Percent mean aerodynamic chord
Some aircraft express center of gravity as a distance from a datum, while others express it as a percentage of mean aerodynamic chord, or %MAC. Converting between the two requires the CG arm, the leading edge of MAC location and the MAC length.
What MAC means
Mean aerodynamic chord is a representative wing chord used for aerodynamic reference. On a tapered or swept wing, it provides a useful standardized location for expressing center of gravity.
The formula
%MAC = ((CG arm − LEMAC) ÷ MAC length) × 100. LEMAC is the arm of the leading edge of the mean aerodynamic chord relative to the aircraft datum.
CG 15 inches aft of LEMAC
If CG arm is 95 inches, LEMAC is 80 inches and MAC length is 60 inches, the CG is 15 inches aft of LEMAC. Fifteen divided by 60 equals 0.25, so the position is 25% MAC.
Datum and MAC references are different
The aircraft datum may be at the firewall, nose, wing or even in front of the aircraft. LEMAC tells you where the aerodynamic reference begins relative to that datum. Do not substitute a chord dimension or wing leading edge that is not the published LEMAC.
Use the correct loading condition
Calculate the actual loaded CG first from total moment and total weight, then convert that CG to %MAC if needed. Fuel burn, passengers or baggage changes can move the final value.
The envelope still controls
A mathematically correct %MAC is not automatically acceptable. Compare the result against the approved CG envelope and any phase-of-flight limits in the aircraft documentation.
Related PilotDesk tools
Moment & CG · Weight & Balance Builder · Ballast and CG