The axes of an aircraft are three imaginary lines that pass through an aircraft’s CG. They can be considered as imaginary axles around which the aircraft turns, passing through the CG at 90° angles to each other. The axis from nose to tail is the longitudinal axis, the axis from wingtip to wingtip is the lateral axis, and the axis that passes vertically through the CG is the vertical axis. Whenever an aircraft changes its flight attitude or position, it rotates about one or more of the three axes. [Figure 4-15]

The motion about the aircraft’s longitudinal axis is “roll,” the motion about its lateral axis is “pitch,” and the motion about its vertical axis is “yaw.” Yaw is the horizontal (left and right) movement of the aircraft’s nose.

The three motions of the conventional airplane (roll, pitch, and yaw) are controlled by three control surfaces: roll is controlled by the ailerons, pitch by the elevators, and yaw by the rudder. The use of these controls is explained in Chapter 5, Flight Controls. Other types of aircraft use different methods. Weight-shift control aircraft control two axes (roll and pitch) using an “A” frame suspended from the flexible wing attached to a three-wheeled carriage; the pilot controls the aircraft by shifting the CG. [Figure 4-16] For more information, see the FAA Weight-Shift Control Flying Handbook, FAA-H-8083-5.

In powered parachutes, control is accomplished by altering the airfoil via steering lines. A powered-parachute wing is a parachute with a cambered upper surface and a flatter lower surface, the two separated by ribs that act as cells which open to the airflow at the leading edge. Cell pressure is greater than the outside pressure, forming a wing that maintains its airfoil shape in flight. The pilot and passenger sit in tandem in front of the engine, located at the rear of the vehicle; the airframe is attached to the parachute via two attachment points and lines. Control is accomplished by both power and the changing of the airfoil via the control lines. [Figure 4-17]