We're looking at the completed Key Facts for the stall chapter — Page 206, Lesson 251

We're looking at the completed Key Facts for the stall chapter. This is the summary of everything you need to know cold, so let's go through it line by line. First, the fundamental definition. A stall involves loss of height and loss of control. That's the consequence. And the cause is airflow separation. So when the airflow detaches from the wing, you get a stall. That separation happens when either the boundary layer has insufficient kinetic energy, or the adverse pressure gradient becomes too great. Let me unpack that. The boundary layer is the thin layer of air right next to the wing surface. It needs kinetic energy—that is, speed—to keep flowing along the surface. The adverse pressure gradient is the pressure rising along the chord, which acts like a hill the airflow has to climb. If the air is too slow, or the hill is too steep, the flow can't make it and it separates. Now, that adverse pressure gradient increases with angle of attack. So as you raise the nose, the pressure hill gets steeper, and eventually the flow separates. The angle of attack at which this happens has two alternative names: the stall angle, and the critical angle of attack. And the coefficient of lift at that point is called CLMAX—the maximum lift coefficient. That's the peak of the lift curve. Here's a key point: a stall can occur at any airspeed or flight attitude. It's not about speed, it's about angle of attack. A typical stalling angle is approximately 16 degrees. That's your reference number. Now recovery. To recover from a stall, the angle of attack must be decreased. That's the primary action. And maximum power is applied during stall recovery to minimize height loss. So you're reducing angle of attack and adding power simultaneously. Then we have the wing-drop handling, and this is a contrast you must remember. On small aircraft, the rudder should be used to prevent wing drop at the stall. On swept wing aircraft, the ailerons should be used to prevent wing drop at the stall. That's a direct contrast—rudder for small, ailerons for swept. After you've recovered, you recover the height lost during the stall with moderate back pressure on the elevator control. Not aggressive—moderate. Now, the indications. The first indications of a stall may be unresponsive flight controls, a stall warning device, or aerodynamic buffet. Buffet is the vibration you feel. And at speeds close to the stall, ailerons must be used with caution to lift a dropping wing. That's because the ailerons can be ineffective or even adverse near the stall. Then we have the acceptable indications of a stall, which are three specific things. One: a nose-down pitch that cannot be readily arrested. Two: severe buffeting. Three: pitch control reaching the aft stop and no further increase in pitch attitude occurs. So those are the three acceptable, unmistakable signs. Finally, the reference stall speed, VSR. That's a calibrated airspeed—CAS—defined by the aircraft manufacturer. And there are two constraints on it. First, VSR may not be less than a 1g stall speed. Second, when a device that abruptly pushes the nose down at a selected angle of attack is installed—that's a stick pusher—VSR may not be less than 2 knots or 2 percent, whichever is greater, above the speed at which the device operates. So the reference stall speed has to be above the stick-pusher activation speed by at least 2 knots or 2 percent, whichever is bigger. That's the complete set of key facts for the stall. Every one of those statements is examinable, so make sure each one is firmly in your memory.