Stalling — Page 166, Lesson 195

Right, let's get into the stalling chapter. This is a revision aid, so we're going to work through the key facts and terminology together. I want you to think of this as us filling in the blanks to build a complete picture of what a stall is, how we recognise it, and how we recover from it. Let's start with the very definition. Stalling involves a loss of lift and a loss of control. That's the fundamental consequence. And because of that, a pilot must be able to clearly and unmistakably recognise a stall. You need to know what it feels and looks like before it becomes dangerous. Now, what actually causes it? A stall is caused by airflow separation. The smooth flow over the wing breaks away from the surface. This separation can occur 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's the energy of motion—to keep flowing along the surface against the rising pressure. The adverse pressure gradient is that rising pressure towards the rear of the wing. If the boundary layer runs out of energy, or the pressure rise is too steep, the flow can't stick to the surface anymore and it separates. And here's the key relationship: the adverse pressure gradient increases with an increase in angle of attack. So as you raise the nose and increase the angle of attack, the pressure gradient gets worse, and eventually you reach the point of separation. Now, the angle of attack at which the stall occurs has a couple of alternative names. It's called the critical angle and the stalling angle of attack. And the coefficient of lift at which a stall occurs is maximum. That's the peak of the lift curve—beyond that point, lift decreases. A very important point for you as a pilot: a stall can occur at any attitude or flight speed. It's not just about slow flight. You can stall at high speed if you pull hard enough. A typical stalling angle is approximately 15°. That's a good rule of thumb for a conventional aerofoil. To recover from a stall, the angle of attack must be decreased. That's the primary recovery action—lower the nose. And during that recovery, maximum power is applied to minimize height loss. You want to get out of the stall and regain airspeed as efficiently as possible. Now, there's a distinction between aircraft types when it comes to preventing a wing drop at the stall. On small aircraft, the aileron should be used to prevent wing drop at the stall. But on swept wing aircraft, the rudder should be used to prevent wing drop at the stall. That's a critical difference—ailerons on a swept wing can be ineffective or even aggravate the situation at the stall, so you use the rudder instead. And once you've recovered the stall, you recover any height lost during the recovery with moderate back pressure on the pitch control. So you've lowered the nose, applied power, and then you gently pull back to regain your lost altitude. Now, let's talk about the indications. The first indications of a stall may be sluggish flight controls, a stall warning device, or aerodynamic buffet. That buffet is the shaking you feel as the airflow starts to break down. At speeds close to the stall, aileron must be used with caution to raise a dropping wing. Again, that caution about aileron use near the stall. There are also acceptable indications of a stall that you need to know. These are the definitive signs: (1) a nose down pitch that cannot be readily arrested. (2) severe buffeting. (3) pitch control reaching aft stop and no further increase in nose-up attitude occurs. So if the nose pitches down and you can't stop it, or you get severe buffeting, or you've pulled the stick all the way back and the nose won't come up any further—those are the accepted signs of a stall. Now, let's move to the regulatory side. The reference stall speed, which is abbreviated VSR, is a CAS defined by the flight manual. CAS is calibrated airspeed. So VSR is a specific speed defined in your aircraft's documentation. There's a rule about VSR: it may not be less than a 1g stall speed. That's the minimum. And here's a more complex one: 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 1 knot or 5 %, whichever is greater, above the speed at which the device operates. So the reference stall speed has to be at least that margin above the stick pusher activation speed. Stall warning is a critical safety feature. It must provide sufficient margin to prevent inadvertent stalling, and it must be clear and unmistakable to the pilot in straight and turning flight. So you have to be able to recognise it in all phases of flight. The acceptable stall warning may consist of the inherent aerodynamic qualities of the aeroplane—that's the natural buffet—or by a device that will give clearly distinguishable indications under expected conditions of flight. And there's a timing rule: stall warning must begin at a speed exceeding the stall speed by not less than 5 knots or 5 % CAS, whichever is the greater. So you get at least that much warning before the actual stall. Now, the artificial stall warning systems. On a small aircraft, it's usually given by a horn or light. On a large aircraft, it's usually given by a stick shaker, in conjunction with horn and a noisemaker. So the stick shaker physically shakes the control column, and you get an audible warning too. An artificial stall warning device can be activated by a flap switch, an angle of attack vane, or an angle of attack probe. These are the different sensors that can trigger the warning. And finally, most angle of attack sensors compute the rate of change of angle of attack to give early warning in the case of accelerated rates of stall approach. So if you're pulling into a stall quickly, the system can warn you sooner by looking at how fast the angle of attack is increasing. That's the full picture of the stalling key facts. We've covered the definition, the cause, the recognition, the recovery, and the regulatory requirements for stall warning. Take a moment to let that sink in, and we can go over any part of it again if you need.