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.

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

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.

All instrument rated pilots are familiar with the term “chop it and drop it.” If the runway environment comes into sight just as the missed approach point is reached, the pilot can cut the power, enter a slip and drop it like it’s hot – or so the theory goes. Although widely practiced and propagated by many a CFI, is it really safe? Furthermore, is it even legal?

Take a look at 14 CFR 91.175: Takeoff and landing under IFR – Operation below DH or MDA:

(c) Operation below DA/ DH or MDA. Except as provided in paragraph (l) of this section, where a DA/DH or MDA is applicable, no pilot may operate an aircraft, except a military aircraft of the United States, below the authorized MDA or continue an approach below the authorized DA/DH unless—

(1) The aircraft is continuously in a position from which a descent to a landing on the intended runway can be made at a normal rate of descent using normal maneuvers, and for operations conducted under part 121 or part 135 unless that descent rate will allow touchdown to occur within the touchdown zone of the runway of intended landing;

(2) The flight visibility is not less than the visibility prescribed in the standard instrument approach being used; and

(3) Except for a Category II or Category III approach where any necessary visual reference requirements are specified by the Administrator, at least one of the following visual references for the intended runway is distinctly visible and identifiable to the pilot…

“Normal maneuvers” are just what you might think: any maneuver one would perform during a stabilized approach and landing. This includes normal heading and altitude bracketing, minor airspeed adjustments and ordinary descent rates (no more than 1,000 feet per minute in most airplanes). If you wouldn’t do it in a run-of-the-mill visual approach, then you shouldn’t do it on an instrument approach. With these standards, the old “chop it and drop it” just doesn’t cut it. The prudent pilot ought to realize when landing with a normal rate of descent using normal maneuvers cannot be accomplished. A missed approach is the appropriate response.