VDP

[sarangan]

I am curious about how to interpret this VDP. Usually, it is the point along the glidepath that intersects the nonprecision MDA. But this one (RNAV 3R at LUK) shows a VDP that is presumably above the MDA. So at what altitude is this VDP located?

http://aeronav.faa.gov/d-tpp/2204/00083r3r.pdf#nameddest=(LUK)

 

[luvflyin]

I am curious about how to interpret this VDP. Usually, it is the point along the glidepath that intersects the nonprecision MDA. But this one (RNAV 3R at LUK) shows a VDP that is presumably above the MDA. So at what altitude is this VDP located?

http://aeronav.faa.gov/d-tpp/2204/00083r3r.pdf#nameddest=(LUK)

There is no Glidepath on that Approach. You could do the ol’ dive n drive if you wanted to. The VDP is telling you even if you get the airport in sight way out there, you should not leave the MDA until the VDP.

 

[flyingron]

It also warns you that if you proceed past that point that you're going to need a steeper than "standard" approach.

 

[RussR]

I am curious about how to interpret this VDP. Usually, it is the point along the glidepath that intersects the nonprecision MDA. But this one (RNAV 3R at LUK) shows a VDP that is presumably above the MDA. So at what altitude is this VDP located?

http://aeronav.faa.gov/d-tpp/2204/00083r3r.pdf#nameddest=(LUK)

VDPs are by definition located at the point where the descent angle intersects the MDA. This one is no different, and looks normal to me. Why do you think it's not?

 

[texasclouds]

The VDP is essentially your missed approach point in a high performance plane. If you don’t see the runway by then, you probably can’t make a stabilized approach to land after that point. Some companies require the pilot to go missed if they don’t have visual of runway environment by VDP.

Now in a 172, sure you could see the runway at MAP, chop power, full flaps, and get her down with a long runway. But they isn’t safely happening in a transport category aircraft.

 

[midlifeflyer]

VDPs are by definition located at the point where the descent angle intersects the MDA. This one is no different, and looks normal to me. Why do you think it's not?

I was wondering the same thing. It looks typical to me too.

 

[luvflyin]

I am curious about how to interpret this VDP. Usually, it is the point along the glidepath that intersects the nonprecision MDA. But this one (RNAV 3R at LUK) shows a VDP that is presumably above the MDA. So at what altitude is this VDP located?

http://aeronav.faa.gov/d-tpp/2204/00083r3r.pdf#nameddest=(LUK)

This should help make it make sense to you. That 3 degree angle and Threshold Crossing Height symbol are not a Glidepath. It’s just letting you know what the angle of descent would be if you made a continuous angle descent from the FAF at the minimum altitude at the FAF. And what altitude you would cross the threshold at if you decided to do the Approach that way instead of the chop n drop method. VDP’s were invented to let a pilot know that there may not be a safe margin of obstacle clearance if they get the airport in sight beyond that point and descend below the MDA way out there. Which is legal by FAR 91.
135 and 121, I don’t know. Maybe they require not leaving the MDA until a VDP.

Visual Decent Point (VDP)
The Visual Descent Point (VDP), is shown by a bold letter “V” positioned above the procedure track and centered on the accompanying dashed line. (See example below.) The VDP is a defined point on the final approach course of a non-precision straight-in approach procedure from which normal descent from the MDA to the runway touchdown point may be commenced.

Vertical Descent Angle (VDA) and Threshold Crossing Heights (TCH)
A VDA and TCH may be published on non-precision approaches. For Copter approach procedures, a Heliport Crossing Height (HCH) will be depicted in place of the TCH. The VDA is strictly advisory and provides a means to establish a stabi-lized descent to the MDA. The presence of a VDA does not guarantee obstacle protection in the visual segment. If there are obstacles in the visual segment that could cause an aircraft to destabilize the approach between MDA and touchdown, the profile will not show a VDA and will instead show a note that states “Visual Segment-Obstacles”.

 

[John Collins]

I think you are confused with the depiction showing a descent path beyond the VDP and then leveling off, suggesting that the VDP is not from the MDA, but the profile view is not to scale. I did a calculation of where the VDP intersects the MDA and found the VDP is located 2.35 NM from the threshold and is rounded to the nearest tenth of a NM, 2.4. So the chart designer is using artistic license to depict the profile view and it should not be taken to be literally to scale.

