All measure distances in their own coordinate system, positive backwards. This can be the actual MAC location of the wing, other aircraft use the leading edge of the wing root as the lengthwise zero point. Next, you need a location for this length unit. The % part just shows that this length is given as a percentage of the mean aerodynamic chord. This is not exactly the mathematic mean of the wings' chord, but a size which includes the damping effect of a pitching wing which scales with the square of its chord. Most jet pilots will set a specific takeoff pitch trim setting for takeoff based on %MAC so that if the pilot loses an engine after $V_1$ the trim will be set for a speed around $V_2$.Īs points out in his comment, it is the Mean Aerodynamic Chord of the wing. If this is the case, there is a specific pitch trim setting for each %MAC and airspeed combination. The same analysis can be made when the CG moves aft. We can increase the tail down force with nose up stabilizer trim. If the CG moves forward, Moment #1 gets larger and to compensate the tail down force must increase. These moments must equalize for the airplane to be in equilibrium. There is a moment between CG and CP and another moment between CP and tail down force. What may not be obvious is that we can use this formula in figuring out why %MAC is important. In the picture below, we can eyeball the %MAC to be around 10% MAC.

If the CG is at the LEMAC, we can say it is at 0% MAC. %MAC is simply a calculation of how far the CG is from the LEMAC. The same process happens for calculating TEMAC. As a result, aerodynamic engineers figure out where the average of the LEMAC by averaging the wing root and wing tip. For swept-wing aircraft it isn't that simple. It is simply the measurement from where the wing starts and ends from the datum. LEMAC and TEMAC are easy to calculate for straight wing aircraft. There is also a trailing edge mean aerodynamic chord (TEMAC) that is calculated. Unfortunately, we need to change where the reference point is from the (datum) to the front of the wing called leading edge mean aerodynamic chord (LEMAC). Pilots, dispatchers or load masters will calculate the weight and balance with respect to the aircraft datum. All one has to do is to figure out the numbers and plug them into the formula. The calculations is rather straight forward and the generic formula is the same for all airplanes. %MAC (Percent Mean Aerodynamic Chord) is a mathematical calculation that shows where the center of gravity is over the wing.

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