Timed VDP

[Tref]

I have heard recently that some pilots are being asked to calculate a timed visual descent point for a non-precision approach. While it's not difficult to do, I've never heard of anyone doing this in actual operations before and according to the AIM, "the VDP will normally be identified by DME." And, "pilots not equipped to receive the VDP should fly the approach procedure as though no VDP has been provided."

I think that timing is too inaccurate to be useful for VDP location identification and that it should not be used. Can anyone provide any references that state otherwise?

Thanks,
Tref

 

[Andy Neill]

I guess the question is, "Upon what will you base your decision to descend once you break out at MDA?" I agree that the timed version will probably be less accurate than using DME, but it still provides SOME useful info.

Let's say the MDA is 400' HAT and my VDP is now 10% of 400 (40) seconds short of the MAP. If you break out more than 40 seconds before MAP, will you begin your descent then or will you wait until your calculated VDP? If you don't break out until 20 seconds after VDP, will you attempt a landing or climb and fly your MA at the MAP?

A Jetstream crew in Missouri descended before the VDP on a DME calculation and hit trees 13 seconds later. All but two died.

 

[Serrano]

A Jetstream crew in Missouri descended before the VDP on a DME calculation and hit trees 13 seconds later. All but two died.

A Jetstream only requires two crew members. I speculate that what you mean to say is that of 15 souls on board there were 13 fatalities.

At the time of that acident Corporate Airlines did not train for or require the use of any VDP proceedures. The Captain called the runway in sight. I have never had a good answer to what he thought he saw, I do not believe that it
was RW 36 at KIRK.

If you do in fact have a runway in sight a timed VDP can cause you an unnessasary missed approach in the event that you screwed up the time or
encountered more tailwinds by forcing you to remain at the MDA untill
the clock runs out. IMHO a timed VDP should only be used as a test for reasonableness to provide some kind of WAG as to where you are on the approach.

The bottom line is that you have to be really careful when you are down
close to the ground in the clouds or the earth will rise and smite thee.

 

[rickair7777]

With no VDP published and no DME, it doesn't hurt to time a homemade VDP...you're going to time the FAF to MAP anyway right? Determining the expected VDP time in advance is a good SA tool...but you don't necessarily just chop & drop when you hit that estimated VDP time either.

 

[Tref]

I appreciate the thoughts, but does anyone have any official reference to timed VDPs? I've never seen one and I'm wondering if one exists.

BTW, the 10% of HAT for a timed VDP is only accurate if you're groundspeed for the entire Final Approach Segment is exactly 120kts. If it's not, then it's time to break out the calculator. Personally, I would prefer to look for a VASI/PAPA indication rather than use a less than reliable timed VDP.

Along the same lines, has anyone ever seen a step down point on a nonprecision approach that was based on time? I don't think one exists. I know that in order to use a VDP, at minimum the approach lights must be in sight, but the idea of obstruction clearance is the same in both cases.

 

[Tref]

With no VDP published and no DME, it doesn't hurt to time a homemade VDP...you're going to time the FAF to MAP anyway right? Determining the expected VDP time in advance is a good SA tool...but you don't necessarily just chop & drop when you hit that estimated VDP time either.

Would you stay at the MDA until your VDP time had passed, even if it looked like you were getting too high for a normal landing? If not and you would descend based on visual cues anyway, then what's the point?

Not being argumentative, and I agree that it might help with SA, but sometimes bad info is worse than no info.

 

[charter dog]

Aim 5-4-5

Here's the rest of what the AIM has to say about VDPs:

f. Visual Descent Points (VDPs) are being incorporated in nonprecision approach procedures. The VDP is a defined point on the final approach course of a nonprecision straight-in approach procedure from which normal descent from the MDA to the runway touchdown point may be commenced, provided visual reference required by 14 CFR Section 91.175(c)(3) is established. The VDP will normally be identified by DME on VOR and LOC procedures and by along-track distance to the next waypoint for RNAV procedures. The VDP is identified on the profile view of the approach chart by the symbol: V.

