Timed VDP

[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".