ATPL · Technical · Principles Of Flight Atpl Ground Training… lesson — Page 145, Lesson 168

We’ve just finished the question bank for Chapter 6, so now we’re moving into a brand-new chapter: Chapter 7, Stalling. This is one of the most important chapters in your entire Principles of Flight syllabus, because a stall is something you will actively manage every single time you fly. Let me give you the roadmap of what this chapter covers, because the structure itself tells you what a professional pilot needs to know. First, we start with the Introduction and then go straight into the Cause of the Stall — that’s the fundamental physics of why a wing stops producing lift. From there we look at The Lift Curve, which is the graphical relationship between angle of attack and lift coefficient. That curve is the backbone of understanding stalls. Then we move into the practical side: Stall Recovery — the exact sequence of actions to get out of a stall. After that, Aircraft Behaviour Close to the Stall and Use of Flight Controls Close to the Stall — because how the aeroplane feels and how you handle the controls in that regime is critical. Next, Stall Recognition — the cues, both visual and physical, that tell you a stall is coming. Then Stall Speed — what determines it and how it changes. Then Stall Warning and Artificial Stall Warning Devices — the systems that alert you, like the stick shaker. We then get into certification: Basic Stall Requirements (EASA and FAR) — the regulatory standards the aeroplane must meet. Then Wing Design Characteristics, split into The Effect of Aerofoil Section and The Effect of Wing Planform — how the shape of the wing influences stall behaviour. There’s a Key Facts summary section, and then we finish with the Super Stall (Deep Stall) and Super Stall Prevention – Stick Pusher — that’s the dangerous condition where the aeroplane can get stuck in a stall, and the device that prevents it. So that’s the full map. Let’s start at the very beginning with the Cause of the Stall. I want you to think about what a stall actually is. It’s not about the engine stopping — that’s a different thing entirely. A stall is an aerodynamic condition where the wing exceeds its critical angle of attack, and the airflow over the wing can no longer remain attached. The lift drops dramatically, and you lose control of the aeroplane. Let me walk you through the physics. As you increase the angle of attack — that’s the angle between the wing chord line and the relative airflow — the lift increases, up to a point. But beyond a certain angle, called the critical angle of attack, the airflow separates from the upper surface of the wing. The smooth flow breaks down into turbulent, separated flow, and the lift coefficient falls off sharply. That’s the stall. Now, here’s the key professional point: the stall is determined by angle of attack, not by airspeed. You can stall at any speed, at any attitude, at any power setting. The airspeed at which the stall occurs is just a symptom of the angle of attack reaching that critical value. That’s why we talk about the stall in terms of angle of attack first and foremost. Let’s look at the Lift Curve — this is the graph that shows lift coefficient on the vertical axis against angle of attack on the horizontal axis. As angle of attack increases from zero, the lift coefficient rises in a straight line — that’s the linear portion of the curve. But then it starts to curve over, reaches a peak, and that peak is the maximum lift coefficient, which we call C_L max. The angle of attack at that peak is the critical angle of attack. Beyond that point, the curve drops — that’s the stalled region. That figure shows you exactly this — at low angles of attack, the lift increases linearly, and then it peaks and falls off. That peak is where the stall begins. Now, Stall Recovery. The fundamental rule is: to recover from a stall, you must reduce the angle of attack below the critical value. That means moving the control column forward — lowering the nose. You also apply maximum power to increase airspeed and reduce the angle of attack further. The sequence is: reduce angle of attack, apply full power, level the wings, and then gently pull out of the resulting dive once the wing is flying again. The key is that you must not pull back on the control column until the stall is broken, because pulling back just increases the angle of attack and keeps you stalled. Let me also introduce the concept of Aircraft Behaviour Close to the Stall. As you approach the stall, you’ll feel aerodynamic buffet — that’s a vibration through the airframe caused by the turbulent airflow separating from the wing. The controls may feel sloppy or less effective, and the aeroplane may start to roll or yaw unexpectedly. This is your warning that you’re getting close. Use of Flight Controls Close to the Stall is critical. You must use smooth, coordinated control inputs. Abrupt or excessive control movements can precipitate the stall or cause a spin. The ailerons, in particular, become less effective and can induce adverse yaw close to the stall, so you use rudder to coordinate. Stall Recognition is about knowing the cues: the buffet, the nose dropping, the loss of control effectiveness, and the stall warning devices. You need to recognise these immediately and instinctively. Stall Speed — this is the speed at which the stall occurs for a given configuration. It’s not a fixed number; it changes with weight, load factor, flap setting, and power. The stall speed increases with weight and with load factor — for example, in a turn, the stall speed increases because the wing has to produce more lift. Flaps reduce the stall speed by increasing the wing’s camber and maximum lift coefficient. Stall Warning — this is the natural warning, like the buffet, and also the artificial devices. Artificial Stall Warning Devices include things like the stall warning horn or light, which activate at a predetermined angle of attack or airspeed before the stall. The most sophisticated is the stick shaker, which vibrates the control column to simulate the buffet. Then we have Basic Stall Requirements (EASA and FAR) — these are the certification rules that specify how the aeroplane must behave in a stall. The aeroplane must give adequate warning, must be controllable, and must recover with normal use of the controls. Wing Design Characteristics — this is about how the wing is shaped to influence stall behaviour. The Effect of Aerofoil Section — different aerofoil shapes have different stall characteristics. Some stall gently, with a gradual loss of lift, while others stall abruptly. The Effect of Wing Planform — the shape of the wing in plan view, like taper and sweep, affects where the stall starts and how it progresses across the wing. Finally, the Super Stall (Deep Stall) — this is a dangerous condition, particularly on T-tail aeroplanes, where the stalled wing and the tail are in a position that prevents the nose from dropping, so the aeroplane remains stalled and cannot recover by normal means. Super Stall Prevention – Stick Pusher is a device that physically pushes the control column forward to force the nose down and break the stall. That’s the whole chapter in a nutshell. Now let’s go back to the beginning and work through it properly, starting with the Cause of the Stall in detail.