1. VDPs are intended to provide additional guidance where they are implemented. No special technique is required to fly a procedure with a VDP. The pilot should not descend below the MDA prior to reaching the VDP and acquiring the necessary visual reference.


2. Pilots not equipped to receive the VDP should fly the approach procedure as though no VDP had been provided.

If someone is requiring pilots to calculate "home-grown" VDPs on the basis of timing, I can't imagine where they found any regulatory or advisory guidance to do so. This may exist, but I just haven't seen it. So here's my take on it. I remember back when they first started publishing VDPs, there was some discussion among myself and some local pilots as to whether they should be calculated where not published. Among the questions raised were:

1) If no VDP is published, and we wish to create our own, what descent angle will be required in order to provide sufficient obstacle protection?​

2) What about NPAs where timing from the FAF is used to identify the MAP?​

The consensus was that:​

1) If a PAPI or VASI serves that runway, then obstacle clearance is provided if the calculated VDP is located at the intersection of the MDA and the PAPI/VASI path. If not, then there would be no known level of protection unless an obstacle analysis was available for that runway.​

2) The timed VDP would be of questionable benefit due to it's inherent potential for inaccuracy.​

Conclusions:​

A published VDP should be considered as though it were a stepdown fix, especially during low visibility and night operations. Descent should never be commenced prior to the VDP during limited visibility. Visual obstacle clearance is of limited value at best during these conditions.​

Any non-published (calculated) VDP should be considered as being for planning purposes only unless an airport obstacle analysis has been completed or a VASI/PAPI path will be followed. Quite useful if used solely to determine a descent point from MDA in conditions which allow good visual obstacle clearance or as a point at which you would expect to intercept the PAPI/VASI path. The timed VDP would have all the value of SWAG! Timing is fine for a MAP where there is nothing to hit at MDA anyway, but will be of little value in determining a descent point from MDA to the runway.​

My favorite use for the "home baked" VDP is on approaches with no straight-in mins published, but a straight-in landing is usually expected to be accomplished. ASE and SMO are good examples.​

MDA at SMO on the VOR or GPS-A is about 500 HAA. Trouble is, you can't descend from 940 HAA until 1.5 NM from the end of the rwy. (2.4 DME) The PAPI is 4.0 degrees and is located about 700' from the end of rwy 21. The PAPI path is at about 200' below you when you begin descent to MDA from 1120 msl. (940 HAA) You'll never make it in a jet without vastly exceeding stabilized approach criteria. If visibility is OK, you can circle if cat D mins exist. If not, see ya! So when must you see the airport by to have a realistic shot at a straight-in? The 4 degree PAPI crosses the intermediate stepdown altitude of 940 HAA at about 2.3 NM from the PAPI. So you need to see the runway by then. That works out to about 3.2 DME, or 0.8 Nm prior to the final stepdown fix. (CULVE 2.4 DME) So that's what we brief: If we don't see the runway by 3.2 DME, we fly the missed approach. OR, if reported ceiling and visibility were determined to be sufficient to meet cat. D mins: We'll circle to the left for rwy 21 at the 740' MDA. (we're cat C except when circling, where cat D must usually be used due to our normally >141 kt circling speed) For this reason, we like to make the straight-in or circle decision prior to beginning the approach and commit to it. I've been involved in the impromptu change of plans during mid-approach before and was not happy with the uncertainty it created. We're paid to do better than that.​

So it turns out that this "home baked" VDP calculation is quite useful for this kind of planning if you take the time to calculate it beforehand. However, I would personally never use timing for this purpose. Fortunately, we have GPS and can calculate a VDP as measured from the ARP if the GPS is not being used as primary navigation for an instrument approach, as might be the case during a visual to a runway not served by vertical guidance or a LOC without DME. Happens all the time with all the podunk airports we frequent!​

Apologies for the long-winded post. It was a boring day on call with no action and I had enough Starbuck's espresso to really get into this!​

Best,​

 

[Tref]

Thanks Charter Dog. I think we're on the same page. BTW, how did you cut and paste the AIM reference? I tried on my initial post and it wouldn't let me.

Tref

 

[charter dog]

BTW, how did you cut and paste the AIM reference? I tried on my initial post and it wouldn't let me.

