Mentor matters: What’s a TDZ again?

The problem is typically attacked in two primary areas. First, by increasing education of performance planning, especially in awareness of issues inherent with making a land-or-divert decision and the need to use safety margins that allow for real world—not test-pilot—performance. Second is the focus on rigorous adherence to stabilized approach criteria, because even a landing calculated to have a conservative safety margin can result in an overrun if flown poorly.

This stabilized approach criteria often has the pilot perform a final land/go-around decision at 500 feet above field level (AFL), if the flight meets a half-dozen criteria. Most important is that airspeed and altitude are within an acceptable window; if so, the approach can continue. Otherwise, a go-around should be performed if the energy of the aircraft is too high to allow for a landing with adequate margins.

Unfortunately, many pilots treat this 500-foot AFL gate as the last box to check before committing to land. In reality, an approach could certainly be stabilized at 500 feet, only to become unstable closer to the runway.
There’s a problem, though, with attempting continuous evaluations during the final 500 feet to touchdown: There’s a tendency for pilots to shift focus to outside visual cues, with a corresponding decrease in scanning the instruments that could alert to excessive airspeed or height.

Fortunately, there is another tool that can be readily applied during the critical last few seconds of landing—one that can prompt a go-around when landing margins are compromised. It’s an awareness of, and adherence to, the touchdown zone (TDZ) as the sole area where the wheels should set down.

The AIM defines the touchdown zone simply enough: “The first 3,000 feet of the runway beginning at the threshold.”It’s common for pilots to have forgotten the definition of the touchdown zone, likely last remembered during a cram session for the instrument rating oral exam. The AIM defines the touchdown zone simply enough: “The first 3,000 feet of the runway beginning at the threshold.” However, there’s a more nuanced and practical working definition of the TDZ we can use—one that’s especially relevant for the relatively shorter runways used by both piston- and turbine-powered general aviation airplanes.

Central to understanding this working definition of TDZ are the pavement markings used in the TDZ on runways served by precision approaches. For these runways, the TDZ will be painted with bars corresponding to their distances from the threshold, in 500-foot increments. After the aiming point markings located 1,000 feet from the threshold, two bars on each side of the centerline indicate the 1,500-foot and then the 2,000-foot point, while a single bar on each side of the centerline are located at 2,500 feet and 3,000 feet from the threshold.

To avoid confusion, a runway is always designed to have at least an 1,800-foot-long area in the center without any TDZ or aiming point markings. If the runway has TDZ markings on both ends (because each end is served by a precision approach), any markings that would extend to within 900 feet of the runway midpoint are left off. If only one runway end features TDZ markings, those markings will be omitted if they extend to within 1,800 feet of the aiming point marking of the opposite runway end. These facts, and some simple math, can help define a rule to assist pilots—especially jet pilots—with snap decisions if they find too much runway rushing under them.

Consider a runway with only one set of TDZ markings that is just short enough that the last TDZ marking (the one that would be at the 3,000-foot mark) needs to be left off. If we do some simplification (e.g. the start of the aiming point markings actually reside 1,020 feet from the threshold; and while TDZ markings are typically 75 feet long and aiming point markings 150 feet long, we’ll use 100 feet for both for simplicity), we see that to have the very end of the 3,000-foot marker just encroach into the 1,800-foot “no markings” zone, the runway would have to be 3,100 feet plus 1,800 feet plus 1,100 feet, or 6,000 feet long. Less than this and the TDZ effectively becomes 2,500 feet long with the last marker absent. For every 500 feet of runway less than 6,000 feet, another TDZ marker is removed. So, a just longer than 5,500-foot runway will only have markings to and including the 2,500-foot point, while a 5,000-foot (plus a smidge) runway will only have the 1,500 and 2,000-foot markers after the aiming point..

This helps with overrun prevention when we work the critical concept of landing safety margins into the equation. A common practice is to require that planned runway requirement not exceed 60 percent of the actual landing distance available. Using this safety factor on a runway just over our 6,000-foot threshold for truncating the TDZ marks, we’d need no more than 3,600 feet of total air and ground distance to stop the airplane—or 2,600 feet of ground roll, assuming the typical 1,000-foot air distance from the threshold to touchdown (also a simplification, but again functional for rule-of-thumb purposes).

Here’s where the math all comes together. If a pilot happens to float a bit and puts the wheels down at the end of the last TDZ mark (3,100 feet from the threshold), he or she still has 2,900 feet left to stop. That’s slightly more than the 2,600 feet calculated as necessary. This math follows as we look at shorter runways with fewer TDZ markings. For example, a slightly longer than 5,500-foot runway with a 2,500-foot painted TDZ gives you 2,900 feet of ground distance past the last TDZ marking (don’t forget the roughly 100 feet of length for the last mark), enough as no more than 2,300 feet (60 percent of 5,500 feet minus a 1,000-foot air distance) of ground roll could be required.

Putting all this math into simple terms—combining a 60-percent safety factor with an operating practice to put the wheels down in the TDZ marking area (or go around)—and the situation stacks in your favor. There are some significant caveats, of course. Many runways don’t have the full TDZ marking set—such as a runway with no approach or only a nonprecision approach, for example. Here a rule of thumb can help us. Treat the TDZ as the first 3,000 feet or half of the runway, whichever is less, and the same math will work out for you. Figuring out how to identify the 2,750-foot point of a 5,500-foot runway is certainly trickier than just ensuring the wheels are down before the end of the 2,500-foot marker, but the basic concept still works. Using runway remaining signs and/or preplanned reference points on the ground will help.

The other major caveat relates to the ability to actually stop the airplane once the wheels are down. If your approach left you floating past the aiming point, there is strong reason to suspect that you flew it too fast. Knowing you have more runway than the necessary stopping distance is only assured if touchdown occurs near the calculated VREF threshold speed. Forcing the airplane down in the TDZ at 40 knots over VREF has historically shown to be a setup for an overrun, so while saying that any landing past the TDZ should be avoided is a good policy—it’s not the same as saying any touchdown in the TDZ will end successfully.

This strategy can add another layer of protection against a runway overrun, but just like proper performance planning and stabilized approach discipline, it’s not a panacea.