This simple calculation will assist you in making a safe descent from MDA to the 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. And conveniently enough, the clouds are right at 500′ AGL.
As you begin your final approach, you leave the final approach fix for MDA. At MDA, you start looking for the runway, but intermittent ground contact makes things rather challenging.
A few seconds later, you wonder how long it’ll be before you’re too high to continue descending to the runway. Suddenly, the runway appears 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. Alarmed, you decide to go around.
Looking back, you realize you never briefed a VDP (Visual Descent Point) because there wasn’t one on the chart.
So, what could you have done differently?
Let’s Revisit 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 until reaching the VDP.
When you reach the VDP, typically, you’ll 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 indicated by a bold V on your approach chart, like the image below.
When You Look at the Chart and There’s No VDP
When instrument procedure designers survey the land during the creation of an approach, they analyze what obstructions penetrate safety clearance tolerances. If obstructions are present, a VDP might be denied during the creation of the instrument approach. That’s why you won’t find a “V” published on every non-precision approach, like in the image below.
If that’s the case, you can use a rule of thumb to find the approximate distance where you’d 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). Dividing this by 300 gives 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 isn’t DME. It’s the VDP’s distance from the runway. You’ll need to add or subtract this from DME readings to accurately locate your descent point.
And there’s another thing to keep in mind. In many cases, the VDP wasn’t published due to terrain or obstacles. Before starting your descent down from MDA to the runway, make sure you have sufficient visibility to not only see the runway but also any obstructions that might be in your path.
What to Do if You Fly Past the VDP
You’ve passed the VDP on an instrument approach, and you can start to see the runway, but you’re high. What should you do? Here are several factors to consider.
Have you ever calculated your own VDP? Let us know in the comments below.
