Visual descent points are handy tools the FAA has included on many GPS approaches to keep you from flying dangerously unstabilized approaches and to prevent pilots from hitting all sorts of obstacles. But there are a number of approaches that don’t have these magic little references. Read on to find out how to compute your own VDPs in one simple formula.
What Are VDPs?
In last Monday’s article we took a look at the requirement for a “normal rate of descent using normal maneuvers” to proceed below the MDA per the FAR/AIM. Unfortunately, there are a lot of missed approach points out there that have the smell of a trap. There is simply no way to make the runway using any semblance of a normal maneuver.
To limit the temptation to proceed with a landing under unstabilized approach conditions and prevent collisions with obstacles along the final approach path, the FAA began publishing visual descent points (VDPs) on many GPS plates. Marked by a “V,” the VDP is the last point at which a descent from the MDA to to the touchdown zone can be made at a stable three degree glideslope. If the runway is not in sight by the VPD, a missed approach should be executed.
Unfortunately, VDP’s are not published on all charts. Luckily, there is an easy technique to compute your own.
Making Your Own VDP
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. Note that 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.