License to Learn: Slow town

“After aligning the airplane with the runway centerline, the final flap setting should be completed; in the absence of the manufacturer’s recommended airspeed, a speed equal to 1.3 V_S0 should be used.”

That’s the FAA’s recommendation from its Airplane Flying Handbook, and it’s a reasonable one. Have you ever wondered why we use 30 percent above stall speed as a reference in these instances? It’s important to know the reason—especially if your high-speed landings require every last grain of salt at Bonneville to come to a complete stop.

Generally speaking, most pilots cross the runway threshold at speeds faster than 30 percent above stall speed (1.3 V_S). Why? For them, it feels safer, especially when there’s ample salt ahead of the airplane. The problem is that some runways are so short that their numbers appear to overlap. Even runways of reasonable length might not be so reasonable at the increasing true airspeeds (thus, groundspeeds) associated with high-density-altitude airports.

Runway length, however, is not the reason for crossing the runway threshold at or below 1.3 V_S. Higher “over the threshold” approach speeds increase the probability that you’ll float, balloon, porpoise, or drift while attempting to land. Crossing the threshold at or below 1.3 V_S means that the typical general aviation airplane experiences increasing drag, not decreasing drag, during the roundout and landing flare.

Let’s examine why this is, using the flaps-up approach model for simplicity. With flaps up, the airplane’s best L/D speed (best glide speed) is found at the bottom of its total drag curve. As you’ll recall from ground school, the total drag curve is the combination of a decreasing parasite drag curve (as airspeed decreases) and an increasing induced drag curve (as airspeed continues to decrease). In a generic Cessna 172, the best glide speed is approximately 66 knots calibrated airspeed (KCAS). The flaps-up stall speed at maximum weight is 49 KCAS. Approaching at 1.3 V_S with flaps up results in an airspeed of 64 KCAS. This puts you near the bottom of the total drag curve. Big surprise? Not really.

At 64 KCAS, as you increase the angle of attack during the roundout for landing, your wing throws its lift rearward, and induced drag increases (yes, ground effect reduces induced drag a bit, but induced drag still increases overall). You decelerate as a result. Your chance of floating during the roundout diminishes because of the relatively quick decrease in your airspeed.

Imagine what happens if you cross the threshold at 80 KCAS (16 knots faster than best glide speed). During the roundout, the parasite drag decreases faster than induced drag increases as you decelerate from 80 to 66 KCAS. This means that you initially don’t slow down as quickly as you’d like, which ultimately increases your chance of floating during landing, and running off the end of the runway. This is one instance where having insufficient salt actually increases your blood pressure. While this example reflects a flaps-up condition, the same principle (albeit with different airspeeds) also applies to landing with full flaps.

Of course, floating isn’t conducive to landing in the shortest distance possible. That’s why more than a few airplane manufacturers actually recommend flying at a little less than 30 percent above stall speed during short-field landings.

Your typical Cessna 172 pilot’s operating handbook recommends short-field landings be flown at approximately 1.3 V_S0, while the typical Cessna 150 uses 1.22 V_S0. The 1979 Piper Turbo Arrow III has short-field landings performed at 1.23 V_S0, which is nearly the same recommendation as for the Mooney M20J. However, the 1985 Cessna 210R POH has short-field landings performed at 1.18 V_S0 (center of gravity at midrange). All of the short-field landing speeds mentioned here place your airplane below its best glide speed.

To be worth your salt as a pilot, it’s important to understand how drag affects you during the roundout and flare. While you’ll typically fly an extended final approach at higher speeds, it’s best to cross the threshold (perhaps at 50 feet or less) in a stabilized condition, no more than 30 percent above the airplane’s present stall speed. Induced drag now becomes your own airspeed “arresting device” during the roundout and flare. Running off the runway becomes less likely as a result. You’re also less likely to meet the airport police—otherwise known as the “real” arresting device.

Rod Machado is a CFII and the owner of a Cessna 150, based in Southern California.

Web: www.rodmachado.com