Every flight manual prints a stall speed, and most pilots read it as a property of the aeroplane: below this number, in trouble. That's the wrong mental model, and it's the one that kills people in a steep turn at 500 ft with the speed indicator looking perfectly fine.
What actually causes a stall
A wing generates lift by deflecting the airflow over the aerofoil. As long as that flow stays attached to the upper surface, lift increases with angle of attack (AoA) — the angle between the wing's chord line and the relative airflow.
Beyond a certain AoA, the flow separates from the surface: lift collapses and drag rises sharply. That's the stall.
This critical angle of attack, typically around 15 to 16° on a conventional aerofoil, is a property of the wing's shape. It doesn't depend on weight, speed, bank angle, or altitude.
Remember: a wing always stalls at the same angle of attack, at widely different speeds depending on the situation. The stall speed printed in the manual is only one particular case — straight-and-level flight, at a given weight.
Why we still talk about a speed at all
In stabilised level flight, lift equals weight. There's then a direct relationship between AoA and airspeed: the slower you fly, the more back pressure you need to keep lift equal to weight. That gives you a minimum speed at which you reach the critical angle of attack — that's VS1 (or VS0 in the landing configuration).
That relationship only holds in this one specific case. As soon as the flight path is no longer level and stabilised, the stall speed shifts.
Load factor changes everything
In a turn, the wing has to generate more lift than the aircraft's weight: the vertical component of lift still has to balance weight, while a horizontal component turns the aircraft. The ratio between lift produced and weight is the load factor, written n.
Stall speed then follows a simple relationship:
Stall speed (in the turn) = VS × √n
| Bank angle | Load factor | Stall speed increase |
|---|---|---|
| 30° | 1.15 | +7% |
| 45° | 1.41 | +19% |
| 60° | 2.00 | +41% |
An aeroplane that stalls at 50 kt wings-level stalls at 71 kt in a 60° banked turn. That is the exact mechanism behind an accelerated stall in the base-to-final turn, ground close below and attention fixed on the runway.
The same logic applies to a hard pull-up: pulling back increases load factor, which increases AoA, which moves you closer to the stall — regardless of indicated airspeed.
What makes stall speed move
- Weight: heavier means more lift required, so a higher stall speed.
- Load factor: turns, pull-ups, turbulence.
- Configuration: flaps increase maximum lift coefficient and lower the stall speed. Retracting flap on a slow final immediately raises it again.
- Centre of gravity: a forward CG raises stall speed, an aft CG lowers it (see mass and balance).
- Wing contamination: ice, frost, insects. A contaminated wing stalls at a lower angle of attack, often with no warning.
Altitude, on the other hand, does not change indicated stall speed: the airspeed indicator measures dynamic pressure, so the stall happens at the same indication regardless of height. True airspeed at the stall, however, is considerably higher at altitude.
Asymmetric stalls and the spin
If one wing stalls before the other — ball off-centre, sideslip, aileron deflected at low speed — the aircraft rolls towards the stalled wing. If the stall is held, the rotation becomes self-sustaining: that's a spin.
Two rules that are non-negotiable at low speed:
- Keep the ball centred. A symmetric stall is benign; a stall with sideslip is not.
- No aileron input near the stall. A lowered aileron increases the angle of attack on the wing that's already closer to stalling, which is exactly the wrong direction.
Recovery: reduce the angle of attack, nothing else first
Since the cause is the angle of attack, the cure is to reduce it: pitch forward, then add power and level the wings once the airflow has reattached.
The order matters. Applying full power before reducing AoA, on a single-engine propeller aircraft, worsens the yaw and increases spin risk.
What this changes in flight
- Don't watch the airspeed indicator alone: watch attitude, bank angle, and the ball. Those are what actually control angle of attack.
- In a low, slow turn — base-to-final, circuit work, a mishandled go-around — every extra degree of bank raises the speed at which you'll stall.
- The stall warning triggers on angle of attack, not on speed. When it sounds and the airspeed still looks comfortable, believe the warning.