This easy calculation will help you make a safe descent from MDA to landing.

Scenario: No VDP? Calculate One Yourself

You're flying an instrument approach through low clouds. The MDA (Minimum Descent Altitude) for your non-precision approach is 500' AGL. The clouds sit, conveniently, right around 500' AGL.

As you begin your final approach, you leave the final approach fix for MDA. At MDA you begin looking for the runway, but intermittent ground contact is making things tough.

A few seconds later, you wonder how long you have until you'll be too high to continue your descent to the runway. Suddenly the runway pops up at your 12 o'clock and you rush to leave MDA, diving towards the runway. You're too high, crossing the threshold around 200' above the runway. Startled, you elect to go-around.

Looking back, you realize you never briefed a VDP (Visual Descent Point), because there wasn't one on the chart.

What could you have done differently?

Let's Review VDPs

According to the AIM, "the VDP is 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."

VDPs are only published for straight-in instrument approaches to specific runways, and if your approach has one, you shouldn't descend below MDA prior to reaching the VDP.

When you reach the VDP, you'll typically be able to follow a 3-degree glide path to the runway, which is the same glide path as most precision approaches.

So how do you know if your approach has a VDP? It's denoted by a bold V on your approach chart, like the image below.

You Look At The Chart And There's No VDP

When instrument procedure designers survey land during the creation of an approach, they'll analyze what obstructions penetrate safety clearance tolerances. If obstructions are present, a VDP might be denied during the creation of the instrument approach. This is why you won't find a "V" published on every non-precision approach, like the image below.

If that's the case, you can use a rule-of-thumb to find the approximate distance where you would start a descent from MDA to the runway: Take the AGL value of the MDA and divide it by 300.

For example, on the Crookston (KCKN) VOR/DME Approach to Runway 13, the lowest MDA takes you to 344 feet above the TDZE (Touchdown Zone Elevation). Divide this by 300, and you'll get 1.15, which is the approximate distance from the runway where you can start a 3-degree descent to the runway.

Remember that the resulting value is NOT DME. It is the VDP's distance from the runway. You'll need to add or subtract this from DME readings to properly locate your descent point.

And there's another thing to keep in mind. In many cases, the VDP wasn't published because of terrain or obstacles. Before you start your descent down from MDA to the runway, be sure you have enough visibility not only see the runway, but also see any obstructions that might be in your path.

You Fly Past The VDP. Should You Go Missed?

You've passed the VDP on an instrument approach, and you start to see the runway, but you're high. What should you do? Here are several things to consider.

Have you ever calculated your own VDP? Tell us in the comments below.