What the Heck Is a VDP?
While recently observing a stage check with an Instrument Pilot student, I was set back when the student pilot looked up and said, "what the heck is a VDP?" Luckily for him, this was early on in his training and not a sufficient reason for the stage check instructor to fail him. His outburst lead me to think, "are us CFI's really teaching VDP's?" Fast forward about two months to my CFII check ride, there it was again, the VDP. Yet this time I'm on the other side, explaining to the DPE what a VDP was. I went all out, explaining calculations and concepts. With a pleased examiner and a new certificate in my pocket, I knew I had to share some information with everyone else.
Let us define the VDP:
"(The VDP) defined point on the final approach course of a non-precision straight-in approach procedure from which normal descent from the MDA to the runway touchdown point may be commenced, provided the approach threshold of that runway, or approach lights, or other markings identifiable with the approach end of that runway are clearly visible to the pilot."
Let us dissect the facts:
1) It is a point on the FINAL APPROACH COURSE.
2) Of a Non-Precision (VOR, LOC, etc.) Straight-in Approach.
3) You descend from the MDA.
4) Only if the threshold, approach lights, or other markings are clearly visible to the pilot.
Cool! So how do we know if we have a VDP? This question has two answers: if it's published (see the above depiction of a non-precision instrument approach,) then we do. It is the black "V" marking that depicts the mileage from the runway or other point. This tells us that we probably have an approach that has a 3-glideslope that intersects with the height above threshold (HAT.) This is part of the overall picture with VDP's, "a point in which the glideslope intersects the HAT." More importantly, a published VDP may act as an advisory that if the pilot has adequate visual reference, he/she may commence the visual aspect of the approach. If they do not they will likely land past the touchdown zone.
So what if we don't have a published VDP (like the VOR23 approach into KBED.) Well, no problem! That's the beauty of the beast, you can calculate your own VDP if it is not depicted!
Here is what you need to think about:
1) All non-precision straight-in approaches have a VDP, even if it is not depicted.
2) You can calculate your VDP if you have a DME, without one, and for timed approaches.
With or without DME use the following equation:
VDP = HAT/300
Why 300 you may ask? Well think about what the math is telling us, we are trying to find where our HAT intercepts our 3-degree glide slope. Since HAT is usually in hundreds of feet, we can't divide by 3, so we will divide by 300! The main issue I see with most students is they can't see why this number exists, and the approach has a 3.5-degree glide slope they still divide by 300. This is not the case, and the divisor must be adjusted according to glideslope.
If on a timed approach:
VDP = timed approach time - (HAT/10)
The HAT is divided by ten because of magic.... Just kidding, but it works out really well. The way you can also think about this is 10% of the HAT is equal to seconds subtracted from the MAP.
Let's talk about what happens if you don't see your minimums at the VDP. You should always continue to the missed approach point (MAP) and continue to your instructed (or depicted) missed approach procedure. Always. This should make sense because going missed prematurely has many complications in its own.
So there you have it, a short post on a few ways to understand VDP's. Just remember that VDP's can be extremely helpful if you want to descend from the MDA sooner, and you can calculate them if they are not depicted on the chart. I would advise going ahead and calculating them for a few different non-precision approaches, just to get into the swing of things.