Hi Tref,

I just did a normal highlight, Ctrl+C on the FAA.gov AIM and pasted it. Then I highlighted it, wrapped the .php quote tags from the reply toolbar around it and voila! Done. I've had trouble in the past with .pdf documents not letting me copy. This one should work.

Best,

 

[DC8 Flyer]

Timed VDPs are in now way shape or form required. They are however easy to use, can make a non-precision approach without DME or a published VDP much easier. When you fly something with some size to it (thats not meant as a cut, just my observation on how helpful timed VDPs are) you will appreciate the timed VDP. It is much harder to get a 275K pound airplane at 145 knots stabalized from the MDA to the runway, than it is a say a baron. Having a fixed point to know where the 3 degree path intersects the inbound course at MDA helps when making the "continue or go missed" decision.

While going missed at the VDP or your homemade timed VDP is not required, it is, IMHO, a good idea, of course no turns until you pass the published MAP.

Calculating the timed VDP is easy, takes a minute or two, so not something to do while inbound on the approach.

Some basic numbers to help understand how
3 degree glide slope = about 300 feet per NM
60 knots = 1 NM per minute

So with a VDP of 400' AGL and a time of 3:30 at 120 knots (7.5 NM from FAF to MAP) from the FAF to the MAP (assuming the MAP is at the end of the runway), and your actual GS for the day is 150 knots, my timed VDP would be about 2 minutes 15 seconds from the FAF (about 5 miles from the FAF or 2.5 miles from the FAF). That gives me 1 minute 15 seconds to go from 400' to the runway at a GS of 150 knots, 3 degree slope at 150 knots is (5 * 150) = 750 FPM. There is plenty of room in the time for me to start down, since I don't instantly go from level at the MDA to 750 FPM, and time to find the runway enviroment and line up, if need be.

The 10% rule would work out nicely here as well, 10% of your HAA, in this case 400' AGL, would be 40 seconds. Take 40 seconds off of 3:30 and you get 2:50, where I came up with 2:15.

Timed VDP is not meant to super accurate, mearly a guidance point, you could sit down and figure it out exactly but that would mean you woud have to fly a constant groundspeed all the way in, which is more difficult than necessary. If you understand how to use the numbers, you can get a fairly accurate time point and make that transistion even easier from MDA to landing or missed.

Hope this helps.

 

[Rez O. Lewshun]

If you actually use a VDP, keep in mind the distance will be most likely greater than the visibility approach minimums for the non precision approach. In that case, if the wx is at visibility mins, at your VDP you will not see the runway. Whereas, if you fly to the MAP you may see the run but will not be in a position to land. (stabilized approach criteria)

Thus, if you actually calculate and use a VDP, the visibility required should be the VDP distance from the runway...or greater....

 

[FL420]

PDP not VDP

This has been discussed before:

http://forums.flightinfo.com/showthread.php?t=73672&highlight=PDP

 

[charter dog]

This has been discussed before:

FL420:

Looks like it's being discussed again!

Thanks for the link to the other discussion though. I suppose that technically, you would be correct in saying that a PDP is not a VDP if the FAA doesn't publish it. You correctly stated that this is not an FAA recognized term. I did find reference to it in some interview prep mental math gouge though.

It does seem like a good way to differentiate between a published VDP with it's attached obstacle protection and a "homemade" version of the same thing which does not, unless PAPI/VASI path is intercepted or an obstacle analysis has been completed.

It is much harder to get a 275K pound airplane at 145 knots stabalized from the MDA to the runway, than it is a say a baron.

