Pilot's tip of the week

Avoiding “Flap Sink”


Subscriber question:

"I just earned my private pilot certificate last month and my mom volunteered to be my first passenger. But when I retracted the flaps at 500 feet AGL in my DA40, the sinking feeling made her nervous. Should I work on my technique or encourage her to get used to it?" — Beck C.


“As an FAA examiner, I know exactly what you’re describing as I regularly experience the same sensation on practical exams. The good news is that we can smooth out your technique, so those accelerations won’t happen. But first, let’s explore why you’re accelerating downward in the first place.

In most of our flying, say at cruise or constant velocity climb or descent, you don’t feel pulled in any direction because all the forces are in balance. That feeling of being in free fall happens when something causes an imbalance. On any given day, the lift generated by your aircraft is proportional to the square of the velocity times the absolute angle of attack.

I know that unpacking math formulas can be painful—but trust me, this one is worth the effort. By retracting your flaps, the angle of attack decreases, so that sudden loss of lift makes you accelerate toward the earth. The key here is to increase the velocity at the same time to compensate for that smaller angle of attack.

You may wonder how this is gonna help since by pushing forward on the stick to increase airspeed, you’re also lowering the angle of attack even more. Yeah, that’s true. But let’s take another look at that formula above. The velocity is squared in that expression, so that means it has a larger impact on lift than angle of attack does.

So how can we apply this information to retracting flaps? Start by getting some practice at altitude alone or with an instructor. Set up your takeoff flap setting and climb speed and then try retracting your flaps. To maintain constant lift, push the stick forward slightly before and during the flap retraction.

You probably won’t get it right the first time or two, but don’t worry. You’ll soon get the timing of the forward stick pressure just right. And that sinking feeling—for you and your mom—will be a thing of the past.”

What's your cue to retract flaps on takeoff (presuming obstacle clearance is no longer a concern)?

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