 

[IK04]

Does the FAA still have the DC-3 used to determine approach angles and VDPs?

I used to watch that thing make approaches and saw it on the ground in Amarillo one day. It was there with the other Flight Check aircraft, an old Falcon or Sabre, I don't remember...

 

[luvflyin]

I think you are confused with the depiction showing a descent path beyond the VDP and then leveling off, suggesting that the VDP is not from the MDA, but the profile view is not to scale. I did a calculation of where the VDP intersects the MDA and found the VDP is located 2.35 NM from the threshold and is rounded to the nearest tenth of a NM, 2.4. So the chart designer is using artistic license to depict the profile view and it should not be taken to be literally to scale.

 

[sarangan]

I was wondering the same thing. It looks typical to me too.

Perhaps it is my lack of understanding, but what I am used to seeing on a non-precision approach that is not co-located with a precision or LPV approach is something like this:


Basically, the VDP is at the MDA at a certain distance that assures a normal descent angle to the runway possible. In other words, it is the intersection between the MDA line and a normal glidepath (if it were available). I always interpreted the solid glidepath line below the VDP as belonging to the co-located LPV approach, not that of the LNAV. That makes sense to me based on the above diagram, and there are plenty of other VOR and LNAV-only approaches that look like this.

But the RNAV 27 at LUK has a solid line that dips below the VDP as if there were an LPV approach. But there isn't. So what exactly does the solid line below the VDP represent?

 

[John Collins]

Does the FAA still have the DC-3 used to determine approach angles and VDPs?

I used to watch that thing make approaches and saw it on the ground in Amarillo one day. It was there with the other Flight Check aircraft, an old Falcon or Sabre, I don't remember...

I don't know the status of an FAA DC-3, but it is a calculation performed by the procedure designer that determines the location of the VDP. Other considerations are used to determine if a VDP is charted or not.

 

[John Collins]

The current IAC standard says:

3.4.5.6 Procedure Track A profile view of the procedure track shall be shown using an 8 weight (.020") line. The approach track shall begin toward the top of the primary facility line, unless otherwise dictated by the procedure, and shall descend to .10" above the underline, where the final approach ends and the missed approach begins. On non-precision only approach procedures, the approach track will begin at the point specified on the procedure source document and descend to the MDA or VDP, thence horizontally to the missed approach point. The segment shall typically be 0.2" in length but length may vary due to the layout of the profile components.

 

[Clip4]

I think where you are confused is an IAP that has both LPV and LNAV minimums provided. The VDP does not apply to the LPV and is only applicable to the LNAV.

On the approach you linked, descent rate 5x ground speed for the 3° angle will set you up nicely.

 

[RussR]

But the RNAV 27 at LUK has a solid line that dips below the VDP as if there were an LPV approach. But there isn't. So what exactly does the solid line below the VDP represent?

View attachment 106676

I see what you're getting at now. Yes, the line should be horizontal after the VDP for a procedure without vertical-guided lines of minima. This appears to be a charting error, you could bring it to the FAA's attention at:

https://www.faa.gov/air_traffic/fli...esults&tab=charts&nasrId=LUK#searchResultsTop

It is important to understand though, that at least on FAA charts, the "glidepath line" in the profile view is purely notional and for illustrative purposes only. It's not to scale and you can't infer anything by the shape of it. The information like the courses and altitudes are what's important.

 

[luvflyin]

I see what you're getting at now. Yes, the line should be horizontal after the VDP for a procedure without vertical-guided lines of minima. This appears to be a charting error, you could bring it to the FAA's attention at:

https://www.faa.gov/air_traffic/fli...esults&tab=charts&nasrId=LUK#searchResultsTop

It is important to understand though, that at least on FAA charts, the "glidepath line" in the profile view is purely notional and for illustrative purposes only. It's not to scale and you can't infer anything by the shape of it. The information like the courses and altitudes are what's important.

Yup. There was someone here a few years ago saying if you were using a Jepp Chart, you had to dive n drive because that's the way they drew the pretty bold black line.

