The angle of attack (AOA) is the angle at which relative wind meets an airfoil. It is the angle that is formed by the chord of the airfoil and the direction of the relative wind or between the chord line and the flight path. The angle of attack changes during flight as the pilot changes the direction of the aircraft. It is one of the factors that determines the aircraft's rate of speed through the air. The increase in angle of attack increases lift up to a point. Too high of an angle of attack results in a loss of lift and can cause an aircraft to stall, the critical angle of attack.
The Critical Angle of Attack (CAA) is the angle of attack at which the air no longer flows sufficiently and smoothly over the upper surface of the airfoil. At this point, the aircraft is said to be in a stall. A fixed-wing aircraft always stalls at the same Critical Angle of Attack, rather than at the same airspeed. The airspeed at which the aircraft stalls is variable, depending on the weight of the aircraft, the load factor at the time and the thrust from the engine. The Critical Angle of Attack will vary on every airplane depending on the design of the wing.
All takeoff and landing operations are critical. Naval Aircraft Carrier operations, commercial flights and even the space shuttle require Angle of Attack systems for the safety of their pilots and crew. Angle of Attack indicators measure the Potential of Wing Lift (POWL) directly and help the pilot fly these operations close to the stall point with greater precision. All Short Take Off and Landing (STOL) operations require the aircraft to be able to operate at full-stall during landings and maximum climb angle during takeoffs. Since the airspeed of maximum performance during these maneuvers varies, airspeed is of less value to the pilot than angle of attack.