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Mythconceptions

May 1, 2012

 

Rod Machado Everyone believes in a myth at one time or another. This is especially true for those convinced that the world will expire on December 21, 2012, because the Mayan calendar abruptly ends on that date. Isn't it possible, however, that the massive, five-by-seven-foot stone calendar simply fell on the guy doing the carving before he could finish the job? That was the original meaning of getting stoned.

Pilots occasionally believe in myths, too. One such mythtake revealed itself at a local FAA safety seminar when an attendee mentioned that he never extends flaps in a turn for fear of stalling. When I asked how this might happen, he simply sat there dumbstruck, never having thought the idea through. I playfully reminded him that he needed to respond before December 21 of this year, while we were still around to hear the answer.

No, extending flaps while turning from base to final approach won’t cause you to stall—unless you let it cause you to stall. Flaps aren’t the culprit here. It’s your reaction (or lack of it) to flap extension while turning that causes you to stall, which is often accompanied by a personal introduction to Ah Uuc Ticab, the Mayan god of the Earth.

Extending flaps allows you to maintain a particular angle of attack at a lower nose-down attitude. Flap extension also creates drag, requiring a lower nose-down attitude to sustain the target airspeed during a descent. While it’s easy to see how much the nose should be lowered in wings-level flight (lower the nose by eight degrees, for instance), it’s much more difficult to determine this value in a descending turn, especially as flaps are deployed.

During a descending turn to final approach, changing your pitch attitude moves the airplane’s nose along some diagonal to the horizon. The angle of the diagonal depends on the degree of bank you’re using at the time. The net result of any pitch change is that the nose moves a little in the horizontal direction and a little in the vertical direction. In a 45-degree bank, the nose moves horizontally as much as it does vertically. If you could sustain a 90-degree bank (request a landing clearance from Ah Uuc Ticab Tower, first), any pitch change would result in only horizontal movement of the nose—there would be no vertical movement whatsoever.

The takeaway point here is that changing your pitch attitude in a descending turn doesn’t provide the same vertical displacement of the nose that you experience in wings-level flight. This is where applying flaps in a turn from base to final approach could bite you in the empennage.

Let’s say that you’re in a descending turn from base to final approach in an airplane whose flaps generate a nose-up pitching moment when deployed (partially or fully). As you extend the flaps in the turn, the airplane’s nose pitches up along a diagonal, from your toes to your nose (as seen from the cockpit). You compensate for this motion by applying forward elevator pressure to sustain the desired nose-down attitude to maintain your approach speed.

The problem is that flap deployment in a turn results in both a horizontal and vertical component of nose movement, instead of pure vertical motion of the nose as experienced in wings-level flight. Therefore, you don’t see the nose pitch up vertically toward the horizon by the same amount it does when applying flaps in wings-level flight. Without the obvious vertical pitch movement toward the horizon, some pilots simply fail to apply sufficient forward elevator pressure to sustain their approach airspeed. Ultimately, this can lead to airspeed decay and eventually a stall.

There is an additional factor that increases an unwary pilot’s potential for stalling as flaps are applied in a turn. The increase in elevator back-pressure necessary to sustain your attitude in a descending turn at steeper banks often masks changes in elevator pressure resulting from flap application. In many airplanes, applying flaps moves the center of lift forward and increases the downwash on the tail. The result is often a change in elevator control forces that helps remind you to adjust your attitude and maintain your approach airspeed. The aft elevator pressure that you’re already applying in the descending turn, however, might mask this flap-induced change in elevator force. The net result is that you could miss a valuable clue (a change in elevator pressure) that reminds or prods you to maintain the correct approach speed.

Ultimately, it’s a myth to believe that applying flaps in a turn results in a stall. On the other hand, it’s absolutely true that a pilot who doesn’t understand how flap application in a turn affects an airplane is more likely to stall. So, during a descending turn, glance out your left window and compare the wing chord to ground movement for an approximate assessment of your angle of attack. And be especially cognizant of your attitude in a descending turn, making a special effort to adjust it immediately to maintain the right approach airspeed. That’s how to fly stall-free when applying flaps in a descending turn.

It’s also how to keep your lifespan calendar from getting rocked and ending abruptly.

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