Center of Gravity (CG)

What Is Center of Gravity?

The Center of Gravity (CG) is the theoretical point through which the entire weight of an aircraft acts vertically downward. It is the three-dimensional balance point of the aircraft — the location where, if the aircraft were suspended, it would remain level. CG position is arguably the most critical parameter in weight and balance calculations, directly affecting stability, control authority, and fuel efficiency.

CG is always expressed as a percentage of the Mean Aerodynamic Chord (MAC) — the average chord length of the wing. A forward CG provides greater longitudinal stability but requires more elevator force and increases drag; an aft CG improves fuel efficiency but reduces stability margins. Both extremes are bounded by the certified forward and aft CG limits in the aircraft's type certificate.

How It Works

The CG interacts directly with the aerodynamic center — typically located at approximately 25% MAC on most subsonic aircraft. The distance between CG and the aerodynamic center, known as the static margin, determines the degree of natural pitch stability.

• Forward CG limit: Determined by the ability of the elevator to rotate the aircraft to the required angle of attack for takeoff and landing. On the Boeing 737-800, the forward limit is typically 9% MAC.
• Aft CG limit: Determined by the minimum acceptable static margin (usually 5–10% MAC for transport aircraft). Exceeding the aft limit can result in uncontrollable pitch-up.
• CG shift in flight: Fuel burn, payload movement, and fuel transfer between tanks all shift the CG during flight. The A330 uses fuel transfer to the tail trim tank to maintain optimum CG near 38% MAC, reducing trim drag by up to 1.5%.
• MTOW interaction: At MTOW, the CG envelope is most restrictive; structural limits and landing gear loads impose additional constraints.

Performance Specifications

CG management directly affects fuel efficiency. Airlines that actively optimize CG (using fuel transfer or cargo positioning) can achieve savings of 0.5–1.0% in total fuel burn — equivalent to approximately 150–300 kg per transatlantic flight on a wide-body aircraft. Wing loading calculations also depend on accurate CG data to determine the critical load distribution across the wing structure.

Aircraft Examples

• Airbus A320: CG envelope spans 16–40% MAC; uses a center-of-gravity management system with trim tank transfers.
• Boeing 747-8: CG limits change with landing gear extension due to the nose gear geometry; limits span approximately 12–35% MAC.
• Concorde: Required significant fuel transfer between fore and aft tanks to compensate for the aft shift of the aerodynamic center from subsonic to supersonic flight (the aerodynamic center moved rearward by about 6–8% MAC at Mach 2.0).
• Airbus A330: Tail trim tank holds up to 6,200 kg (13,668 lb) of fuel specifically for CG optimization.