 

[sarangan]

Yup. There was someone here a few years ago saying if you were using a Jepp Chart, you had to dive n drive because that's the way they drew the pretty bold black line.

While the shape of the black line does not mean much, the end points do matter. In the example I pointed out, the black line extends below the VDP. I was glad to hear someone else say this may be a charting error.

 

[RussR]

While the shape of the black line does not mean much, the end points do matter. In the example I pointed out, the black line extends below the VDP. I was glad to hear someone else say this may be a charting error.

I agree that it's charted incorrectly and should be fixed. But from the flying perspective, I'm failing to see the practical impact of it. The MDA is 1280 so you wouldn't go below that anyway. The VDP is at 2.4 nm to RW03R, and the MAP is at RW03R. Those are the details you need. That black line on the FAA charts is so notional anyway that it could be completely removed and all the data you need is still there.

 

[Clip4]

There is nothing wrong with the chart. When the VGSI and descent angles not coincident that is how the VDP is charted. Search other charts with the same situation.

 

[sarangan]

There is nothing wrong with the chart. When the VGSI and descent angles not coincident that is how the VDP is charted. Search other charts with the same situation.

I am not disagreeing with you, but could you point me to another chart that has a similar depiction? ie, one that only has a non-precision approach and where the solid line extends below the VDP.

 

[dtuuri]

It makes sense to me that where more than one approach procedure are on the same chart, like "ILS or Loc" and LPV and LNAV, then the procedural track should extend to the end of the longest one. So in the OP's case — it's a mistook. It goes below mins.

 

[Clip4]

I am not disagreeing with you, but could you point me to another chart that has a similar depiction? ie, one that only has a non-precision approach and where the solid line extends below the VDP.

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

 

[dtuuri]

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

My theory is holding. The LP MDA is past the VDP. Have to say the 20 ft. lower is not to scale, though.

 

[Clip4]

My theory is holding. The LP MDA is past the VDP. Have to say the 20 ft. lower is not to scale, though.

My point is there is nothing wrong with KLUK approach and it’s not a charting error.

 

[dtuuri]

My point is there is nothing wrong with KLUK approach and it’s not a charting error.

Are you standing by the assertion it has to do with the VGSI? Here's the approach from the opposite direction. VGSI same as ILS, VDP associates with the LOC approach, procedural track goes all the way to ILS DA since it's the longest. No?

00083IL21L (skyvector.com)

 

[sarangan]

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

That's a great find! Thank you.

But my question remains the same. If solid line = instrument course and dotted line = missed approach course, why do these charts depict the visual segment below the VDP with a solid line?
I am implicitly assuming that VDP altitude is the same as the MDA altitude. The AIM description seems to support that interpretation. It says "The pilot should not descend below the MDA prior to reaching the VDP.".

 

[luvflyin]

That's a great find! Thank you.

But my question remains the same. If solid line = instrument course and dotted line = missed approach course, why do these charts depict the visual segment below the VDP with a solid line?
I am implicitly assuming that VDP altitude is the same as the MDA altitude. The AIM description seems to support that interpretation. It says "The pilot should not descend below the MDA prior to reaching the VDP.".

The VDP is not an altitude. It is a distance. Yes, the distance occurs at a predictable altitude so to speak, if you are flying a predictable angle of descent. But I think it is important to make the distinction to ensure that it’s intent, what it is trying to accomplish, is achieved.

 

[sarangan]

The VDP is not an altitude. It is a distance. Yes, the distance occurs at a predictable altitude so to speak, if you are flying a predictable angle of descent. But I think it is important to make the distinction to ensure that it’s intent, what it is trying to accomplish, is achieved.

I am not so sure about that, or perhaps that's where my confusion lays. VDP is a point in space. Yes, it does have a distance from the runway, but it is also at a certain height above the runway. How else would someone calculate angle of descent without a defined height and distance from the runway?

 

[luvflyin]

I am not so sure about that, or perhaps that's where my confusion lays. VDP is a point in space. Yes, it does have a distance from the runway, but it is also at a certain height above the runway. How else would someone calculate angle of descent without a defined height and distance from the runway?

