MAC & envelope

031 Mass & BalanceChapter 4

CG limits in transport aircraft are often expressed as a percentage of the Mean Aerodynamic Chord (MAC) rather than a raw arm from the datum.

Mean aerodynamic chord

Definition — MAC

The Mean Aerodynamic Chord — a representative wing chord length used for aerodynamic and loading reference. Defined by the manufacturer in the AFM.

Definition — LEMAC

Leading Edge of MAC — reference point for %MAC calculations.

Definition — TEMAC

Trailing Edge of MAC. MAC length = TEMAC − LEMAC (distance along the longitudinal axis).

Expressing CG as %MAC

Formula — CG as percentage of MAC

%MAC = (CG distance from LEMAC ÷ MAC length) × 100

%MAC = (CG − LEMAC) ÷ MAC × 100

Key fact

CG can be referenced relative to the datum (arm in metres/inches) or as %MAC. Both describe the same point — the loading manual converts between them.

Key fact

The CG safe range includes both forward and aft limits. The envelope is drawn on a mass vs CG chart (or mass vs %MAC) in the AFM — must not be exceeded in any flight phase.

CG envelope

Key fact

The CG envelope shows approved CG range at each mass. Limits ensure adequate longitudinal stability, control authority, and structural loading. A safety margin is applied for crew/pax movement and fuel consumption.

Key fact

At higher masses, the allowable CG range may narrow. At lower masses, the envelope may allow a wider CG band — always read the chart, never assume.

Recalculation triggers

Key fact

Recalculation of mass/CG is required when cumulative changes exceed ±0.5% of MAC or ±0.5% of MLM (max structural landing mass). For fleet masses, individual aircraft DOM and CG may not differ by more than ±0.5% from the fleet values.

Fuel and CG envelope

Key fact

Fuel burn moves the CG along a pre-calculated path depending on tank sequence (inner wing tanks often emptied first to reduce root bending and manage CG travel). The fuel index tables map fuel mass to CG shift.

Exam trap

%MAC uses distance from LEMAC, not from the datum (unless the datum happens to coincide with LEMAC). Convert CG arm to LEMAC reference before applying the formula.

Exam trap

The safe range includes both limits — forward AND aft. “CG within limits” means inside the envelope at the current mass, not just below an aft limit.