DC 8 Flyer:

And at 145knots, starting the descent 10 seconds early or late would place you about .4 nm short or long of the desired descent point. Looked at another way, (break out the calculator!) let's say your average ground speed prediction for the approach is 135 kts, but the actual groundspeed turns out to be 125 knots. (it happens) On a 5 mile segment from the FAF to the PDP, you calculate 2:13 as the time it will take from the FAF to the PDP. At 125 kts actual groundspeed, 2:13 would find you .38 nm short of the PDP. If your PDP was based on a 3.0 degree descent angle from a 500' HAA, then this timed segment would have you begin descent at a point which would result in a descent angle of only 2.4 degrees. If you are counting on this timed PDP to provide a 3 degree glidelope. the timing will need to be based on a very accurate groundspeed. Here, 10 knots of error in the speed estimate changed the descent gradient from 318'/nm to 254'/nm. This level of potential inaccuracy should be considered when deciding how much faith to place in this method. In order for a PDP/VDP to be of any real practical value, it's geographical position must be fixed. No way to be sure it is with the timed version without a groundspeed readout. Enough with the math!

Anyway, like most bizjet guys, I've become spoiled by the "boxes O' PFM" they gave us to play with, but am glad to remember how to do it the hard way. Helps me to spot entry errors!

I wouldn't presume to tell you how to fly your diesel 8, and no disrespect to your viewpoint is intended. I just thought I'd give my reasons for not preferring to use this method. Part of the reason is of course that I always have a better alternative in the form of GPS. I realize that not everyone does, and so makes due with what they do have. As we all must.

Best,

 

[Rez O. Lewshun]

Hate the timing method. Love the distance fix.

 

[rickair7777]

Would you stay at the MDA until your VDP time had passed, even if it looked like you were getting too high for a normal landing? If not and you would descend based on visual cues anyway, then what's the point?

Maybe. Better to go missed than get too low on final at a small, unfamiliar airport at night. Besides, if everything is working right, you should be pretty much on PAPI/VASI at your PDP.

Actually what I use for a do-it-yourself VDP (or PDP) is the distance-to-runway readout on my FMS: AGL/300 = PDP miles from threshold. This is also useful for night visual approaches to uncontrolled fields.

If you don't have an FMS or DME, then the time works.

 

[Tref]

Timed VDPs are in now way shape or form required. They are however easy to use, can make a non-precision approach without DME or a published VDP much easier. When you fly something with some size to it (thats not meant as a cut, just my observation on how helpful timed VDPs are) you will appreciate the timed VDP. It is much harder to get a 275K pound airplane at 145 knots stabalized from the MDA to the runway, than it is a say a baron. Having a fixed point to know where the 3 degree path intersects the inbound course at MDA helps when making the "continue or go missed" decision.

While going missed at the VDP or your homemade timed VDP is not required, it is, IMHO, a good idea, of course no turns until you pass the published MAP.

Calculating the timed VDP is easy, takes a minute or two, so not something to do while inbound on the approach.

Some basic numbers to help understand how
3 degree glide slope = about 300 feet per NM
60 knots = 1 NM per minute

So with a VDP of 400' AGL and a time of 3:30 at 120 knots (7.5 NM from FAF to MAP) from the FAF to the MAP (assuming the MAP is at the end of the runway), and your actual GS for the day is 150 knots, my timed VDP would be about 2 minutes 15 seconds from the FAF (about 5 miles from the FAF or 2.5 miles from the FAF). That gives me 1 minute 15 seconds to go from 400' to the runway at a GS of 150 knots, 3 degree slope at 150 knots is (5 * 150) = 750 FPM. There is plenty of room in the time for me to start down, since I don't instantly go from level at the MDA to 750 FPM, and time to find the runway enviroment and line up, if need be.

The 10% rule would work out nicely here as well, 10% of your HAA, in this case 400' AGL, would be 40 seconds. Take 40 seconds off of 3:30 and you get 2:50, where I came up with 2:15.

Timed VDP is not meant to super accurate, mearly a guidance point, you could sit down and figure it out exactly but that would mean you woud have to fly a constant groundspeed all the way in, which is more difficult than necessary. If you understand how to use the numbers, you can get a fairly accurate time point and make that transistion even easier from MDA to landing or missed.

Hope this helps.

You do realize that in your example the difference between 2:50 and 2:15 is 35 seconds and at 150kts the difference is 1.5nm? You think that works out nicely? I mean, I know it's called a "nonprecision approach" but isn't that a bit much?

Also, (maybe I just have a delicate ego) but I kinda thought a 747-400 was "something with some size to it." An 875,000lb Baron would be interesting.

