What is actually happening in a stall

Why a wing stops lifting when tilted too far, and why it happens all at once.

A stall happens when the airflow over the top of a wing can no longer follow the surface all the way back to the trailing edge, so instead of thinning out gradually the flow tears away from the wing in one go, and the wing loses most of its lift within a fraction of a second.

The physics of airflow separation

As a wing tilts to a steeper angle to generate more lift at a given speed, the air travelling over its top surface has to turn through a sharper corner just behind the leading edge and then keep following the surface as it curves away underneath it, into a region where the pressure is rising rather than falling. Air flowing into rising pressure is fighting against its own momentum, since the pressure ahead is pushing back on it, the same way a ball rolling up a slope fights gravity, and the air nearest the wing's surface, already slowed by contact with the surface, runs out of momentum first. Past a certain angle, that slowed air near the surface stops moving forward altogether and starts moving backward, and the main flow above it can no longer stay attached to the wing. It lifts away from the surface and tumbles into a churning wake instead of following the curve. This is what engineers mean by boundary layer separation, driven by what is called an adverse pressure gradient, and once it happens, the low pressure that the attached flow was creating over the top of the wing collapses, and with it most of the wing's lift.

Where you have already seen this

When water or tea is poured from a jug too slowly, it does something odd: instead of leaving the spout in a clean arc, it clings to the underside of the spout and dribbles down the outside of the jug. The liquid is following the curved surface for the same reason the air over a wing does, because the surface is bending the flow and the flow has enough momentum to bend with it, right up until it does not. Poured fast enough, the stream breaks cleanly away from the spout instead of clinging to it. A wing at a shallow angle behaves like the fast pour, flow breaking away cleanly at the trailing edge where it is meant to. A wing pushed to a steep angle behaves like the slow pour clinging to the spout, except that the point where the flow gives up and peels away keeps creeping forward as the angle increases, further and further from the trailing edge, until it lets go near the front of the wing and takes almost all of the lift with it.

The one number worth remembering

For most ordinary wing shapes, the angle at which this separation happens, called the critical angle, sits at around fifteen degrees of tilt relative to the oncoming air, and that figure barely changes with how fast the aircraft happens to be flying. This is the detail that catches people out: a stall is not something that happens because a wing is flying too slowly, it happens because a wing is tilted too steeply, and slow flight only matters because a slower wing has to tilt to a steeper angle to make the same amount of lift. An aircraft can stall at high speed just as easily as at low speed if it is pulled to a steep enough angle quickly enough, which is why pilots are trained to watch their angle rather than trusting a speed reading alone.

What this changes in practice

Because separation spreads outward from the point where it starts rather than appearing evenly across the whole wing, a stall rarely switches lift off like a light going out everywhere at once. It usually begins at one section of the wing, often near the root or the tip depending on the wing's shape, and spreads from there, which is why aircraft can be designed so that the wing root stalls before the tip. Keeping lift and aileron control alive at the tips for longer after the root has already stalled gives the pilot something to fly with even as the aircraft as a whole is losing lift, rather than losing roll control and lift at exactly the same moment.

What this does not explain

The critical angle and the sudden loss of lift explain why a stall feels abrupt rather than gradual, but they do not by themselves explain everything that follows, since a wide, churning wake of separated, turbulent air still trails behind a stalled wing and can still buffet the tail surfaces. That buffet, often felt through the airframe as a shudder just before the wing gives up completely, comes from turbulent air already torn loose from the wing striking the tail surfaces downstream, which is a separate effect from the loss of lift itself even though the two often arrive together.