VDP

[Dadanawa]

Visual Descent Point.

VDP.

The latest point on the approach from which a stabilised descent at a 3 degree slope can be made. This point is known as the Visual Descent Point or VDP.

Formula:

DH-TCH➗GS➗106.13 (constant)

From the approach example below:

DH = 687' (decision height)
TCH = 55 (threshold crossing height)
GS = 3.5 (glide slope angle)

VDP = (687-55)➗3.5➗106.13

= 632➗3.5➗106.13

= 180.6➗106.13

= 1.7 nm from threshold
https://cimg1.ibsrv.net/gimg/pprune.org-vbulletin/771x1324/f87415ea_f84c_4665_8ed4_2add78f2b3d2_34bd3c56cfbe43d07cdb023 20f14412d3e1af5ff.jpeg

[TeeS]

Hi Dadanawa
I'm not quite sure whether you are asking a question or making a statement but whatever, it seems to be some unnecessary arithmetic done to calculate a number that is not really needed.

If you don't fly CDFA approaches, you level off at the MDA of 980ft and if you do not have the required visual references by the missed approach point at D1.9 ISAB then you go around. If you do have the required visual references and it is safe to do so, you land.
If you do fly CDFA approaches, you add a figure that your operator calculates for your aircraft (lets assume 50ft), to the MDA of 980ft to produce a 'derived DA' of 1030ft. If you do not have the required visual references at the derived DA, you go around. If you do have the required visual references and it is safe to do so, you land.

Presumable there may be some ISA temperature corrections to consider but in the UK that doesn't concern me too often and being an escapee from Rotorheads I only have to think at 100kts :O

Cheers

TeeS

[Dadanawa]

Thanks for your input TeeS,

Where it comes in handy is when the reported visibility is less than the VDP distance to the threshold. In that case you can expect not to see the threshold and accordingly, prioritize your go-around plan.

Having said this, a good set of approach lights might be visible, even though the visibility is less than the VDP distance.

We are required to calculate VDP for Non Precision Approaches at our airline.

All the best,

E W.

[back to Boeing]

So this is the third time in 6 years you’ve started this exact topic using exactly the same calculations. so what’s your point?

[TeeS]

Thanks Dadanawa

I can see where the logic behind the thought process would have started but in real use it seems to add lots of complication at a stage of flight where I like things to be as simple as possible.

The calculation produces a distance in nm from the threshold (which is also shown on the plate as 1.2 + 0.5 = 1.7NM); however, if I want to compare that to a visibility in m then I've got to multiply by 1852. If I'm using that distance during the approach to tell when my 3.5 degree flight path meets the MDA, I will have to do another bit of mental arithmetic to know what the DME should be reading since the DME does not appear to be zeroed to threshold in this case. If I'm using the ISAB then I've got to add 0.2 NM and for the SBH I add 2.0NM - oh no I don't because it is already on the plate as D1.9 ISAB / D3.7 SBH.

Of course, if you have added something like 50ft to the MDA to obtain a derived DH then your calculation would have to become VDP = (687 + 50 - 55) / (3.5 * 106.13) which gives a VDP of 1.84NM from threshold or D2.0 ISAB / D3.8 SBH.

It's just too many numbers for no real benefit in my mind.

Cheers
TeeS