VDP and what they want to hear

Fly764

Well-Known Member
OK heres a question regarding VDP, I have heard numerous other CFIs debate this, and I am kind of wondering what others who have interviewed at airlines have said to the following vdp question, Can you descend before or after VDP? For those who have interviewed, have you said yes or no and with what reasonings.
 
strictly legally speaking... can you descend after vdp but before the MAP? YES. however. that's best only done in a light airplane where you can still land and get stopped with loads of runway remaining. a transport category aircraft is required to touch down in the touchdown zone. if you're going 140kts and trying to descend after the VDP and still touch down in the TDZ you're gonna either be not stabilized or just plain out of luck.

as we all know the vdp gives us a normal 3 degree descent, so practically speaking, if you are at the vdp in a transport cat airplane... and don't see the runway, I would just go missed.

i'm sure there will be plenty of people who reject this idea and i have faith they'll let us hear it...
 
strictly legally speaking... can you descend after vdp but before the MAP? YES. however. that's best only done in a light airplane where you can still land and get stopped with loads of runway remaining. a transport category aircraft is required to touch down in the touchdown zone. if you're going 140kts and trying to descend after the VDP and still touch down in the TDZ you're gonna either be not stabilized or just plain out of luck.

as we all know the vdp gives us a normal 3 degree descent, so practically speaking, if you are at the vdp in a transport cat airplane... and don't see the runway, I would just go missed.

i'm sure there will be plenty of people who reject this idea and i have faith they'll let us hear it...

+1 to that Allen

In the Eagle interview, I got this question and on top of that I showed them I can use this formula:

MAP - [(HAT)/(Descent Angle)]/100 = VDP

They seem to love that...
 
The work computer is slow so I can't get the AIM to come up. IIRC it is in the AIM you "Should or Shall" not descent before a PUBLISHED VDP (if you can ID the point).

Easy math for where a VDP is for 3 degree slope, number of miles from the your HAT divided by 3 equals DME distance of the VDP, ie 600 foot MDA 600/3 = 2 miles from threshold for VDP.

There is a rule for timing only, but I can't recall, getting old.
 
Easy math for where a VDP is for 3 degree slope, number of miles from the your HAT divided by 3 equals DME distance of the VDP, ie 600 foot MDA 600/3 = 2 miles from threshold for VDP.

.

beat ya to it
 
There is another formula you can use to come up with your own VDP:

HAT/300= DME

You then simply add: DME + MAP DME= VDP

you can do the same thing with time, if you don't have DME....knowing your GS

I've read somewhere that people call this calculated VDP actually a "PDP" (Planned descent point).

Make sure you don't use it on a circling approach!
 
MDH/1000x3 gives you distance in NM from the TDZ. Hardest part sometimes is figuring out where the TDZ is.

I use a rough rule of 0.2NM from the threshold.
 
strictly legally speaking... can you descend after vdp but before the MAP? YES. however. that's best only done in a light airplane where you can still land and get stopped with loads of runway remaining. a transport category aircraft is required to touch down in the touchdown zone. if you're going 140kts and trying to descend after the VDP and still touch down in the TDZ you're gonna either be not stabilized or just plain out of luck.

as we all know the vdp gives us a normal 3 degree descent, so practically speaking, if you are at the vdp in a transport cat airplane... and don't see the runway, I would just go missed.

i'm sure there will be plenty of people who reject this idea and i have faith they'll let us hear it...

As mentioned below by me, you don't have to compute a 3 degree angle, you can do shallower if you like, obstructions allowing of course.

+1 to that Allen

In the Eagle interview, I got this question and on top of that I showed them I can use this formula:

MAP - [(HAT)/(Descent Angle)]/100 = VDP

They seem to love that...

You can compute the VDP for whatever descent angle you like too.....2 degrees, 3 degrees, 4 degrees.....just make sure of any hazards as mentioned before. And an easy way to remember the formula is GuS wears a HAT, as depicted above.
 
The work computer is slow so I can't get the AIM to come up. IIRC it is in the AIM you "Should or Shall" not descent before a PUBLISHED VDP (if you can ID the point).

Easy math for where a VDP is for 3 degree slope, number of miles from the your HAT divided by 3 equals DME distance of the VDP, ie 600 foot MDA 600/3 = 2 miles from threshold for VDP.

There is a rule for timing only, but I can't recall, getting old.

Interview at 9E is easy. We use LIDO, and most of our charts have a VDP already computed and printed on the chart. It's one of the about 4 things I actually like about those charts.

