The Point of No Return

VDPs: Beyond the dive and drive

January 1, 2001

To paraphrase Henry Higgins, from My Fair Lady, "Why can't a nonprecision approach be more like an ILS?" A precision approach offers a three-dimensional chute down to the runway—as opposed to a minimum descent altitude (MDA) that lures pilots into descending as quickly as possible and then changing configuration to motor on to the missed approach point.

But yes, Professor Higgins, there are nonprecision approaches out there that imitate, to a certain extent, the profile given on a typical ILS.

Take, for example, the GPS Runway 21 approach into Carlsbad, New Mexico. If this approach had been built like a standard VOR approach, once you reached the final approach fix at TIBQU waypoint (pronunciation guesses, anyone?), five miles from the runway threshold, you would begin a descent from 4,900 feet to 3,580 feet. How you accomplished that descent would be between you and your maker, because the chart would give you no guidance. Perhaps you'd take the time before you started the approach to figure out that if you maintained a descent rate of 528 feet per minute (assuming your groundspeed's 100 knots), you'd arrive at 300 feet agl (the MDA) one mile from the runway threshold (the minimum required visibility), at which point you could continue that stabilized approach all the way down final, assuming you had the runway in sight. Perhaps you'd do that, or maybe you'd just take a wild stab at the appropriate descent rate and hope it all came out OK in the wash.

Instead of leaving you to guess at the right rate, the GPS Runway 21 approach, like many new GPS approaches, contains a tool to help you manage your descent and avoid the "dive and drive" that instrument pilots of small airplanes have been taught since the inception of the nonprecision approach. This tool is known as a visual descent point, or VDP.

While we've been ingrained with the mindset to get down as quickly as possible on a nonprecision approach, doing so makes for an unstabilized profile (see " The Stabilized Approach," November 2000 Pilot). Power changes often lead to unwanted changes in airspeed and add to the already high workload on the final segment.

When the Wide Area Augmentation System (WAAS) becomes fully available (see " Future Flight: Headwinds for WAAS," May 2000 Pilot), and our avionics are revised to meet the new standard, we shall have precision-like approaches where before there were none. But until that day arrives—and for many small airplane operators it always seems it will be later rather than sooner—the VDP can serve us well.

Airline crews are trained to calculate VDPs on each nonprecision approach they fly. Depending on the type of aircraft flown, the VDP is either calculated by the pilots via a formula or completed by the flight management system and backed up by a hand calculation. This procedure helps the pilots to determine beforehand the point on the approach at which they should begin their descent from the MDA, given the runway environment in sight. Knowing this figure ahead of time creates a safety-oriented mindset: If the pilots don't have sight of the airport at the VDP, they know they probably can't continue the approach in a stabilized manner. The conscientious crew will go missed and try again.

The FAA (formerly NOS) and Jeppesen chart VDPs on certain nonprecision approaches that fit a particular model: There must be a final approach fix or stepdown fix, and the derived VDP must come after this point. The FAA calculates the VDP such that the angle between the VDP and the point at which an airplane should cross the runway threshold is typically between three and four degrees, like the glideslope on an ILS approach.

We know that a stabilized approach is safer and easier on both the pilot and the airplane. In an effort to increase pilots' ability to make smoother nonprecision approaches, the FAA is reversing what was a disturbing trend.

VDP controversy

Over the past few years, the FAA began deleting visual descent points from approaches, citing a desire to reduce its own charting workload. However, airline industry response made it clear that VDPs were a necessary component in the push to reduce controlled flight into terrain (CFIT) accidents. But VDPs aren't without their downside.

Part of the controversy surrounding VDPs lies in their practical application. Often, a VDP is placed a couple of miles out on the final approach course. This is valid in one sense, in that by design, the VDP is the point at which the pilot can begin or continue a stabilized descent from the MDA to the runway threshold. However, because of a higher MDA on some nonprecision approaches, in order to mimic a reasonable (three- to four-degree) glideslope, the VDP must be farther from the runway threshold than the minimum visibility for the approach. Therefore, you wouldn't be able to see the runway threshold in order to make a stabilized visual descent if the in-flight visibility is near minimums.

