Whenever an airplane changes its flight attitude or position in flight, it rotates about one or more of three axes, which are imaginary-lines, that pass through the airplane's center of gravity. The axes of an airplane can be considered as imaginary axles around which the airplane turns, much like the axle around which a wheel rotates. At the point where all three axes intersect, each is at a 90° angle to the other two. The axis, which extends lengthwise through the fuselage from the nose to the tail, is the longitudinal axis. The axis, which extends crosswise from wing tip to wing tip, is the lateral axis. The axis, which passes vertically through the center of gravity, is the vertical axis.
The airplane's motion about its longitudinal axis resembles the roll of a ship from side to side. In fact, the names used in describing the motion about an airplane's three axes were originally nautical terms. They have been adapted to aeronautical terminology because of the similarity of motion between an airplane and the seagoing ship.
In light of the adoption of nautical terms, the motion about the airplane's longitudinal axis is called "roll"; motion about its lateral axis is referred to as "pitch." Finally, an airplane moves about its vertical axis in a motion, which is termed "yaw"—that is, a horizontal (left and right) movement of the airplane's nose.
The three motions of the airplane (roll, pitch, and yaw) are controlled by three control surfaces. The ailerons control roll; pitch is controlled by the elevators; yaw is controlled by the rudder.
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