Visual Descent Points
VDP: “A 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.”
A normal descent from the MDA to the runway touchdown point is key to the definition of a VDP above. During a straight-in, non-precision approach the VDP will be located prior to the Missed Approach Point if one is in fact published. The reality is that if a pilot reaches the missed approach point he may not be in a position to land when he sees the airport. This is especially true for higher performance aircraft like jets and turbo props, but this does not exclude slower general aviation aircraft. A pilot attempting to dive at the runway can destabilize the approach, especially at night with low visibility, and non-stabilized approaches are a leading cause of accidents. The Visual Descent Point allows for a 3 degree descent to the runway, and thus a stabilized approach.
Making Your Own VDP
What happens when there is no VDP published? It is simple to create your own, and this is something that is routinely used by professional pilots for these types of non-precision approaches.
First, figure out the height above terrain (HAT) of the MDA. Now, divide the HAT by 300. The number you get is the distance from the runway threshold (in nautical miles) of your visual descent point. In mathematese: VDP = HAT / 300
Let’s take an example. Consider the localizer approach to runway 2R in Nashville where the HAT at the MDA is 550 feet. To make the mental math easy (aren’t we busy enough up there?) let’s round it up to 600 feet. Recall that VDP = HAT / 300, so we have to compute: 600 / 300 = 2
The VDP for this approach is 2 miles from the runway threshold. But wait, there’s more. How will you know when you are precisely 2 miles from the threshold? Notice that the runway threshold is at a DME of 1.5 from the localizer. Just add 1.5 + 2 to get our DME reading of 3.5 at the visual descent point.
Although most newer aircraft have advanced avionics packages with GPS glideslopes, many of us still have to fly the old “dive and drive” type straight-in, non-precision approaches from time to time. If so, learn how to quickly compute these VDPs, and fly safer, more stabilized approaches.