Why the VDP?

Mavmb

Well-Known Member
I'm trying to study up on some of the instrument world because I never get to see any of it now. So can someone explain to me what the VDP is or why it's used for the faster airplanes on approaches?
 

Derg

New Arizona, Il Duce/Warlord
Staff member
Well if it's a charted VDP, you're not able to begin descent until reaching that point. Sometimes there are obstacles, terrain features or whatever that may preclude you from begining descent from MDA with the runway environment in sight.

Here's a little trivia for ya, Mike -- we stopped doing the "dive & drive" non-precision approaches at Delta. We do a thing called "CANPA" (Constant Angle, Non-Precision Approach".

Basically, we'll set 250 in the radio altimeter, MDA+50 in the barometric altimeter bug, and we'll start down at the FAF about approx 800 FPM. Almost more of a poor-man's ILS approach.

Works like a charm!
 

MikeD

Administrator
Staff member
[ QUOTE ]
Well if it's a charted VDP, you're not able to begin descent until reaching that point. Sometimes there are obstacles, terrain features or whatever that may preclude you from begining descent from MDA with the runway environment in sight.

Here's a little trivia for ya, Mike -- we stopped doing the "dive & drive" non-precision approaches at Delta. We do a thing called "CANPA" (Constant Angle, Non-Precision Approach".

Basically, we'll set 250 in the radio altimeter, MDA+50 in the barometric altimeter bug, and we'll start down at the FAF about approx 800 FPM. Almost more of a poor-man's ILS approach.

Works like a charm!

[/ QUOTE ]

Cool. With the CANPA, do you reach MDA at the VDP, or just a little prior?
 

IrishSheepdog

Sitting in the median
When I was doing my Commercial and Multi-engine training, I started plugging the numbers into the Garmin 430's vertical nav page and was doing these. Some instructors objected (the ones who had no idea what I was doing, after trying to explain), while others thought it was the coolest thing ever.

I still enjoying doing them this way... right there on the GPS it tells you the vertical speed needed to maintain to the point specified. It also works well for crossing restrictions. Too bad when I got to fly a Lear 25 we didn't have the 430... I ended up with a pegged VSI trying to make a restriction!
 

Cheechako

Well-Known Member
VDPs are almost mandatory for large high-performance airplanes. The chop-and-drop is not an option (something in the regs about leaving MDA should only be done with the runway environment in sight and a normal, stabilized approach to landing can be made). Many times the VDP comes long before the MAP. If the runway enviornment is not in sight by the VDP, it's time to go missed.

We calculate VDP a couple of ways:
Height above Touchdown (HAT) divided by 300 = distance TDZ.
10% of HAT = seconds from TDZ for a timed approach.
 

E_Dawg

Moderator
For the smaller planes you can do a VDP pretty easily by just taking the AGL altitude at MDA (either HAA or HAT), cutting off the last digit, and converting the first two into seconds. That works for planes of any speed, BTW (kind of a poor man's VDP, it's actually called a planned descent point when you make your own; but really it's the same thing).

As an example:
MDA = 620' MSL
HAT = 350' AGL
Time from FAF to MAP = 3:35

So you take the HAT, chop off the last digit and get 35 seconds.

That means that if you don't see the runway environment when you're 35 seconds from the MAP, start climbing because after that you won't be able to make it at a normal rate of descent. Just remember to not actually follow the missed until reaching the MAP
.

There's also an easy way to do it for DME.... but I forgot
 

onthebeach

New Member
Computing VDP using time and DME

>>HAT = 350' AGL
Time from FAF to MAP = 3:35

So you take the HAT, chop off the last digit and get 35 seconds. <<

The way you have written this is correct, but doesn't show the principle behind the computation. The rule of thumb you have outlined is used to compute an approximate VDP when none is shown in the IAP. The basis is that, at most final approach speeds, a 3 degree descent angle will result in about 600 fpm descent rate...that is, 10 feet per second. Therefore if you divide the HAA (not HAT, for most NP approaches...) by 10, you will get the number of seconds to subtract from your timing so that you will know an approximate VDP. In the example, if you had not sighted one of the required visual cues for further descent from MDA *or* landing (91.175) by the 3:00 minute point (3:35 - 0:35), you could not reasonably expect to land using a normal rate of descent and normal maneuvers (91.175). This is because, in this example, that 35 seconds at a descent rate of 10 feet per second would be necessary to perform a stabilized, visual final approach from the MDA to the runway.

>>That means that if you don't see the runway environment when you're 35 seconds from the MAP, start climbing because after that you won't be able to make it at a normal rate of descent. Just remember to not actually follow the missed until reaching the MAP.<<

Good points.

>>There's also an easy way to do it for DME....<<

Here is the rule of thumb if you are calculating an approximate VDP and the MAP is based on DME: Each 100 feet of HAT equals 0.3 NM. This is because a descent of 1000 feet for every 3 nautical miles over the ground is approximately equal to a 3 degree descent profile. So, in the example, 350' HAT calls for allowing 0.3 + 0.3 + 0.3 + 0.15 = 1.05 NM. Round it to 1 nm. Here's how you use this 1 NM figure: If the DME to the MAP is based on a navaid that is on the final approach course...that is, if the DME *increases* on final approach, *subtract* the 1 NM from the MAP DME. When you get to this point, you should be able to see one of the required items (91.175); if not, a missed is almost certain. On the other hand, if the DME *decreases* on final approach (that is, the IAP is based on a terminal navaid, *or* a navaid that is on the "far" side of the airport), you would *add* the 1 NM to get the approximate VDP.
 
Top