It is not, by definition, a height. It is a distance. On this Approach it is 2.4 NM to RW03R. It has no defined altitude. Where altitude plays into it is using that geographic point over the surface of the earth to determine when you should or should not make a certain altitude change with your airplane. That is that you should not descend below the MDA until reaching that point.

 

[Clip4]

I am not so sure about that, or perhaps that's where my confusion lays. VDP is a point in space. Yes, it does have a distance from the runway, but it is also at a certain height above the runway. How else would someone calculate angle of descent without a defined height and distance from the runway?

You are over thinking this. When you get to the MDA land the airplane if you have the required flight visibility and runway elements. If you don’t, climb to the missed altitude without any turns until you pass the MAP.

 

[midlifeflyer]

I am not so sure about that, or perhaps that's where my confusion lays. VDP is a point in space. Yes, it does have a distance from the runway, but it is also at a certain height above the runway. How else would someone calculate angle of descent without a defined height and distance from the runway?

Of course it happens to be a certain height above the runway since it's calculated for the purpose of enabling a normal descent from the MDA.

The Visual Descent Point (VDP) is a defined point on a straight-in, non-precision approach from which you can descend below the MDA, as long as you have the required visual reference. If a VDP is available, it will be indicated by a "v" on the profile view portion of the instrument approach procedure chart. Do not descend below MDA before reaching the VDP.​

ou can calculate it if you want to do the math. But from a practical navigation perspective, it's a distance. How far from the runway at MDA do I have to be to make that "normal" descent?

 

[dtuuri]

[Y]ou can calculate it if you want to do the math.

Well, YOU can do the math: FAA Order 8260.3E (17SEP20)

Me? Not so much.

 

[dtuuri]

Well, YOU can do the math: FAA Order 8260.3E (17SEP20)

Me? Not so much.

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?

 

[RussR]

Well, YOU can do the math: FAA Order 8260.3E (17SEP20)

Me? Not so much.

I'd say there are basically three levels of approximation. The charted value is 2.4 nm.

1) HAT divided by 300. In the LUK 3R case, 799/300 = 2.66 nm

2) "Flat earth math": For this you need to know the threshold elevation, which is not published on the chart. But it is on the airport diagram as 481. In this case that also happens to be the touchdown zone elevation, but that's not always the case.

We need the Threshold Crossing Height of the VGSI, which from the chart is 44. Note that if the TCH of the VGSI and of the procedure are within 3 feet, the non-coincident note containing the TCH of the VGSI will not be published, so we'd either have to look in the Chart Supplement or just estimate again using the procedural TCH. (The actual value here is probably not 44 feet, but 43.7 or something like that, so it's >3 feet different.)

We have the glidepath angle of 3 degrees.

We could use "tangent" here, but since it's an approximation anyway, I'll use 318 ft/nm for a 3 deg glidepath.

Take the MDA, subtract the VGSI TCH and the threshold elevation, and divide by 318.

(1280-44-481)/318 = 2.37

3) The round-earth formula contained in the 8260.3E, which takes the same inputs as 2) but accounts for the curvature of the earth.



Inputting the numbers above, I get 2.36 nm.

HOWEVER, even this is an approximation because the values for TCH and THRe were rounded to the nearest foot because that's what's published. The real values may have a decimal place or two. Finding the exact numbers isn't something the casual user could really do. They're available on form 8260-9 for each procedure, but this is generally only easily available when a new procedure is on the IFP Gateway's Coordination tab, for a few months before it's published.

Note that the difference between the easy estimate in #1 and the almost-precise estimate in #3 is 0.3 nm. That's 12 seconds at 90 knots, not a big difference. That's about how much time it's really going to take to start a descent anyway. So in practice, the simple "HAT/300" is pretty good.

 

[RussR]

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?

As I previously stated, I don't believe they are charted correctly. I'm not an expert on the charting standards, but from John Collin's reference to the charting standards above:

"the approach track will begin at the point specified on the procedure source document and descend to the MDA or VDP, thence horizontally to the missed approach point."

I think it's pretty clear that if there is a VDP and it's a non-vertically guided procedure, the line should be horizontal after the VDP, since the VDP is determined by the intersection of the MAP altitude and the VGSI angle.

If there is more than one line of non-vertically-guided minima, then the lowest is used to calculated the VDP - so there will be no MDAs lower than the one at the charted VDP, and therefore the line should not go down after the charted VDP.

