Aerodynamic Warnings — Page 443, Lesson 548

I want to walk you through aerodynamic warnings, and we're starting with stall protection. This is a big one, because it's the system that stops the aircraft from ever getting into a stall in the first place. Let me set the scene. A stall protection system may be fitted to large commercial aircraft, and its whole job is to prevent them from entering the stall. Now, the way it does that depends on the aircraft type. In fly-by-wire systems, the flight computer simply will not allow the aircraft to approach the stall — it puts a hard limit on the pilot's inputs. But on other aircraft, 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. So the moment the aircraft slows down past 1.2 times the stall speed, the autopilot system pushes the power levers forward automatically. Now, there's a special case for T-tail aircraft. On aircraft which have a T-tail, a stick-pusher may be fitted. The stick-pusher is a device that physically pushes the control column forward in the event that the aircraft slows to 2 knots above the stall speed. So it acts before you even reach the stall — 2 knots above it — and it forces the nose down. Why does a T-tail need this? Because pushing the nose down prevents the aircraft entering a deep stall, and a deep stall is the dangerous one, because from a deep stall there is little or no chance of recovery. So the stick-pusher is a last-resort mechanical intervention to keep you out of that unrecoverable condition. Now let's look at the components. There's a block diagram of the component parts of a stall warning system and an angle of attack sensor shown at Figure 33.5. I want you to keep that figure in mind as we go through the sensing side. The heart of it is angle of attack sensing. The angle of attack — abbreviated AoA — is also called the alpha angle, written with the Greek letter α, and it's also known as the aerodynamic incidence. Here's the precise definition: it is the angle between the chord line of the wing of an aircraft and the direction of the relative airflow. The chord line is the straight line from the leading edge to the trailing edge of the wing, and the relative airflow is the air coming at the wing as the aircraft moves through it. So the AoA is the angle between that chord line and that airflow direction. Why does this matter? Because the angle of attack is a major factor in determining the magnitude of lift generated by a wing. Lift increases as the AoA increases — up to some critical value. But past that critical value, lift begins to decrease. And here's the mechanism: the decrease happens due to separation of the slow-moving air — that's the boundary layer — from the upper surface of the wing. That separation, in turn, results in separation and turbulence of the main airflow. And when that happens, the wing assumes a stalled condition. So let me tie that together. The boundary layer is the thin layer of slow-moving air right next to the wing surface. At low angles of attack it stays attached, and the wing produces lift cleanly. As you increase the angle of attack, the lift keeps growing — until you hit that critical angle. Beyond it, the boundary layer separates from the upper surface, the main airflow becomes turbulent and separated, and lift collapses. That's the stall. And the whole point of the stall protection system — whether it's the fly-by-wire computer refusing to let you get there, the AFCS slamming the throttles to full power below 1.2 VS, or the stick-pusher shoving the column forward 2 knots above stall on a T-tail — is to keep the wing from ever reaching that separated, stalled condition. That's the core of stall protection and angle of attack sensing.