In the wind

The simple answer is, you don’t. Not reliably, anyway. The one-G stall speeds on the airspeed indicator and in the pilot’s operating handbook are accurate only at a specific weight and as long as you don’t climb, descend, or turn. For a more accurate view of how much lift the wing is producing, look to the angle of attack indicator.

An airfoil can stall at any airspeed, in any attitude—fast, slow, upside-down, or sideways. Yet it always stalls when it surpasses its critical angle of attack. AOA indicators measure the angle between the wing’s chord line and the relative wind and deliver a visual representation of the lift the wing is producing to the pilot. As the angle of attack increases, the indication progresses from green to yellow to red, which warns of an impending stall and loss of lift. These useful instruments used to be relegated to the world of jets, but lower-cost AOA gauges have proliferated in smaller general aviation aircraft over the past decade.

Angle of attack is independent of speed, weight, and attitude, and it’s not only relevant at the edge of a stall. Each airfoil has a specific AOA for best glide and range and another for best endurance, among other performance parameters.

The data for AOA indicators comes from a probe or vane on the outside of the airplane. The accuracy of the data may be affected by airspeed, attitude, installation, and other factors, so most air data computers correct for common errors.

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