For example, the MDA on the LOC/ DME Runway 23 approach at Cumberland, Maryland, is 700 feet agl. In order to maintain the approximately 3.5-degree descent angle established on the final approach course, the VDP lies 1.6 nautical miles from the runway threshold. Minimum visibility on the approach is one statute mile. Other approaches show even more drastic differences between the required visibility and the distance to the threshold from the VDP.

However, it seems few pilots crash on nonprecision approaches because of the placement of VDPs, but more often because they descend from the MDA prematurely—and often, purposefully—in an attempt to find the runway. When pilots descend below the glideslope on a precision approach, it's generally because they haven't stabilized the descent, they simply lose track of what they're doing, or they have some key piece of the approach misinterpreted or dialed up incorrectly. On a nonprecision approach, there's no glideslope to use as a barometer for how stabilized your approach is—unless that approach has a VDP.

Some approaches aren't suitable for VDPs, because of obstructions under the final approach segment that necessitate a stepdown fix too close to the runway. For example, if a cell phone tower penetrates a 20:1 slope close enough to the runway, a stepdown fix may be inserted in order to maintain obstacle clearance. If this fix is within a couple of miles of the runway, a VDP probably won't work because the descent angle would be too steep. In other words, the minima won't be adjusted to accommodate a VDP.

You may proceed below the MDA visually on an approach when you have the runway environment in sight and can maintain visual contact all the way to touchdown. If sight of the runway or its environs (actual runway lights, runway markings, or the pavement itself) is lost during the descent, a missed approach must be executed.

A poor man's VNAV

Anyone can use a VDP, which is a simplified version of the more elaborate system that it mimics: barometric vertical navigation (VNAV) procedures. VNAV procedures are designed to give virtual glideslope information to specially equipped and certified operators on otherwise nonprecision approaches. This is accomplished by establishing a vertical path angle (VPA) for each approach, which is computed using the difference in altitude between the minimum altitude at a given waypoint—the final approach fix (FAF) is standard—and the elevation of the threshold crossing height divided by the length of the final approach segment.

The FAA plans to add more VDPs to aid pilots on nonprecision approaches, and this is exciting because it holds promise for helping to reduce CFIT accidents. A VDP can discourage pilots from "diving and driving" in many situations. Because the VDP isn't regulatory, however, pilots can choose to go beyond the VDP, while remaining at the MDA, to extract more use from their airplanes—important to those who fly small aircraft that don't particularly need to follow a three-degree glideslope to make a stabilized approach.

Any hazard associated with using a visual descent point is the same as those that apply any time you choose to descend from the MDA. Obviously, if you ever hope to make a landing from an IFR approach, you're going to have to descend from the MDA at some point. We'll take a look at these concerns to see what you're up against when making the visual portion of your descent.

Visibility concerns

When you make the decision to move below the MDA, you need to consider several factors. First, the runway should be in sight from your current position on the approach. This seems straightforward, but if you reflect upon recent forays into actual IMC, you probably recall that visibility doesn't always help you out by staying constant.

Also, the underside of the lowest cloud layer may be ragged, causing you to fly through curtains of scud before you find the altitude at which the clouds truly break off. At many airports, especially those strategically located on bottom land, ground fog can throw a wrench into the equation by cutting you off from the prize at the last moment. Precipitation in the area often means varying visibility as well—either because of the precip itself, or through its by-product, precipitation fog.

Therefore, your decision to begin your visual descent, whether from the VDP or later during the approach, must be based upon a belief (or simply a fervent hope) that the visibility will not only stay at or above the minimum required, but allow you to see the runway the entire time you're descending toward it.

Missed approach

Another hairy situation occurs when you must go missed from below the MDA. Missed approach procedures are designed to commence from the MDA at the missed approach point (MAP). Any missed approach procedure needs to begin at this point, so your first order of business upon losing sight of the runway—assuming you're breaking off the approach for that reason—is to get yourself to the MAP safely.

