Ablation. The melting, erosion, evaporation, or vaporization of nose cone materials of vehicles entering a planetary atmosphere at high speeds.

Aerodynamics. The science that studies the motion of air and other gases and the forces that act on bodies moving through them.

Aeronautics. The science or art of operating both lighter-than-air and heavier-than-air craft.

Aerospace. The earth's atmosphere and the space above it, in which aircraft, missiles, and spacecraft fly.

Afterburner. An auxiliary combustion chamber at the rear of a jet engine into which additional fuel is injected and burned, utilizing unburned oxygen in the exhaust gases to increase thrust.

Aileron. A movable control surface, usually on the trailing edge of a wing, used to control the rolling of an airplane.

Airfoil. A surface designed to obtain a reaction from air, such as a, wing for lift, fin for stability, elevator for control, and propeller blade for thrust.

Airspeed. Speed of an aircraft relative to the air in contrast to ground speed, or its speed relative to the ground.

Airway. A designated air route equipped with navigational aids.

Angle of attack. The angle formed by the chord of an airfoil and the line of air flow, or relative wind.

Angle of incidence. Angle made by the chord of an airfoil and the longitudinal axis of the aircraft; that is, the angle at which the airfoil is attached to the airplane.

Area rule. A method of aircraft design for minimizing drag. The cross-sectional area, of transonic aircraft is distributed to resemble that of an ideal body of minimum drag by lengthening the nose, adding a, tail blister, or indenting the fuselage in a Coke-bottle shape.

Aspect ratio of airfoil wing. Ratio of span of a wing to its mean chord.

Attitude. The position of an aircraft or spacecraft as determined by the relationship of its axes to some reference line, plane, or system of reference axes.

Automatic direction finder (ADF). A radio compass that automatically indicates the direction of the station to which it is tuned.

Automatic pilot, or autopilot. A device that maintains an aircraft in straight and level flight on a set course.

Aviation. The art or practice of designing, developing, manufacturing, or operating heavier than-air craft.

Axis. A line passing through an airplane about which it may revolve. An airplane has three mutually lateral, from wing tip to wing tip, measuring pitch; and vertical, from back to belly, measuring yaw.