Definition
The mean aerodynamic chord (MAC) is a single reference chord line used to represent the aerodynamic behavior of an entire wing. It is the chord of an imaginary rectangular wing that would produce the same lifting force and pitching moment as the actual wing. The MAC is used as the reference line against which the airplane's center of gravity (CG) position is expressed, typically as a percentage of MAC.
Plain English
Wings are usually tapered or swept, so they don't have one obvious 'width' from front to back. The MAC is a calculated average wing width that engineers and pilots use as a standard reference line. Where the airplane balances (its center of gravity) is described as a percentage along this line.
Context Anchor
Seen in weight-and-balance data, airplane flight manuals, and center-of-gravity limit charts, especially for airplanes with tapered or swept wings.
Derivation
Chord' comes from the Latin chorda, meaning 'string' -- the straight line drawn from the leading edge to the trailing edge of a wing, like a string stretched across a curve. 'Mean' simply means 'average.' So MAC is the 'average chord' that represents the whole wing.
Why Pilots Care
Center of gravity limits are expressed as a percentage of MAC, directly affecting stability and control.
Analogy
MAC is like using one standard ruler for a wing whose width changes from place to place. It gives pilots and engineers one consistent reference line for balance.
Intuition Check
Mean' does not mean 'unkind' or 'intended'; here it means average. 'Chord' does not mean a musical chord; on a wing, it means the front-to-back distance.
Example Sentence 1
The aircraft's loading chart showed the center of gravity at 22% MAC, well within the approved range.
Example Sentence 2
Weight and balance data listed the MAC length to establish the forward and aft CG limits.