Angle of attack
Angle of attack (Greek alpha) is a term used in aerodynamics to describe the angle between the wing's chord and the direction of the relative wind, effectively the direction in which the aircraft is currently moving. The amount of lift generated by a wing is directly related to the angle of attack, with greater angles generating more lift (and more drag as it increases the frontal area). This remains true up to the stall point, where lift starts to decrease again because of airflow separation. Planes flying at high angles of attack can suddenly enter a stall if, for example, a strong wind gust changes the direction of the relative wind, an effect that is seen primarily at low speeds.
Using a variety of additional aerodynamic surfaces — known as high-lift devices — like leading edge extensions, fighter aircraft have increased the potential flyable alpha from about 20° to over 45°, and in some designs, 90° or more. That is, the plane remains flyable when the wing's chord is at right angles to the direction of motion.
Some aircraft are equipped with a built-in flight computer that automatically prevents the plane from lifting its nose any further when the maximum angle of attack is reached, in spite of pilot input. This is called the angle of attack or alpha limiter. The pilot may disengage the alpha limiter at any time, thus allowing the plane to perform tighter turns (but with considerably higher risk of going into a stall). A famous example of this is Pugachev's Cobra, a maneuver which can only be performed by the MiG-29, the Su-27/Su-33 and some prototype Western aircraft. It consists of the pilot disengaging the alpha limiter and pulling the aircraft to a 90°–110° angle of attack, then back down to zero. In a properly performed Pugachev's Cobra, the plane maintains a straight and level flight throughout the maneuver.