If they pull out a Jepp chart in the interview, tell them it's outdated and you need to see current charts. :)
 
DME + MAP DME= VDP

Just for clarification on my part here. Is the VDP calculated from the MAP or from the runway threshold? I.E. You have an MDA of 600. 600/300 = 2NM. The runway threshold is at 1.5 DME. The MAP is .5 DME from that. So would you add 1.5 to the 2 NM, or 2 (1.5 + .5) to the 2NM?

GuS wears a HAT

Having a blond moment here. Never heard/seen this before. GuS wears a HAT would equate to HAT/GS, GS being the descent angle?
 
Having a blond moment here. Never heard/seen this before. GuS wears a HAT would equate to HAT/GS, GS being the descent angle?

Hehe.....blondie. :D

Yes, GuS wears a HAT being the mnemonic to help remember the formula: divide desired GlideSlope angle into the Height Above Touchdown. Or otherwise, GuS wears a HAT. The distance you come up with from there is calculated back from the runway threshold, or begining of the runway. Some people say calculate it back from the MAP, but the MAP and the runway beginning may not be coincidental. For instance, look at the TACAN 29R at KTUS. HAT is 477' with a 3.55 degree GS. Doing GuS wears a HAT and dividing that by 100, you get 1.34...or nominally 1.3 miles. Working back 1.3 miles from the runway, you get a VDP DME of 0.7 from the TUS VORTAC, or about 0.4nm short of the MAP at HOSVO. In this instance, if you calculated the VDP back from the MAP (looking at the distance markers back from the runway in the profile view), you'd get a VDP way back at 1.6 DME from the VORTAC or back near GAVET, which is about 2.5nm from the runway threshold. If you were to commence your descent at that point, you'd be shallow with about a 2 degree glidepath and nowhere near the 3.55 degrees you did the calculation for; because you ran the calculation back from the MAP instead of the runway threshold (you're calculating a descent angle to get to the runway, but if the MAP and runway threshold aren't coincident, then your VDP is inaccurate. Is it dangerous? It could be if there are obstructions that your shallow and inaccurate VDP now won't keep you clear of. Thats why you calculate back from the runway threshold and not the MAP, even though they're oftentimes coincidental, but not always. Again, to be perfectly accurate, you want to measure your desired glidepath to the runway, not to somewhere short of the runway that computing from a non-coincident MAP will lead you to.

http://204.108.4.16/d-tpp/1011/00430VDT29R.PDF

Now, looking at a more "normal" IAP where the MAP and approach end of the runway are concident, you'll see where I'm correct. Take the TACAN 11L into KTUS. You have a HAT of 361' and a 3.05 GS angle. Applying GuS wears a HAT and again dividing that by 100, you get 1.18....or 1.2nm. Calculating back from the runway threshold, you get 3.6 DME (2.4 DME at the rwy threshold + 1.2 DME back= 3.6 DMEVDP), although the plate is showing 3.4 DME (or 1.0 NM back from the runway threshold) for that calcuation, which tells me that the VDP depicted would give you a 3.5 degree glidepath to the runway threshold.

http://204.108.4.16/d-tpp/1011/00430VT11L.PDF
 
the reason for the difference(3.6 vs 3.4) is that the VDP on the chart is calculated to the TDZ....not the threshold.
 
the reason for the difference(3.6 vs 3.4) is that the VDP on the chart is calculated to the TDZ....not the threshold.

Correct, which is the 0.1 to 0.2-ish that I didn't account for in that instance, but should be accounted for for landing in a TDZ. With runways without a formal TDZ, I'll go from the threshold. My main point being don't compute solely off the MAP in all cases.
 
Just for clarification on my part here. Is the VDP calculated from the MAP or from the runway threshold? I.E. You have an MDA --- HAT --- of 600. 600/300 = 2NM. The runway threshold is at 1.5 DME. The MAP is .5 DME from that. So would you add 1.5 to the 2 NM, or 2 (1.5 + .5) to the 2NM?


Good question. I do not have the answer though. I just use the MAP DME. Calculating your own PDP (Planned descent point) however does not have any regulatory criteria to maintain from the TERPs handbook. Like my reference book says "it's more like a pilot aid to use in the cockpit to help get the job done right". I think this assumes the pilot knows the performance of the aircraft he/she flies.
If I were to answer just based on the last aircraft I flew (PA-28...) I can probably say that it doesn't matter, because I can probably land the damn thing in that .5NM you mentioned. I understand however that it is different for a much larger aircraft.

What do you think would make more sense using? The actual distance from the treshold?

cheers
 
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