Vdp?

[cbattags]

Can anyone give a good explanation for a VDP and its purpose.

 

[CLECA]

After further review, 121.651 (4) is cited below. I would imagine that upon reaching the MDA you can't start down from the MDA to landing maybe due to some type of obstruction, but that really doesn't establish anything otherwise they would just make that the final segment??????? So in other words,,,,,,,,,,,,, I dunno? Or maybe because if you wait until the MAP you would be too steep to make a "normal approach and landing"

121.651 (4) When the aircraft is on a straight-in nonprecision approach procedure which incorporates a visual descent point, the aircraft has reached the visual descent point, except where the aircraft is not equipped for or capable of establishing that point, or a descent to the runway cannot be made using normal procedures or rates of descent if descent is delayed until reaching that point.

 

[User546]

First glance I thought you were inquiring about some rogue V-Speed I've never heard of before.

Simply put, it stands for Visual Descent Point, and it's the place at which - given your out of the clouds and got the field ahead - you can descent from the published portion of a non-precision approach to set yourself up for a normal straight in landing (putting you in charge of obstacle clearance). You see these typically, as CLECA said, when the Missed Approach Point is located at the threshold of a runway where a normal descent and landing might not be possible if you flew the entire published procedure.

And per Rod Machado's "Instrument Survival Guide" he adds this: "Descents from a VDP provide a descent of 300 to 400 feet per nautical mile (approx 3 to 4 deg angle) to the first 3,000 feet of the landing surface."

 

[DC8 Flyer]

There are two ways to find a VDP as well, DME and Timing.

If you have an approach that uses DME it, simply divide your MDA by 300, ie a 600' MDA would equal a 2 mile DME VDP. The tricky part comes in where you have to figure out where the DME source is and how much to "add" onto that 2 miles to get 2 miles from the end of the runway. For example you are doing a LOC only approach to a 6000' long runway with a 600' MDA. So your distance from the end of the runway for a 3 degree slope would be 2 miles, but you have to add 1 mile for the length of the runway. So at 3 DME you would be at your VDP.

Second way is if you only have timing for your approach, off field NDB for example. Take your MDA (600 feet again) and drop the zero, or divide by 10, does the same thing. So we get 60 (60 seconds). Subtract that from your timing, say 3:00. So your VDP would be 2:00 This is off a 600 Foot/min assumption so if you are faster than 120kts GS (5 * GS = descent rate for a 3 degree glideslope), say 160 knots = 800 FPM for 3 degree slope, you have to start down 13 seconds earlier, so 1:47. I got that from 600 FPM = 100 feet/10 seconds, increase by 200 FPM (600 to 800 FPM) 800 FPM = 130 Feet/10 seconds.

That looks like a lot of math mumbo jumbo but just use 600 FPM as a baseline of 100'/10 seconds, so 1200 FPM is 200'/10 seconds.

180KTS GS equals 900 FPM descent rate, equals 150'/10 seconds = 15 seconds earlier VDP.

 

[CLECA]

There are two ways to find a VDP as well, DME and Timing.

If you have an approach that uses DME it, simply divide your MDA by 300, ie a 600' MDA would equal a 2 mile DME VDP. The tricky part comes in where you have to figure out where the DME source is and how much to "add" onto that 2 miles to get 2 miles from the end of the runway. For example you are doing a LOC only approach to a 6000' long runway with a 600' MDA. So your distance from the end of the runway for a 3 degree slope would be 2 miles, but you have to add 1 mile for the length of the runway. So at 3 DME you would be at your VDP.

Second way is if you only have timing for your approach, off field NDB for example. Take your MDA (600 feet again) and drop the zero, or divide by 10, does the same thing. So we get 60 (60 seconds). Subtract that from your timing, say 3:00. So your VDP would be 2:00 This is off a 600 Foot/min assumption so if you are faster than 120kts GS (5 * GS = descent rate for a 3 degree glideslope), say 160 knots = 800 FPM for 3 degree slope, you have to start down 13 seconds earlier, so 1:47. I got that from 600 FPM = 100 feet/10 seconds, increase by 200 FPM (600 to 800 FPM) 800 FPM = 130 Feet/10 seconds.

That looks like a lot of math mumbo jumbo but just use 600 FPM as a baseline of 100'/10 seconds, so 1200 FPM is 200'/10 seconds.

180KTS GS equals 900 FPM descent rate, equals 150'/10 seconds = 15 seconds earlier VDP.

Excellent post I never thought about a calculated VDP , I just assumed he meant published VDP......and you know what happens when one assumes

 

[Bjammin]

Good Post DC-8! On the money!

A VDP on an approach chart, notated by a "V" on the descent profile line, means that is the point from which you should not start down to the runway until you reach the DME indicated and that will give you the descent angle shown in the corner of the profile.

Say if you break out of the clouds and now see the runway. Don't descend below the MDA until the VDP beacuse there may be an obsticle clearence issue if you do.

 

[Doc Holiday]

Excellent post I never thought about a calculated VDP , I just assumed he meant published VDP......and you know what happens when one assumes

There are companies out there that show you the same method as mentioned above for "calculating" a VDP (by using time and HAT as the variables). However they are frequently referred to as PDPs (planned descent point). This is because a VDP is published on a chart (read: cannot just be made up) and takes obstacle clearance into consideration (although it is still only intended to provide additional guidance). If an airplane is not equipped to define the VDP (usually using DME), the approach should be flown as if no VDP exists.

A VDP/PDP is an excellent idea, but neither will do you any good when the visibility is down to minimums. For example, many VOR approaches use a mile visibility as the minimum for cats A and B, slightly higher as the the category goes beyond that. If the HAT is 600, a VDP/PDP would be roughly 2 miles from the runway, in wich case you will not have the required visual cues in sight. If that is the case, consideration needs to be given to circling because staying at MDA until the runway environment is in sight and attempting to land straight in would required an excessive (or impossible) rate of descent.

The argument for or against circling is a whole different discussion as it depends on several factors. However, the point is a VDP/PDP is an excellent tool in deciding the viability of safely and legally completing a non precision approach when it is used as a planning aid before you get down to mins and have to decide on your next course of action quickly.

Reference AIM 5-4-5