in the profile view. It was being viewed at as if you must be following that line as if it was a glidepath or glideslope. You do not. All the VDA does is let you know what the angle of descent will be if you choose to do a constant angle of descent approach. If you choose.
 
RussR said:
I think we're discussing semantics here. No, the VDP is not a certain height above the runway, if by that you mean "it's always 500 feet above the runway" or something like that. But in the sense that I mean it, and I believe @midlifeflyer does too, it IS a certain height above the runway, that height determined by the MDA, and is therefore different for each procedure. But in the LUK case the VDP is at 799 feet above the touchdown zone. Therefore 799 is a "certain height". MDA, after all, is a direct input into the VDP formula.
Click to expand...
I do not mean by that it’s always 500 feet above the runway or something like that. I mean that the VDP is NOT a height. It is a distance from the runway.
 
luvflyin said:
I’d say the VDA is very relevant. This whole thing started out with where the big V, the VDP was located on the chart relative to the bold black line in the profile view. It was being viewed at as if you must be following that line as if it was a glidepath or glideslope. You do not. All the VDA does is let you know what the angle of descent will be if you choose to do a constant angle of descent approach. If you choose.
Click to expand...

You were responding to my post about how to calculate the VDP. If there is a VGSI like there is at LUK, the VDA is not even an entry into the official formula for the VDP.
 
dtuuri said:
Btw, TERPS 2-6-5.b. SEEMS to indicate these examples are incorrect when they depict the VDP farther from the runway than the lowest category's MDA:

"When dual or multiple lines of NPA minimums are published, use the lowest MDA from any CAT to calculate the VDP distance."
​
So, @RussR, what am I missing if they are charted correctly?
Click to expand...

The variable of speed for aircraft categories A thru D and the corresponding descent rates from AGL to TCH.
 
Last edited:
luvflyin said:
I do not mean by that it’s always 500 feet above the runway or something like that. I mean that the VDP is NOT a height. It is a distance from the runway.
Click to expand...

It is a distance from the runway that is calculated based on you being at a certain height, i.e. the MDA. The VDP is not particularly useful if you are not at the MDA at that point.
 
RussR said:
It is a distance from the runway that is calculated based on you being at a certain height, i.e. the MDA. The VDP is not particularly useful if you are not at the MDA at that point.
Click to expand...
Exactly what I’ve been getting at. How height gets into it is the height your airplane is at when you get to the VDP and what you do about it. But the VDP does not have a height
 
Last edited:
RussR said:
You were responding to my post about how to calculate the VDP. If there is a VGSI like there is at LUK, the VDA is not even an entry into the official formula for the VDP.
Click to expand...
Yeah. How to calculate the distance from the runway the VDP will be at
 
Well, here is the answer I received from the FAA.

========================================================
AI-211928
Category: Charts & Publications > US Terminal Procedures
Airport: KLUK
State: OH
Procedure Name: RNAV( GPS) 3R
Inquiry: There has been some discussion about whether the VDP is correctly depicted on this chart. Nearly all non-precision approaches show the VDP (if one is available) at the bottom of the final descent, followed up a horizontal line to indicate that the the airplane should remain at the MDA until reaching the MAP. In this case, the instrument approach (solid black line) appears to continue below the VDP, which doesn't make much sense. There is also another one like this at a nearby airport, RNAV 21@K62. Could you please clarify if this is a charting errror, or if I am interpreting it incorrectly?
========================================================

FAA Response:

Hello Andrew,

You are correct that the profile track should level off after the VDP. We will update both of the charts that you referenced for the 6/16/22 publication date.

Thanks very much for bringing these to our attention.

Ron Haag
Manager, Terminal Charting Sub-Team A
 
Clip4 said:
I just started at KLUK and searched surrounding airports. Here is an LP /LNAV with the same situation. All the chart is advising is not to descend below MDA until you pass the VDP.

https://aeronav.faa.gov/d-tpp/2204/06864R21.PDF
Click to expand...

Using a chart that has both nonprecision and precision procedures on it does not support your argument that the non-precision only chart OP posted is not an error.

It is a chart error. From the Aeronautical Chart User's Guide:
"On non-precision only approach procedures, the approach track descends to the MDA or VDP point, thence horizontally to the missed approach point."
 
