Aerodynamic Warnings — Page 443, Lesson 548

Let’s start with the big picture. We’re looking at aerodynamic warnings, and the first thing I want you to understand is the difference between a warning and a protection. A warning tells you something is about to happen. A protection system actually does something about it, automatically, without waiting for you. So, stall protection. A stall protection system may be fitted to large commercial aircraft, and its whole job is to prevent the aircraft from entering the stall in the first place. Now, how it does that depends on the type of aircraft. In a fly-by-wire system, the flight computer simply will not allow the aircraft to approach the stall. The computer is the gatekeeper — it won’t let the aircraft get there. In other aircraft, ones that aren’t fly-by-wire, the system works differently. There, an output from the AFCS — that’s the Automatic Flight Control System — advances the throttles to full power if there is a deceleration to below 1.2 VS. Let me unpack that. VS is the stall speed. So if the aircraft slows down to below 1.2 times the stall speed, the AFCS automatically pushes the throttles forward to full power. It’s using thrust to arrest the deceleration and keep you away from the stall. Now, there’s a third scenario, and this one is about a specific type of aircraft. On aircraft which have a T-tail, a stick-pusher may be fitted. A T-tail, as the name suggests, has the horizontal stabiliser mounted on top of the fin, forming a T shape. The stick-pusher is a device that physically pushes the control column forward. When does it act? In the event that the aircraft slows to 2 knots above the stall speed. So you’re creeping down toward the stall, and at 2 knots above it, the stick-pusher shoves the control column forward. Why? Because pushing the column forward lowers the nose, which reduces the angle of attack, which moves you away from the stall. And the reason this matters so much on a T-tail aircraft is that it prevents the aircraft entering a deep stall. A deep stall is a condition from which there is little or no chance of recovery. On a T-tail, the stalled wing can blank out the tailplane, so you lose pitch control entirely — that’s the deep stall. The stick-pusher is there to stop you ever getting into that situation. Now let’s move to the components. I want you to picture a block diagram of the component parts of a stall warning system, along with an angle of attack sensor. That’s shown at Figure 33.5. The heart of the whole system is angle of attack sensing. Let me give you the exact definition, because this is fundamental. The angle of attack, abbreviated AoA, or alpha, which is the Greek letter α, is also known as the aerodynamic incidence. It is the angle between the chord line of the wing of an aircraft and the direction of the relative airflow. So, the chord line is an imaginary straight line from the leading edge of the wing to the trailing edge. The relative airflow is the direction the air is moving relative to the wing — effectively the opposite of the aircraft’s flight path. The angle between those two is the angle of attack. And why does this matter so much? Because it is a major factor in determining the magnitude of lift generated by a wing. Here’s the relationship: lift increases as the AoA increases, up to some critical value. At that critical value, lift is at its maximum. But if you keep increasing the AoA beyond that critical value, lift begins to decrease. Why? Because of separation of the slow-moving air — and that slow-moving air is called the boundary layer — from the upper surface of the wing. That separation, in turn, results in separation and turbulence of the main airflow. So the smooth flow over the top of the wing breaks down, and the wing assumes a stalled condition. That’s the stall — not a loss of engine power, but an aerodynamic breakdown of the lift-producing flow over the wing. So the angle of attack sensor is what measures this critical angle, and the stall warning system uses that information to warn you — or, as we saw, to actively protect you — before the wing reaches that stalled state. That’s the core of aerodynamic warnings: sensing the angle of attack, knowing where the critical value is, and acting before you get there.