What Actually Causes An Aircraft To Stall?

A photo showing the disrupted airflow over a wing at a high angle of attack.

When an airplane stalls, it stops flying properly and starts to descend because the wings are not producing enough lift to keep it airborne. This happens for a number of reasons, but a lot of it has to do with the way air flows over the wings, and it’s something that pilots need to be aware of in order to stay safe. Stalls usually don’t just happen out of the blue, since there are a lot of factors that result in a stall, including speed, wing design, and how the airplane enters the air.

Lift is what keeps a plane airborne, and it’s created by the wings when air flows over them. The curved top section and the flat bottom of the wing makes air travel faster over the top than the bottom. The faster the air, the less pressure, and it is the pressure difference between the top and the bottom of the wing that creates the lift. But this will only work if the air has a smooth course over the wing’s surface because if it isn’t smooth, lift will be reduced, and the plane will have much greater difficulty staying aloft.

One of the biggest causes of a stall is an airplane having too high of an angle of attack, which is the angle between the wing and the incoming air. When the pilot pitches the nose of the plane up, the angle of attack is larger because the wing is more pitched into the air, while when the angle is shallow, air sticks to the surface of the wing and flows smoothly, producing a lot of lift. When the angle is too steep, the air no longer adheres to the curvature of the wing, which causes the air to "separate" and become turbulent. This process of air separation and turbulence is called flow separation, and it decreases the pressure difference that produces lift, causing a stall. All wings have a limit (a critical angle of attack) at which this is true, and when broken, the plane will stall.

Speed enters into this too because it affects the angle of attack. At high speeds, an airplane will need a smaller angle of attack in order to generate enough lift. Air flows quicker over the wings, generating very powerful pressure differences, even if the nose is only slightly up. But as the airplane slows down, the pilot has to increase the angle of attack so that the plane’s wings produce the same amount of lift. Less speed means that there is less air moving over the wings, so the wing must meet the air at a sharper angle to compensate for that. If the speed gets too low, the angle of attack may reach that point of criticality, where the air breaks off and a stall starts. This is precisely why takeoff and landing stalls happen more commonly, since that is when planes are flying the slowest.

The wing design, which determines the angle of attack at which stalls happen, also changes where and when stalls take place. A long, thin wing, like on a glider, can handle high angles and can sustain flight at those angles for a significantly longer period of time than a short, thick wing. Some aircraft have devices such as flaps or slats that modify the wing's shape during flight by extending or retracting. Slats extend the front edge of the wing and project out from it, while flaps project from the bottom of the rear end, but both of them allow the wing to generate more lift at low speeds or higher angles of attack. When those systems fail or are used ineffectively, the plane stalls more easily. Deformation of the wing, such as through denting or ice buildup, also disrupts airflow and decreases the critical angle, resulting in a higher rate of stalls.

Air density, which is determined by things like weather, temperature, and altitude, is also something to consider when dealing with stalls. Thin air, like at high altitudes or during hot weather, has fewer molecules available for the wing to push against, resulting in less lift being created. To account for that, the plane has to have greater speed or greater angle of attack. If the plane is already slow and the pilot pulls up too hard in thin air, the wing will reach its critical angle earlier than it would near the ground or in cold weather, where the air is denser. Another cause for a stall is the manner in which the plane is being turned, or banked. When a plane banks one direction to turn, the wings are no longer producing lift straight up (some of it is produced sideways). A tight turn needs more banking, and if the plane is already at a slow speed, that extra tilt will push the wings to their limits, causing a stall.

Actions by pilots connect all of these causes together, since they're the ones, for the most part, who control speed, angle of attack, and controls. If they pull too firmly on the yoke at low speeds, they will add to the angle of attack and catch a stall very quickly. Stalls happen when pilots make mistakes or don’t pay attention to how the plane is doing, especially during situations like bad weather or having to land on a short runway.

To wrap up, an airplane stalls because the wings stop generating enough lift, which all comes down to airflow being disrupted. The angle of attack getting too great is the biggest trigger, but speed, wing design, air density, turns, and decisions made by pilots all play their part. Usually, aircraft do not stall spontaneously; rather, it happens in a series of steps, and understanding those steps makes flying so much more safer.