Aircraft CG Envelope & Weight-and-Balance Limits
A weight-and-balance calculation answers two separate questions: Is the airplane within an approved weight limit? and Is the loaded center of gravity within the approved CG limits at that weight? Passing one test does not imply passing the other.
The underlying calculation
The arithmetic is simple. The engineering significance is not. The resulting weight-and-CG point must be compared with the approved loading envelope for the actual airplane, configuration, and applicable phase of operation.
What the envelope represents
The envelope is a boundary established through aircraft design, analysis, and certification. Its horizontal coordinate may be CG arm, station, or percent mean aerodynamic chord; its vertical coordinate is usually airplane weight. A point inside the envelope represents a loading condition that satisfies the approved combination of weight and longitudinal CG limits. A point outside the envelope is not made acceptable by being “close.”
Why the forward CG limit exists
In a conventional airplane, moving the CG forward generally requires a greater balancing tail force and corresponding wing lift to maintain trim. That can increase control forces and induced drag. More importantly, an excessively forward CG can leave insufficient elevator authority for rotation, flare, or other high-angle-of-attack conditions.
The exact handling and performance consequences depend on the airplane. The approved forward limit is therefore not a generic comfort boundary; it is an aircraft-specific operating limit.
Why the aft CG limit exists
Moving the CG aft reduces the restoring pitching moment that contributes to longitudinal static stability. As the CG approaches the aft limit, the airplane generally becomes less longitudinally stable and may require less control force to change pitch attitude. Beyond the approved aft limit, stall and recovery characteristics can become unacceptable and control margins can be reduced.
An aft CG is not “better because it reduces drag.” Any performance benefit is subordinate to the certified stability and control limits.
Why the allowable CG range can change with weight
Many envelopes are not rectangles. Sloping boundaries reflect the fact that structural, control, and stability constraints can change with total weight. The same CG arm may be acceptable at one weight and outside the approved envelope at another.
This is why a pilot should compare the actual loading point with the plotted or tabulated envelope rather than memorizing a single forward and aft arm.
Fuel burn changes both weight and moment
Fuel has a station location, so burning fuel removes both weight and moment. The direction and magnitude of CG movement depend on the fuel-tank location relative to the loaded CG. Some airplanes move forward as fuel burns; some move aft; some change very little. Multiple tanks or transfer systems can make the path more complex.
A correct preflight calculation should therefore consider any loading condition the approved data require—not merely the ramp or takeoff point.
Payload changes are not interchangeable
Removing 100 pounds from a forward baggage station and removing 100 pounds from an aft baggage station produce the same weight change but different moment changes. Weight alone cannot describe the loading condition. This is the physical reason the CG calculation exists.
Percent MAC versus arm in inches
Light general-aviation airplanes often express CG as an arm measured from a datum. Larger or transport-category aircraft commonly use percent mean aerodynamic chord:
These are different coordinate systems for describing the same longitudinal CG location. Use the system published for the airplane; do not mix datum-based arm values with percent-MAC limits without the required geometric conversion.
Common errors that produce a mathematically clean but wrong answer
- using an outdated empty weight or empty-weight moment;
- placing a passenger, baggage item, or fuel load at the wrong station;
- mixing pounds with kilograms or inches with another arm unit;
- using total fuel capacity instead of the actual usable/loading quantity called for by the calculation;
- forgetting that moment indexes such as “moment/1000” are scaled values;
- checking maximum weight but not the CG envelope;
- checking only takeoff loading when the airplane can move outside limits later in flight.
What PilotDesk can and cannot do
PilotDesk can perform moment arithmetic and help visualize a loading point. It cannot know whether the station arms, empty-weight data, envelope, fuel density, or loading limits entered by the user are current for a particular aircraft. Those inputs must come from the current weight-and-balance record, POH/AFM, equipment list, approved supplements, or other applicable aircraft records.
Technical basis
The FAA Weight & Balance Handbook explains that safe operation depends on both total weight and the distribution of that weight, and that loading outside established CG limits can seriously impair controllability. See the current FAA Aviation Handbooks & Manuals and the PilotDesk Sources & Methods.