How to Calculate a Visual Descent Point

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.

Visual Descent Point Symbol

A Visual Descent Point (VDP) symbol

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

Nashville-Approach-PlateLet’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.


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About the author
Pat Flannigan is a professional pilot and aviation blogger. He has been flying for fifteen years and is currently working as an airline pilot in the United States.

7 Replies to How to Calculate a Visual Descent Point

  1. GC says:

    Hi there. Interesting article regarding the VDP. Just want to specify that HAT is the height above touchdown (zone elevation) and not above terrain. HAT is found on straigh in approaches while HAA (height above airport) is found in the same spot but while circling.

    Is there any FAA restriction -that you may think of- in regards of commencing a descent prior to reaching the VDP (assuming I have all the items in 91.175) if the weather allows it? I’d tend to say no…think it’s a little tricky…

    cheers

  2. mahdi says:

    tnx. very usefull

  3. Stan hessler says:

    On a precision approach heights (agl) are referenced to TDZE. On a non precision approach heights (agl) are referenced to the ARP. Depending on the terrain and airport layout not making this distintion may prove costly.

  4. Alex says:

    VDP is not the last point where a descent can be started, rather is the first point where you should start a descent below MDA. Furthermore, VDPs are only intended to provide additional guidance and no special procedures required when flying them. Meaning that starting a missed approach is not required (and not advisable since its before MAP)

    Read AIM 5-4-5 f. (Pg.5-4-19)

  5. t timblin says:

    PLEASE READ!!!

    The AIM, under section” 5-4-5, f. 1., Visual Decent Points,” states that “the pilot should not descend below the MDA prior to reaching the VDP and acquiring the necessary visual reference.”

    Which is CONTRARY to what this article states, which is, “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.”

    First, there are no regulatory or presumed requirements suggesting the need to go missed at the VDP anywhere in the FAR/AIM. Simply, a missed approach should be executed at the missed approach point.

    Second, the AIM states that descents should be made NO SOONER than reaching the VDP. The recommendation as read from the AIM says nothing about descending AFTER the VDP and does not suggest that a descent should not, or could not, be made after the VDP; in fact it’s just the opposite, descents should only be made at or after the VDP. Just because an approach glide-slope might exceed 3˚, it doesn’t mean a stable approach can not be made. Accordingly, many approaches in the US have glide slopes greater than 3˚ in which stable approaches are made.

    An examiner could bust you for descending before the VDP.

Please, share your thoughts and opinions

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