Tref

 

[DC8 Flyer]

You do realize that in your example the difference between 2:50 and 2:15 is 35 seconds and at 150kts the difference is 1.5nm? You think that works out nicely? I mean, I know it's called a "nonprecision approach" but isn't that a bit much?

Also, (maybe I just have a delicate ego) but I kinda thought a 747-400 was "something with some size to it." An 875,000lb Baron would be interesting.

Tref

Put your ego back in the flight kit. I said no cut intended for the aircraft size, and since I didn't quote you, I wasn't speaking to you directly, but you chose to take offense. The timing is simply a SA tool not meant to be precise or gospel, my airline uses it, works well for all of us. The time difference I noted was to show the difference between crunching the actual numbers and using a rule of thumb. They both work and give you a good idea where abouts to start looking for a landing or start thinking of missed. You don't take the 747 to the MAP and then look down at 400' AGL and try to make the runway do you?

Try it the next time you have to do a non precision and the weather is "good", you may like it. If not, no problem if I use it, is there?

 

[charter dog]

If you actually use a VDP, keep in mind the distance will be most likely greater than the visibility approach minimums for the non precision approach.

Hey Rez, that's so true. Here is the case where using a VDP/PDP has the effect of raising the minimum visibility and ceiling required for a straight-in above that which is published because it is a circling approach. At least from a practical standpoint if you hope to achieve a stabilized approach, as in the case of the SMO example. We would need 1000-2 1/2 (Cat C) for the straight-in from the PDP ( I'm starting to like this term!) and only 600-2 (Cat D) to circle. I know from personal observation that people drive it in there on a homemade baro VNAV profile to make it straight-in all the time. If GLS, LPV or baro VNAV mins ever get published for SMO, it'll even be legal!

Best,

 

[Tref]

Put your ego back in the flight kit. I said no cut intended for the aircraft size, and since I didn't quote you, I wasn't speaking to you directly, but you chose to take offense. The timing is simply a SA tool not meant to be precise or gospel, my airline uses it, works well for all of us. The time difference I noted was to show the difference between crunching the actual numbers and using a rule of thumb. They both work and give you a good idea where abouts to start looking for a landing or start thinking of missed. You don't take the 747 to the MAP and then look down at 400' AGL and try to make the runway do you?

Try it the next time you have to do a non precision and the weather is "good", you may like it. If not, no problem if I use it, is there?

The reason I thought you were replying to me is because I started the thread and you didn't quote anyone else.

Also, you can do whatever you like, but do you really think that inaccurate data increases SA? I think that it decreases it.

 

[flx757]

Inaccurate data, good idea or bad...our Company Flight Manual spells it out pretty clearly.

"For situational awareness, calculate a VDP (when one is not depicted). If the runway is not in sight untal after passing the calculated VDP, the descent glidepath is greater than 3-degrees and a descent rate greater than 600 FPM is probably needed. Caution should be used when leaving the MDA prior to the calculated VDP without glidepath guidance.
- There may be limiting obstacles.
- A shallow glidepath can lead to visual illusions during the transition to landing and can cause a long landing.

Then there is a discussion and examples on how to calculate a VDP using either distance or time.

Personally, I believe this whole discussion can be made moot by eliminating the "dive and drive" non-precision approach in favor of a constant path angle type approach. These are safer and can help reduce CFIT accidents during the approach phase. I am in disbelief that my company still has not moved in this direction even though they keep promising that this change is "coming".

 

[DC8 Flyer]

The reason I thought you were replying to me is because I started the thread and you didn't quote anyone else.

Also, you can do whatever you like, but do you really think that inaccurate data increases SA? I think that it decreases it.

I guess I wouldn't call it inaccurate data. I pulled a quick example out off the top of my head to show the numbers and how the "long hand" version and the 10% rule can be fairly close. We can argue the difference in 45 seconds, but the fact remains, if you sit down and figure it out to the inch, it is all out the window as soon as your groundspeed changes. Calculating a point at which you start thinking more about going missed than continuing helps me personally, in getting the big picture. Maybe it doesn't work for you, thats fine, but maybe my method will help someone out. I never claimed it was dead on balls accurate, but simply a way to keep track of your progress along the final approach course relative to "glide slope" intercept.

