Center of Gravity Limits and Envelopes

What Is the Center of Gravity Envelope?

The CG envelope is a graphical depiction of the forward and aft CG limits across the aircraft's allowable mass range. The envelope is constructed during certification, reflecting the tested boundaries where the aircraft remains controllable, stable, and structurally sound. Each mass condition—zero fuel, ramp, take-off, and landing—must have its CG position plotted and checked to ensure it falls within the approved envelope.

Forward and Aft CG Limits

• Forward Limit: If the CG is ahead of this line, the aircraft becomes nose-heavy. This increases control forces, may reduce elevator authority, and can make it difficult or impossible to rotate for take-off or flare for landing.
• Aft Limit: If the CG is behind this line, the aircraft becomes tail-heavy. This can lead to pitch instability, reduced longitudinal stability, and, in severe cases, uncontrollable flight attitudes.

Effects of CG Position on Performance

• Forward CG: Increases stability but reduces elevator effectiveness and increases stall speed. More power is required for take-off and climb, and the aircraft may have a shorter range due to higher drag.
• Aft CG: Reduces stability but can improve cruise efficiency and decrease stall speed. However, it makes the aircraft more sensitive to pitch inputs and harder to recover from stalls or spins.

Operational Use

Pilots must plot all relevant mass and CG positions on the envelope for each flight. If any point falls outside the envelope, the load must be adjusted—by moving passengers, baggage, or fuel—until all conditions are within limits. This process is crucial for both safety and regulatory compliance.

Key Points for the ATPL Exam

• The CG is referenced to a fixed datum, often expressed as a percentage of the mean aerodynamic chord (MAC) or a distance from a reference point.
• The CG is allowed to move within defined limits, not fixed in one position.
• The envelope may have different boundaries for take-off, landing, and zero-fuel conditions, reflecting different control requirements at each phase.
• The utility category may have a more restrictive envelope for certain operations.
• Always verify the plotted CG positions for every mass condition before flight release.