Clip4 said:
The variable of speed for aircraft categories A thru D and the corresponding descent rates from AGL to TCH.
Click to expand...

The VDP is not calculated based on rate of descent, it's based on angle.
 
Last edited:
sarangan said:
Well, here is the answer I received from the FAA.

========================================================
AI-211928
Category: Charts & Publications > US Terminal Procedures
Airport: KLUK
State: OH
Procedure Name: RNAV( GPS) 3R
Inquiry: There has been some discussion about whether the VDP is correctly depicted on this chart. Nearly all non-precision approaches show the VDP (if one is available) at the bottom of the final descent, followed up a horizontal line to indicate that the the airplane should remain at the MDA until reaching the MAP. In this case, the instrument approach (solid black line) appears to continue below the VDP, which doesn't make much sense. There is also another one like this at a nearby airport, RNAV 21@K62. Could you please clarify if this is a charting errror, or if I am interpreting it incorrectly?
========================================================

FAA Response:

Hello Andrew,

You are correct that the profile track should level off after the VDP. We will update both of the charts that you referenced for the 6/16/22 publication date.

Thanks very much for bringing these to our attention.

Ron Haag
Manager, Terminal Charting Sub-Team A
Click to expand...
POA gets another Chart fixed. Good job.
sarangan said:
Well, here is the answer I received from the FAA.

========================================================
AI-211928
Category: Charts & Publications > US Terminal Procedures
Airport: KLUK
State: OH
Procedure Name: RNAV( GPS) 3R
Inquiry: There has been some discussion about whether the VDP is correctly depicted on this chart. Nearly all non-precision approaches show the VDP (if one is available) at the bottom of the final descent, followed up a horizontal line to indicate that the the airplane should remain at the MDA until reaching the MAP. In this case, the instrument approach (solid black line) appears to continue below the VDP, which doesn't make much sense. There is also another one like this at a nearby airport, RNAV 21@K62. Could you please clarify if this is a charting errror, or if I am interpreting it incorrectly?
========================================================

FAA Response:

Hello Andrew,

You are correct that the profile track should level off after the VDP. We will update both of the charts that you referenced for the 6/16/22 publication date.

Thanks very much for bringing these to our attention.

Ron Haag
Manager, Terminal Charting Sub-Team A
Click to expand...
 
Tarheelpilot said:
This thread has become a staple of my daily entertainment since it started. I hope you guys keep it going a while longer.
Click to expand...
I'll try. I stepped away for day and now I need to go through 25 complicated posts. I guess that's why they pay me the big bucks. Oh, wait...

As I read TERPS paragraph 2-6-5.b. (see my post #33) my mind envisions this:

VDP.jpg​
If TERPS is saying to chart VDP at the lowest MDA on combined NP approach charts, doesn't that make it impossible by definition to make a "normal descent" from the higher MDA? (Geez, I hope IFR students have their eyes closed in this thread while we CFIIs hash this out.)

I.e., don't they have it just backwards?
 
Last edited:
sarangan said:
Well, here is the answer I received from the FAA.

========================================================
AI-211928
Category: Charts & Publications > US Terminal Procedures
Airport: KLUK
State: OH
Procedure Name: RNAV( GPS) 3R
Inquiry: There has been some discussion about whether the VDP is correctly depicted on this chart. Nearly all non-precision approaches show the VDP (if one is available) at the bottom of the final descent, followed up a horizontal line to indicate that the the airplane should remain at the MDA until reaching the MAP. In this case, the instrument approach (solid black line) appears to continue below the VDP, which doesn't make much sense. There is also another one like this at a nearby airport, RNAV 21@K62. Could you please clarify if this is a charting errror, or if I am interpreting it incorrectly?
========================================================

FAA Response:

Hello Andrew,

You are correct that the profile track should level off after the VDP. We will update both of the charts that you referenced for the 6/16/22 publication date.

Thanks very much for bringing these to our attention.

Ron Haag
Manager, Terminal Charting Sub-Team A
Click to expand...

The charting sub team has a lot to correct because I have seen this on a lot of charts.
 
dtuuri said:
I'll try. I stepped away for day and now I need to go through 25 complicated posts. I guess that's why they pay me the big bucks. Oh, wait...

