Vdp

[c152]

How do you calculate a VDP?

 

[Guillotine007]

VDP's are published on some non precision approaches...but you can calculate an approx VDP by:

taking 10% of your HAT/HAA
then subtracting that from your FAF to MAP time.
This will allow a (in theory) 3 degree "GS" to the runway @ 120 kts.

if i screwed that up, i'm expecting someone to correct me.

Klein

 

[Flymach2]

Hi...

You shouldn't have to calculate a VDP as they are 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.)

If you're asking how to calculate a 3 degree descent angle from the VDP to the runway...

Divide your groundspeed by 2 and then multiply the result by 10.

Groundspeed 140/2=70x10=700FPM descent.

Regards

 

[Deftone45075]

Actually, knowing how to calculate a VDP is quite important. I have sen numerous approaches that do not publish this point, and therefore it is good judgement to calculate and use the VDP. The method Guilotine posted works very well. There is also a methed to establish your VDP in DME, it can be found in the "rules of thumb" section on this site.

You may very well be asked to calculate a VDP on an interview as well.

 

[Flymach2]

Hi...

Approaches without a published VDP have not been assessed for terrain clearance below MDA and may not provide a clear vertical path to the runway. This means that obstacle clearance could be less during the visual segment and the 3 degree calculation could be inaccurate as well as dangerous. This is not a blanket statement and there may be some approaches which would allow for a "calculated" VDP at a 3 degree descent angle with no issues arising.

Regards

 

[Deftone45075]

The above is very good point. However, you are visual when descending below the MDA. If terrain is an issue, you would react accordingly. The VDP is a reference at which you make a decision: if you reach the VDP and do not have the runway in sight, you make the decision to continue to the MAP.

If there was enough terrain that an approach would have to be much steeper than 3 degrees from the MDA, than they would not allow an MDA that low since you would not be in a position to land using normal maneuvering. I am not aware of an approach in a terrain environment where the published MDA is so low that you would have to make much more than a 3 degree glidle path. I find it hard to believe any such approach would be approved.

 

[regionalcap]

Flymach2,
I disagree with your statement about not calculating a VDP. For a non-precision approach, our POH specifically states that we are to calculate one if a VDP is not published. We are required on checkrides to provide this information. Deftone hit the nail on the head about its purpose. It is there to give you a reference for a normal position to land.

 

[Flymach2]

Hi...

Regional and Def,

As I mentioned earlier, my comment was not intended to be a blanket statement and I was just pointing out is that the typical formulas which provide for a 3 degree approach angle from the MDA may not be suitable for some approaches which do not have a published VDP. This does not necessarily prevent flying the normal angle, it only means that obstacle clearance in the visual segment could be less and greater care should be exercised in looking for obstacles in the visual segment.

Regards

 

[Sniper@YourFeet]

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.)

THAT is a VDP.

If you're asking how to calculate a 3 degree descent angle from the VDP to the runway...

That is a PDP, a pilot determined descent point. And you should always calculate one if you are not an 'approved operator' - their FMS gives them a glideslope to the runway, even on NP approaches.

 

[LJDRVR]

Sniper@YourFeet beat me to it. Only the TERPS weenies are allowed to calculate a VDP as defined in the AIM. The rest of us may calculate a planned descent point (PDP) to approximate the same thing.

Flymach2, I have to respectfully disagree with what you said when you assert that non-precision IAP's have not been assesed for obstacle clearance below the MDA. If that's the case, you could never land. Think about it.

VDP's & PDP's provide you with a "jumping off" point from which to depart the MDA for the runway environment ONLY if 91.173 is satisfied, in other words, you're in the visual segment of the approach. The only situation in which I would be wary of using a 3 degeee PDP/VDP is if that runway's PAPI or VASI was published at a >3 degree angle. Maybe one of you TERPS nerds (And I mean that in a good way) can provide the reference, but I'm pretty sure that a non-precision instrument runway with anything sticking up into the obstacle clearence plane will either have visual guidance, (a 3.5 degree PAPI/VASI for example) or won't be certifyable for straight-in minimums in the first place.

VDP/PDP's do two things for me. They help me make the appropriate descision about when to depart the MDA for the runway. (It's poor visibility, most of us are not used to approaching the runway environment that low, it takes the guesswork out if it.) Secondly, whether operating under 135 or 121 we are required to depart the MDA at a normal rate of descent. (No chop and drop) By computing my own PDP, I have a point that if I reach without having the runway in sight, I'm going to go missed, even though the MAP may be the co-located VOR. This removes all temptation to maneuver or circle or run off the end or otherwise kill yourself doing something stupid when you finally see the runway directly below you from an altitude of 500'.

Anybody else?