I give myself a buffer in the time, because we do a config change before leaving the MDA for the runway, flaps 35 to 50, that takes a little bit to restabalize and get going down at the proper rate so waiting until the last second will leave you high and diving for the runway so I lead it a bit.

 

[Tref]

Personally, I believe this whole discussion can be made moot by eliminating the "dive and drive" non-precision approach in favor of a constant path angle type approach. These are safer and can help reduce CFIT accidents during the approach phase. I am in disbelief that my company still has not moved in this direction even though they keep promising that this change is "coming".

That's how we did it in the 400 (and the A330.) We were not allowed to fly level at the MDA. The MDA was treated more like a DA(H) and once you "touched" MDA you had to go around. It took a bit more planning, but I think it was much safer.

 

[Tref]

I guess I wouldn't call it inaccurate data. I pulled a quick example out off the top of my head to show the numbers and how the "long hand" version and the 10% rule can be fairly close. We can argue the difference in 45 seconds, but the fact remains, if you sit down and figure it out to the inch, it is all out the window as soon as your groundspeed changes. Calculating a point at which you start thinking more about going missed than continuing helps me personally, in getting the big picture. Maybe it doesn't work for you, thats fine, but maybe my method will help someone out. I never claimed it was dead on balls accurate, but simply a way to keep track of your progress along the final approach course relative to "glide slope" intercept.

I give myself a buffer in the time, because we do a config change before leaving the MDA for the runway, flaps 35 to 50, that takes a little bit to restabalize and get going down at the proper rate so waiting until the last second will leave you high and diving for the runway so I lead it a bit.

I guess I understand you a little better now. So you don't use it like a real VDP, ie. you can't desend before this point, but more of an "expect to be on the glidepath around this point, plus or minus a bit." Fair enough, but really, we're both still looking out the window for our primary cues. Wouldn't you agree?

 

[DC8 Flyer]

I guess I understand you a little better now. So you don't use it like a real VDP, ie. you can't desend before this point, but more of an "expect to be on the glidepath around this point, plus or minus a bit." Fair enough, but really, we're both still looking out the window for our primary cues. Wouldn't you agree?

Agree 100%. To be perfectly honest however, when it is down and the wx is right at mins for a non precision, I find it very hard to "visually" fly the airplane down without some sort of vasi/papi guidance. It has always been a weakness of mine, the whole runway in the window thing just doesn't work for me, so I have to use math, ie distance from the runway to make it "look" right. Hopefully my company will get the approval to start using the FMS's we have to their full potential, nice VNAV all the way to the runway!

 

[Tref]

Agree 100%. To be perfectly honest however, when it is down and the wx is right at mins for a non precision, I find it very hard to "visually" fly the airplane down without some sort of vasi/papi guidance. It has always been a weakness of mine, the whole runway in the window thing just doesn't work for me, so I have to use math, ie distance from the runway to make it "look" right.

I know exactly what you mean.

Hopefully my company will get the approval to start using the FMS's we have to their full potential, nice VNAV all the way to the runway!

It's the only way to fly! Or, you could have a Flightpath Trajectory Indicator on the PFD like in the Airbus. Then you can say, "gimme the bird." Works great every time.

 

[FL420]

Hopefully my company will get the approval to start using the FMS's we have to their full potential, nice VNAV all the way to the runway!

Many companies fly equipment capable of performing Constant Angle Non-Precision Approaches(CANPA's) but don't want the extra costs of initial and recurrent training in CANPA's for each pilot. You must also have approval in the OpsSpecs.

When the Wx is right down to minimum visibility you might find yourself doing more missed approaches doing a CANPA instead of a dive and drive approach. That is because the MAP on a CANPA is at DA(H) up to two miles from the runway threshold while the MAP on a dive and drive is usually at the runway threshold. A non-commercial operator flying a typical GA aircraft to a long runway has a better chance of getting aboard safely using the dive and drive when not constrained by a requirement to land in the touchdown zone from a stabilized approach.