As I read TERPS paragraph 2-6-5.b. (see my post #33) my mind envisions this:

If TERPS is saying to chart VDP at the lowest MDA on combined NP approach charts, doesn't that make it impossible by definition to make a "normal descent" from the higher MDA? (Geez, I hope IFR students have their eyes closed in this thread while we CFIIs hash this out.)

I.e., don't they have it just backwards?
Click to expand...
Maybe the rationale is they gotta put it somewhere so do it in the way where someone might be a bit high and have to steepen the approach a bit instead of finding themselves a tad bit low. I dunno, just a thought. How much difference is there between MDA’s on these combined NP Approaches? I haven’t gone looking but my memory says not a significant amount. Do you have an example?
 
luvflyin said:
Maybe the rationale is they gotta put it somewhere so do it in the way where someone might be a bit high and have to steepen the approach a bit instead of finding themselves a tad bit low.
Click to expand...
No doubt that's why and it made sense before the push for constant angle descents to the runway. With dive and drive, if you have to choose a single VDP to serve two masters (2 MDAs), you wouldn't want the lower "driver" descending where the higher one does. Perfectly set up on an LNAV+V advisory glide slope on the other hand would require the higher MDA pilot to level off at MDA until the VDP that was calculated for the lower MDA. By definition, then, it wouldn't be a normal descent.

luvflyin said:
How much difference is there between MDA’s on these combined NP Approaches? I haven’t gone looking but my memory says not a significant amount.
Click to expand...
I have been looking and you have a good memory. :) What I've seen are constant MDAs regardless of approach speed category. There must be examples, but I couldn't find them after about twenty minutes of sifting through an old approach book. Otherwise, why the language in TERPS 2-6-5.b? I did notice that the visibility changes with the category. Faster aircraft need more of it. The interesting thing is the visibility requirement is often less than half the length of the distance between the VDP and the runway. So, if you only "need" half as much visibility for a legal straight-in approach as you need for a "normal descent" from VDP, then maybe the definition of VDP could use a little editing?

luvflyin said:
Do you have an example?
Click to expand...
No, and I need to go work on my car. ;)
 
dtuuri said:
I'll try. I stepped away for day and now I need to go through 25 complicated posts. I guess that's why they pay me the big bucks. Oh, wait...

As I read TERPS paragraph 2-6-5.b. (see my post #33) my mind envisions this:

If TERPS is saying to chart VDP at the lowest MDA on combined NP approach charts, doesn't that make it impossible by definition to make a "normal descent" from the higher MDA? (Geez, I hope IFR students have their eyes closed in this thread while we CFIIs hash this out.)

I.e., don't they have it just backwards?
Click to expand...

Here is a typical example of two MDAs on one chart, one for the LP line and one for the LNAV line. Note there is only 20 feet of difference. While the VDP is determined by the lower MDA, the higher MDA would only position it .06 nm further away - so not a big deal.

upload_2022-5-11_7-20-27.png

Here's another one where the difference is more significant. These two MDAs are caused by the top line having a missed approach climb gradient requirement. At 120 ft difference, the higher MDA will have a VDP that is about 0.4 nm further out.

upload_2022-5-11_7-25-35.png

I suppose in the "multiple MDA" scenarios they elected to require only the lower VDP to be published simply to reduce chart clutter and potential confusion of having two VDPs depicted - but that's my opinion on the reason, not fact.
 
Last edited:
dtuuri said:
No doubt that's why and it made sense before the push for constant angle descents to the runway. With dive and drive, if you have to choose a single VDP to serve two masters (2 MDAs), you wouldn't want the lower "driver" descending where the higher one does. Perfectly set up on an LNAV+V advisory glide slope on the other hand would require the higher MDA pilot to level off at MDA until the VDP that was calculated for the lower MDA. By definition, then, it wouldn't be a normal descent.


I have been looking and you have a good memory. :) What I've seen are constant MDAs regardless of approach speed category. There must be examples, but I couldn't find them after about twenty minutes of sifting through an old approach book. Otherwise, why the language in TERPS 2-6-5.b? I did notice that the visibility changes with the category. Faster aircraft need more of it. The interesting thing is the visibility requirement is often less than half the length of the distance between the VDP and the runway. So, if you only "need" half as much visibility for a legal straight-in approach as you need for a "normal descent" from VDP, then maybe the definition of VDP could use a little editing?


