Weight and balance isn't paperwork bolted onto flight planning, it's a direct determinant of whether the airplane will fly the way its performance charts and handling characteristics assume it will. An airplane loaded within its weight and CG limits behaves the way its type certificate says it should; one loaded outside those limits is, to some degree, an experimental aircraft you're finding out about in real time.
Every performance number in the pilot's operating handbook, takeoff distance, climb rate, stall speed, controllability, is calculated for an aircraft at or below a certified maximum weight and within a certified range of center of gravity positions. Load the aircraft outside either limit and those numbers stop being guaranteed; the airplane may still fly, but its actual behavior is no longer the behavior the manufacturer tested, certified, and published data for.
| Term | Meaning |
|---|---|
| Empty weight | The aircraft as built, including unusable fuel and full operating fluids, but no crew, passengers, baggage, or usable fuel |
| Useful load | Maximum gross weight minus empty weight — everything you can add: crew, passengers, baggage, and usable fuel |
| Maximum gross weight | The certified maximum total weight for takeoff (sometimes different from maximum landing weight in larger aircraft) |
| Ramp weight | Weight at engine start, including fuel that will be burned during taxi and runup |
The center of gravity (CG) is the single point at which the aircraft's entire weight can be considered to act, the balance point. It's measured as a distance from a fixed reference point called the datum, which the manufacturer defines arbitrarily (often the firewall, the leading edge of the wing, or some distance ahead of the nose) purely as a consistent zero point for measurement.
The arm is the horizontal distance from the datum to an item's own center of gravity (positive aft of the datum, negative forward of it, depending on convention). The moment is weight multiplied by arm, essentially a measure of how much that item's weight, at that distance, pulls the overall balance point in its direction. Sum every item's weight to get total weight; sum every item's moment and divide by total weight to get the aircraft's overall CG location.
The CG envelope is a certified range, typically plotted as a graph of weight versus CG location, or expressed as a forward and aft limit that can shift slightly with total weight. The computed CG for a given loading must fall inside this envelope at all points during the flight, not just at takeoff, since fuel burn and any mid-flight weight shifts (like a passenger moving, unlikely, or cargo shifting) can move it.
An aft CG reduces the download the tail must produce to balance the aircraft, which reduces trim drag and can slightly improve cruise efficiency, but it comes at a real cost: reduced static and dynamic stability, a lower stall speed but often a more abrupt, less predictable stall break, reduced elevator authority for recovery, and in extreme cases, an inability to recover from a stall or spin at all.
A forward CG generally increases stability, sometimes to the point of requiring more elevator (and more back-pressure) to raise the nose for takeoff or to flare for landing, and increases stall speed slightly. Taken to an extreme, an excessively forward CG can mean insufficient elevator authority to flare at all, particularly at low speed and full flaps, a genuine landing hazard.
Exceeding maximum gross weight increases stall speed, lengthens takeoff and landing distance, reduces climb performance and service ceiling, increases structural loads during maneuvering or turbulence (reducing the margin to the aircraft's certified load factor limits), and can exceed the structural limits of the landing gear on a hard landing.
Fuel is rarely located exactly at the aircraft's CG, so as it burns off, the CG shifts, forward or aft depending on the fuel tank's position relative to the empty-aircraft CG. Most small aircraft are designed so this shift stays comfortably inside the envelope across a normal flight, but it's part of why a loading that's legal at takeoff must also be checked (or reasoned through) for landing, especially on aircraft with unusual fuel tank placement or with cargo intentionally loaded near a limit.
An aircraft has an empty weight of 1,500 lbs at an arm of 39 inches (moment 58,500 in-lbs). Add a pilot and passenger totaling 340 lbs at an arm of 37 inches (moment 12,580), 40 gallons of fuel (240 lbs) at an arm of 48 inches (moment 11,520), and 30 lbs of baggage at an arm of 95 inches (moment 2,850). Total weight: 1,500 + 340 + 240 + 30 = 2,110 lbs. Total moment: 58,500 + 12,580 + 11,520 + 2,850 = 85,450 in-lbs. CG = 85,450 ÷ 2,110 ≈ 40.5 inches aft of datum. Compare that result against the aircraft's specific CG envelope for 2,110 lbs to confirm it's within limits before flight.
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This guide is intended for educational and training purposes only. It is not a substitute for official FAA publications, an authorized ground school, or qualified flight instruction, and should not be used as a sole source for real-world flight planning, dispatch, or operational decisions.