Using a VDP

Most IFR pilots and students can identify a visual descent point (VDP) on the profile view of an instrument approach chart, as highlighted on the RNAV (GPS) RWY 13 approach at KABR.

And pilots typically can recite an accurate definition of the term, based on the explanations provided in references such as the AIM, the Instrument Procedures Handbook, and the Instrument Flying Handbook.

VISUAL DESCENT POINT− A defined point on the final approach course of a nonprecision straight-in approach procedure from which normal descent from the MDA to the runway touchdown point may be commenced, provided the approach threshold of that runway, or approach lights, or other markings identifiable with the approach end of that runway are clearly visible to the pilot. (P/CG)

In my experience, however, few pilots can explain in detail how they would use a VDP while flying a nonprecision, straight-in approach.

Some FAA guidance provides too-subtle hints. See, for example, the following excerpts from the Instrument Procedures Handbook and the Instrument Flying Handbook.

For short runways, arriving at the MDA at the MAP when the MAP is located at the threshold may require a missed approach for some aircraft. For non-precision approaches, a descent rate should be used that ensures the aircraft reaches the MDA at a distance from the threshold that allows landing in the TDZ. On many IAPs, this distance is annotated by a VDP. (IPH 4-37)

The visual descent point (VDP) is a defined point on the final approach course of a nonprecision straight-in approach procedure. A normal descent from the MDA to the runway touchdown point may be commenced, provided visual reference is established. The VDP is identified on the profile view of the approach chart by the symbol “V.” (IFH 1-21)

By definition, if you fly past the VDP at the MDA, you won’t be able to fly a stable, normal descent that puts your wheels on the pavement in the runway touchdown zone. If you continue at the MDA to the missed approach point, which is often at the runway threshold, you may not be able to land safely even if the runway appears through the mist.

But the definitions don’t describe a plan for using a VDP when you’re flying a nonprecision approach.

I teach IFR pilots to use the VDP as a decision point, much like the DA for a precision approach. In other words, when you arrive at the VDP, if you don’t see the visual references required by 14 CFR § 91.175, you should start the missed approach procedure:

• Continue along the final approach track until you reach the MAP, and then fly the charted headings and courses.
• Climb to the initial missed approach altitude (unless, as is rarely the case, you must observe an altitude restriction; see, for example, the ILS or LOC RWY 14R at KBFI in Seattle).

Continuing beyond the VDP toward the missed approach point, hoping that you’ll see the runway environment and still be able to land, sets you up for an unstable descent, per the definitions above. You might be able to get a low-performance airplane like a C172 onto a long runway if you leave the MDA between the VDP and the threshold, but in a faster, complex aircraft, pressing on to a landing sets you up for a long landing and perhaps a overrun. Overruns are a continuing problem, especially in jets and other high-performance aircraft, as noted in this AOPA article. They’re also unnecessarily frequent in piston aircraft.

For a demonstration of how to include a VDP in an approach briefing, see Briefing IFR Procedures.

Using a VDP requires preparation. Although VDPs are published on charts, as the AIM notes, VDPs do not appear in the waypoint list when you load an approach in a GPS navigator, such as a Garmin GTN 750.

Note that the Active Flight Plan for the RNAV (GPS) RWY 13 at KABR (see profile view above) includes GUBDE (the FAF), LOWOB (a step-down fix used when flying to the LNAV MDA), and the MAP at the runway 13 threshold. But the VDP, 1.1 nm from the runway, doesn’t appear in the list.

To confirm your position relative to the VDP while flying this approach, you must monitor the distance from the MAP. Or as a paragraph buried deep in AIM 1-1-17 puts it:

If a visual descent point (VDP) is published, it will not be included in the sequence of waypoints. Pilots are expected to use normal piloting techniques for beginning the visual descent, such as ATD [along track distance]. (AIM 1-1-17)

If you have a WAAS-capable GPS navigator, on most approaches you can follow advisory vertical guidance (+V) to the MDA and then on to the runway touchdown zone. An advisory glidepath typically will intercept the MDA near a VDP, but you must monitor the descent carefully. If you use the +V guidance, you also must confirm that you meet any crossing restrictions, especially in the final approach segment, and that you do not descend below the MDA prematurely. Remember that most autopilots will track an advisory glidepath below the MDA unless you intervene.

For more information about advisory vertical guidance while flying conventional approaches, see VOR Approaches with LNAV+V.

Careful review of the profile view for an approach that includes a VDP typically shows that the visual descent point is farther from the runway than the published minimum flight visibility. For the approach at KABR, the VDP is 1.1 nm (1.27 sm) from the threshold. The visibility requirement for category A and B aircraft is 1 sm. In theory, you could continue about another quarter mile toward the runway in hopes of seeing the runway environment. But at a typical C172 IFR approach speed of 90 KIAS, you are traveling 152 ft/sec, and you’ll cover 0.25 nm in about 10 seconds. In this case, using the VDP as the decision point at which you’ll begin the missed approach is not only prudent, it complies with 14 CFR § 91.175(c)(2), which states that you can continue to a landing only if the required visibility is present.

The VDP for other approaches may not so closely approximate the published visibility minimums. See, for example, the RNAV (GPS) Y RWY 16R approach at KPAE.

The VDP is 1.1 nm from the threshold, but the visibility requirement for the straight-in LNAV-only approach is RVR 2400 or one-half sm. You could, in theory, continue about another one-half miles in hopes of picking up the approach lights or other visual references. But do the math. At typical light-piston approach speeds, you’d cover that distance in about 20 seconds, and you’d be that much closer to the runway, still some 400 ft above the touchdown zone. Using the VDP at 1.1 nm from the end of the runway as the decision point ensures that you can make a smooth, stable descent if the runway environment appears. By definition, starting down from the MDA beyond the VDP guarantees that you’ll have to fly a steeper, probably less steady, descent.