No, and I need to go work on my car. ;)
Click to expand...
Yeah. The visibility minimum is really a that doesn’t have anything to do with it thang. The VDP serves no purpose if you don’t see the airport before you get to it.
 
dtuuri said:
What I've seen are constant MDAs regardless of approach speed category. There must be examples, but I couldn't find them after about twenty minutes of sifting through an old approach book. Otherwise, why the language in TERPS 2-6-5.b?
Click to expand...

Straight-in (i.e. not "Circling") MDAs are typically constant across approach categories because in the final segment, the MDA is a simple formula that doesn't depend on approach speed at all - just obstacle height plus require obstacle clearance for that type of approach. After all, regardless of how fast you're going, you're just going in a straight line - so if you level off at the MDA, you're going to not hit anything regardless of how fast you cover the ground.

Where you do sometimes get varied MDAs by category is if there is a turn involved in the missed approach. Since faster aircraft use up a larger turn radius, sometimes that brings in taller obstacles. One of the solutions for a missed approach obstacle problem is to raise the MDA so that the "starting altitude" for the missed approach is higher. This can result in higher MDAs for higher Cats.

See example here, for the no-CG-required line of minimums.

upload_2022-5-11_7-46-54.png
 

Attachments

  • upload_2022-5-11_7-46-33.png
    upload_2022-5-11_7-46-33.png
    157.1 KB · Views: 2
luvflyin said:
Yeah. The visibility minimum is really a that doesn’t have anything to do with it thang. The VDP serves no purpose if you don’t see the airport before you get to it.
Click to expand...
That’s not quite true. It’s not just there to prevent early descent it’s there to help decide when you can make a normal descent to landing. Meaning not to early or to late. It’s possible to see the runway after VDP but prior to MAP and not be in position to land safely. Having that VDP is mighty useful.

edited for clarity.
 
Last edited:
RussR said:
Here is a typical example of two MDAs on one chart, one for the LP line and one for the LNAV line. Note there is only 20 feet of difference. While the VDP is determined by the lower MDA, the higher MDA would only position it .06 nm further away - so not a big deal.

View attachment 106743

Here's another one where the difference is more significant. These two MDAs are caused by the top line having a missed approach climb gradient requirement. At 120 ft difference, the higher MDA will have a VDP that is about 0.4 nm further out.

View attachment 106744

I suppose in the "multiple MDA" scenarios they elected to require only the lower VDP to be published simply to reduce chart clutter and potential confusion of having two VDPs depicted - but that's my opinion on the reason, not fact.
Click to expand...
Here’s the whole plate if anyone’s interested. I can see the *LNAV only note for FAXEM. It’s redundant for the VDP though. It by definition does not apply to LNAV/VNAV. Don’t hurt to have a reminder though.

upload_2022-5-11_5-54-24.png
 
RussR said:
See example here, for the no-CG-required line of minimums.
Click to expand...
Coincidentally, that Medford RWY 14 approach was in the very book I was paging through looking for examples. Had I flipped pages for another half hour I'd have seen it. Unfortunately, it wouldn't have done me any good because it was an old book and the minimums were the same for all categories: MDA = 3060'.

@RussR, thank you so very much for your time and effort crunching the numbers and finding examples. We're lucky to have you contribute as much as you do. :yes:
 
luvflyin said:
Here’s the whole plate if anyone’s interested. I can see the *LNAV only note for FAXEM. It’s redundant for the VDP though. It by definition does not apply to LNAV/VNAV. Don’t hurt to have a reminder though.
Click to expand...

The " *LNAV Only" notes will not be seen on charts going forward, as the requirement for this note was removed from the 8260.19I, which was effective 6/29/20.
 
dtuuri said:
Well, YOU can do the math: FAA Order 8260.3E (17SEP20)

Me? Not so much.
Click to expand...
This was a thing in a Air Force. If a VDP wasn't published, we were taught to "roll your own."

We used the mnemonic "Gus wears a hat" to remember how to calculate it.

upload_2022-5-11_8-38-37.png

So in one of the examples above: HAT=799 and if you want a 3° glideslope, you'd have (to make the math easy) 798/3 = 2.66 NM from the end of the runway.

This is (was) a good technique for the way we used to fly NP approaches with the "dive and drive" method, especially in bigger, faster planes. You just knew that if you didn't see the runway by the VDP, you'd probably be going around. And, conversely, if you saw teh runway before the VDP, you should wait until hitting the VDP to start down, otherwise you'd be drug in.