Can you meet the climb requirements to return to MDA before MAP? Be sure to analyze the procedure before you head down, and make sure you can answer this question affirmatively. If not, you automatically place yourself below the missed approach path.

For example, consider the GPS Runway 33 approach into Buena Vista, Colorado. If you get the chance to see Buena Vista in good weather, you'll enjoy stunning views of the Collegiate Peaks off to the west. Of course, they're not that far to the west, which is one reason why the approach is structured the way it is.

The missed approach assumes you'll begin at an altitude of 9,540 feet or above. Your instruction is to then climb to 9,850 feet, before beginning a climbing right turn to 16,000 feet, and then proceed direct to TENZE waypoint and hold. The airport elevation is 7,946 feet, with terrain just east (or right) of the approach course at 8,503 feet, up to more than 10,000 feet nearby. If you go missed at 8,500 feet, you need to climb 1,350 feet by the time you reach the MAP and commence the climbing turn. Otherwise, you run the risk of embedding yourself in Triad Ridge, which reaches 9,000 feet or better just east of the airport.

If you're making a circle-to-land approach, the VDP won't do you much good, and you'll have to make other plans for your descent—which brings up an important point: If you're flying a circling approach, getting the approach stabilized takes on an entirely different character. To keep the descent methodical, you must revert back to adapting the approach as best you can to a traffic pattern, and set yourself up to be stabilized once you get on final.

When looking at a descent from the MDA, consider what can happen if you lose sight of the airport and don't execute the missed approach properly. An instrument training flight departed Chico, California, one fall evening, in a Piper Seneca. The flight plan was filed to Brackett Field in LaVerne, California. Weather at Brackett Field was reported at 1947Z to be one-mile visibility with mist obscuring three-eighths to four-eighths of the sky, with an overcast ceiling at 1,000 feet.

The pilot in command, a CFI, was talking with Southern California Approach (SOCAL) and receiving vectors to GOLDI Intersection for the VOR-A approach. SOCAL handed the Seneca off to the local controller, and the pilot was instructed to report GOLDI inbound. After a couple of minutes, the local controller noted that the airplane appeared to be circling the airport to the west. Over the next few minutes, the controller asked the pilot as many as seven times if he had the airport in sight, and the pilot answered "yes" four times. When asked if he wanted to cancel IFR, the pilot said no, but at some point, the local controller informed SOCAL that the pilot had indeed canceled IFR and then instructed the pilot to remain VFR.

A couple of minutes later, the pilot informed the local controller that he was executing the missed approach—now from a point three miles west of the airport. It was soon noted by the local controller that if the pilot was back in IMC, he needed to climb immediately to 4,000 feet msl and turn to a southerly heading, because of sharply rising terrain to the north of the airport. The pilot stated that he was attempting to head back to the VOR on a southeasterly heading. Soon after, radar contact was lost, and the Seneca was found at 2,053 feet msl in the Angeles National Forest, about 3.5 miles north of the airport.

This accident illustrates the siren song that fells pilots as prey to the duck-under syndrome. The pilot was flying into a familiar airport, on a familiar approach, and the ceiling and visibility were reported just the right side of legal. Most likely, the pilot believed that since he knew the area well, he could poke around below the MDA looking for the airport. He had sight of the airport for a while, but lost it during his excursions from the published approach procedure. The discipline of establishing a visual descent point for himself on the approach and sticking to it—staying above the MDA and going missed if the runway wasn't in sight at that point—would have kept the pilot and his student out of harm's way.

Planning your visual descent from the MDA appears on the surface to be nearer to an art than a science. However, establishing a visual descent point that takes into account the aircraft you operate, your currency, and the approach in question can help to focus your plan. After analyzing the procedure on the ground before you fly, and assessing the weather and your comfort level while you're on the approach itself, you can use the VDP to your advantage. Simply starting your descent at the VDP to expedite the approach could leave you lost and low.


Links to additional information about visual descent planning may be found on AOPA Online ( www.aopa.org/pilot/links/2001/links0101.shtml). E-mail the author at julie.boatman@aopa.org.