 

[luvflyin]

Of course it happens to be a certain height above the runway since it's calculated for the purpose of enabling a normal descent from the MDA.

The Visual Descent Point (VDP) is a defined point on a straight-in, non-precision approach from which you can descend below the MDA, as long as you have the required visual reference. If a VDP is available, it will be indicated by a "v" on the profile view portion of the instrument approach procedure chart. Do not descend below MDA before reaching the VDP.​

ou can calculate it if you want to do the math. But from a practical navigation perspective, it's a distance. How far from the runway at MDA do I have to be to make that "normal" descent?

By the very definition you quoted, it does not happen to be a ‘certain height above the runway.’ You will be at a certain height when reach that point. It is then that you make a decision about what to do with the ‘height’ of your airplane.

 

[RussR]

By the very definition you quoted, it does not happen to be a ‘certain height above the runway.’ You will be at a certain height when reach that point. It is then that you make a decision about what to do with the ‘height’ of your airplane.

I'm not sure what you're saying, but the VDP is by definition at a certain height above the runway, and it's that height above the runway determined by the published MDA. The VDP is just the point at which, if you are AT the MDA, you will intercept the VGSI glidepath *. So if you're higher or lower than the MDA, you're not going to intercept the VGSI at the VDP, and can't really consider it to be a useful point if you are not at the MDA (and there, a "certain" height above the runway, as indicated by the published HAT).

* with caveats for if there is not a VGSI.

 

[luvflyin]

I'd say there are basically three levels of approximation. The charted value is 2.4 nm.

1) HAT divided by 300. In the LUK 3R case, 799/300 = 2.66 nm

2) "Flat earth math": For this you need to know the threshold elevation, which is not published on the chart. But it is on the airport diagram as 481. In this case that also happens to be the touchdown zone elevation, but that's not always the case.

We need the Threshold Crossing Height of the VGSI, which from the chart is 44. Note that if the TCH of the VGSI and of the procedure are within 3 feet, the non-coincident note containing the TCH of the VGSI will not be published, so we'd either have to look in the Chart Supplement or just estimate again using the procedural TCH. (The actual value here is probably not 44 feet, but 43.7 or something like that, so it's >3 feet different.)

We have the glidepath angle of 3 degrees.

We could use "tangent" here, but since it's an approximation anyway, I'll use 318 ft/nm for a 3 deg glidepath.

Take the MDA, subtract the VGSI TCH and the threshold elevation, and divide by 318.

(1280-44-481)/318 = 2.37

3) The round-earth formula contained in the 8260.3E, which takes the same inputs as 2) but accounts for the curvature of the earth.

View attachment 106694

Inputting the numbers above, I get 2.36 nm.

HOWEVER, even this is an approximation because the values for TCH and THRe were rounded to the nearest foot because that's what's published. The real values may have a decimal place or two. Finding the exact numbers isn't something the casual user could really do. They're available on form 8260-9 for each procedure, but this is generally only easily available when a new procedure is on the IFP Gateway's Coordination tab, for a few months before it's published.

Note that the difference between the easy estimate in #1 and the almost-precise estimate in #3 is 0.3 nm. That's 12 seconds at 90 knots, not a big difference. That's about how much time it's really going to take to start a descent anyway. So in practice, the simple "HAT/300" is pretty good.

There is not a glidepath angle of 3 degrees. There is a Vertical Descent Angle, VDA, of 3 degrees. And the distinction between the two is pertinent.

 

[RussR]

There is not a glidepath angle of 3 degrees. There is a Vertical Descent Angle, VDA, of 3 degrees. And the distinction between the two is pertinent.

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.

 

[luvflyin]

I'm not sure what you're saying, but the VDP is by definition at a certain height above the runway, and it's that height above the runway determined by the published MDA. The VDP is just the point at which, if you are AT the MDA, you will intercept the VGSI glidepath *. So if you're higher or lower than the MDA, you're not going to intercept the VGSI at the VDP, and can't really consider it to be a useful point if you are not at the MDA (and there, a "certain" height above the runway, as indicated by the published HAT).

* with caveats for if there is not a VGSI.

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.