Now... what wasn't taught is that there are reasons TERPS guys and gals won't publish a VDP. One of them being if there are obstacles in the path. When I read this an an instructor, I made sure I taught all my students how to calculate the VDPs, but also to caution them that there may be a good reason why there isn't on on the approach plate.

Edit: I just saw @RussR describe the HAT/GS approximation.
 
Sluggo63 said:
We used the mnemonic "Gus wears a hat" to remember how to calculate it.

View attachment 106750

So in one of the examples above: HAT=799 and if you want a 3° glideslope, you'd have (to make the math easy) 798/3 = 2.66 NM from the end of the runway.
Click to expand...
I Like that. :) I always used the "three times your altitude in 'thousands' of feet" thumb rule for when to start descents. That would be .799 X 3 = 2.4 nm. I never used it in this situation (no VDP), but had I done so it would have been a conservative .26nm later descent than Air Force pilots. I worked the heck out of that little rule, though, throughout the descent in order to keep a running check on whether my rate was sufficient or too excessive.
 
dtuuri said:
I Like that. :) I always used the "three times your altitude in 'thousands' of feet" thumb rule for when to start descents. That would be .799 X 3 = 2.4 nm. I never used it in this situation (no VDP), but had I done so it would have been a conservative .26nm later descent than Air Force pilots. I worked the heck out of that little rule, though, throughout the descent in order to keep a running check on whether my rate was sufficient or too excessive.
Click to expand...

The "Gus wears a HAT" method results in a 300 ft/nm descent. The "3 times your altitude in thousands" results in a 333 ft/nm descent. Since the "real" value is 318 ft/nm, your method is actually slightly closer! Though it does result in a slightly steeper descent. However, the 300 ft/nm is probably a better overall estimate for VDP location due to the inherent delays in descent (inertia etc.) and the fact that neither estimation method accounts for the typically 50-55 feet of threshold crossing height.
 
Sluggo63 said:
This was a thing in a Air Force. If a VDP wasn't published, we were taught to "roll your own."

We used the mnemonic "Gus wears a hat" to remember how to calculate it.

View attachment 106750

So in one of the examples above: HAT=799 and if you want a 3° glideslope, you'd have (to make the math easy) 798/3 = 2.66 NM from the end of the runway.

This is (was) a good technique for the way we used to fly NP approaches with the "dive and drive" method, especially in bigger, faster planes. You just knew that if you didn't see the runway by the VDP, you'd probably be going around. And, conversely, if you saw teh runway before the VDP, you should wait until hitting the VDP to start down, otherwise you'd be drug in.

Now... what wasn't taught is that there are reasons TERPS guys and gals won't publish a VDP. One of them being if there are obstacles in the path. When I read this an an instructor, I made sure I taught all my students how to calculate the VDPs, but also to caution them that there may be a good reason why there isn't on on the approach plate.

Edit: I just saw @RussR describe the HAT/GS approximation.
Click to expand...
Was there a particular reason you dove n drove instead of constant angle? Do you know if they still do it?
 
luvflyin said:
Was there a particular reason you dove n drove instead of constant angle? Do you know if they still do it?
Click to expand...
"Constant angle" wasn't really a thing back then. There weren't any "computed" glide angles or anything shown, and I think the thinking was to get down to MDA as soon as you could to try and pick out the runway environment and be able to descend stably to land.

I wish I had an old copy of the AF Instrument reg to look at to see what it said about non-precision approaches.
 
Sluggo63 said:
"Constant angle" wasn't really a thing back then. There weren't any "computed" glide angles or anything shown, and I think the thinking was to get down to MDA as soon as you could to try and pick out the runway environment and be able to descend stably to land.

I wish I had an old copy of the AF Instrument reg to look at to see what it said about non-precision approaches.
Click to expand...

My copy of AFMAN 51-37 (Instrument Flying) from 1960 doesn’t mention anything about trying to figure a constant descent rate or angle. In fact, pretty much all it says is upon crossing the FAF, “immediately descend to the published minimum altitude”, which implies a certain sense of urgency, as in “get it down”, which I interpret as dive and drive.
 
You must log in or register to reply here.
Back
Top