Alternating current.

The ability to determine present position in space independently, and is most often used by pilots.

The actual distance between an aircraft and the terrain over which it is flying.

Pressure measured from the reference of zero pressure, or a vacuum.

Force involved in overcoming inertia, and which may be defined as a change in velocity per unit of time.

A magnetic compass error apparent when the aircraft accelerates while flying on an easterly or westerly heading, causing the compass card to rotate toward North.