What causes a plane to stall?

A stall is not an engine failure

In a car, a stall means the engine dies. So when people hear that a plane stalled, many picture the engines cutting out and the plane dropping. Others take "stall speed" to be a fixed minimum speed, and think a stall can only happen when a plane flies too slowly. Both ideas are common, and both miss what a stall is.

In aviation, a stall happens at the wing. When the wing meets the air at too steep an angle, the air stops following its top surface, and the lift drops. In German it is called Strömungsabriss or Überziehen, the flow tearing away. The engines can be running at full power the whole time. In the model of an A320 at 3,000 m and 350 km/h, level flight needs 43 kN of thrust. Push the thrust to full, 143 kN, more than three times as much, and pull the wing to 17°: the wing stalls anyway, and the plane sinks about 5 m/s with its nose high in the air.

How pilots recover from a stall

Because a stall is caused by the angle, the cure is the angle too. The pilot pushes the nose down to reduce the angle of attack, until the air follows the wing again. Only then are the wings rolled level, and power is added to regain speed and climb back. This order is the core of the stall recovery taught in the FAA's Airplane Flying Handbook.

The instinct does the opposite. A plane that sinks with its nose up seems to need more nose up, but pulling back only holds the wing past its limit. In the model, a stalled A320 in a 45° turn stays stalled as long as the angle is held, even while it gains speed; lowering the angle to about 12° or rolling the wings level makes the wing fly again. Real stalls are messier than the model shows: one wing often stalls before the other, the plane rolls towards it, and in light aircraft that can lead into a spin. The model doesn't show spins.

Warning signs before a stall

A wing usually warns before it stalls. As the air starts to separate, the swirls shake the wing and tail, and the plane buffets. Controls feel soft, and the plane sinks although the nose is up. Most planes also have a stall warning, triggered by an angle-of-attack sensor or a vane at the wing's leading edge: a horn or a voice calling "stall".

Many jets, among them the Boeing 737, add a stick shaker that rattles the pilots' control column shortly before the stall (SKYbrary: Stall Warning Systems). Airbus airliners like the A320 work differently: their flight controls normally limit the angle of attack, and they warn with an aural "stall" call.

What changes the stall speed: weight, flaps, altitude, ice

A heavier plane needs more lift, so it reaches the critical angle at a higher speed. In the model, at 3,000 m, the A320's stall speed is 276 km/h at 50 tonnes, 312 km/h at 64 tonnes and 344 km/h at its maximum of 78 tonnes. Flaps and slats work the other way: they make the wing more curved and larger and let it reach higher angles, so the plane can fly slower. At sea level and 64 tonnes the stall speed falls from 269 km/h clean to 193 km/h with full flaps, which is why they come out for takeoff and landing.

Altitude raises the true stall speed, the speed through the air, because thinner air gives less lift at the same speed: 269 km/h at sea level, 312 km/h at 3,000 m and about 550 km/h at 11 km, where the A320 cruises at about 830 km/h. The airspeed indicator measures how hard the air pushes, not the true speed, so it shows about the same stall speed at every height. Near the speed of sound the wing's maximum lift also shrinks, so in the model the critical angle at cruise is only 7.4°. Ice on the wing spoils its shape and can make it stall at a smaller angle and a higher speed; the model doesn't simulate ice.

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