Weight and balance is one of the most safety-critical calculations in aviation maintenance. An aircraft that is too heavy or whose center of gravity (CG) falls outside approved limits can be impossible to control, even by a skilled pilot. As an Aviation Maintenance Technician (AMT), you are legally responsible for understanding the weight-and-balance system well enough to perform reweighs, document equipment changes, and sign off on maintenance that affects an aircraft's loaded condition. Before any computation can be performed, however, you must have a firm grip on five foundational terms: empty weight, center of gravity (CG), datum, arm, and moment. These are the vocabulary of every weight-and-balance document in existence, from the original Type Certificate Data Sheet (TCDS) to a post-alteration weight-and-balance report.
This article defines each term precisely, explains the physics behind it, and shows how the five concepts lock together into the calculation process you will use on the job and on the FAA AMT General knowledge test.
The Datum: Your Starting Line
The datum is an imaginary vertical plane chosen by the aircraft manufacturer as the zero-reference point for all horizontal measurements on the aircraft. Think of it as the origin on a number line laid along the longitudinal (nose-to-tail) axis of the airplane. Every other measurement—every arm—is stated as a distance from this plane.
Manufacturers choose the datum location arbitrarily, and it varies widely between aircraft types. Common datum locations include the leading edge of the wing, the firewall, the nose of the aircraft, or even a point some distance ahead of the nose (sometimes described as a station ahead of the aircraft, producing only positive arm values throughout). The exact datum location for each aircraft is published in the Type Certificate Data Sheet and in the Aircraft Specifications or Aircraft Flight Manual weight-and-balance section.
Why does the datum matter? Because it makes all measurements consistent and reproducible. Any technician, anywhere in the world, using the same aircraft model's documentation will be working from the same reference plane. If the datum were left to individual interpretation, arms—and therefore moments—would be meaningless.
Arm: The Horizontal Distance
An arm (sometimes called a moment arm) is the horizontal distance, measured in inches, from the datum to any given item on the aircraft. Arms follow a sign convention: items located aft of the datum carry a positive arm value, and items located forward of the datum carry a negative arm value. If the datum is placed at or ahead of the nose, all arms are positive, which simplifies arithmetic and reduces sign errors.
Every component that contributes to the aircraft's weight has an arm: fuel tanks, baggage compartments, passenger seats, engines, avionics boxes, and structural members all have published or measured arm values. These arms are listed in the aircraft's equipment list and weight-and-balance data. When a technician adds or removes equipment during an alteration, the arm of each affected item must be determined and documented before the new CG can be calculated.
Weight: The Force at Each Station
Weight, measured in pounds, is the actual gravitational force acting on each item. In weight-and-balance work the relevant weights include the weight of each component, the weight of occupants, the weight of fuel (at 6 lb/gal for aviation gasoline, or approximately 6.7 lb/gal for Jet-A), and the weight of cargo. Each of these weights acts downward through its own arm, creating a turning effect about the datum.
Moment: Weight Times Arm
A moment is the product of a weight and its arm. Expressed as a formula: Moment = Weight × Arm. The result is stated in pound-inches (lb-in), sometimes written as inch-pounds. Moments tell you not just how much an item weighs, but how strongly it acts to rotate the aircraft about the datum reference point.
Consider two 50-pound batteries. If one is installed 20 inches aft of the datum, its moment is 1,000 lb-in (aft, positive). If the second is installed 40 inches aft, its moment is 2,000 lb-in. Even though both batteries weigh the same, the farther one has twice the rotational influence on the aircraft's balance. This is exactly the lever principle that governs all weight-and-balance math: a small weight far from the fulcrum can move the balance point just as much as a large weight close to it.
When moments are summed algebraically (positive aft, negative forward), the total moment describes the net turning tendency of the entire loaded aircraft about the datum. Dividing the total moment by the total weight then locates the point where that net tendency acts—which is the center of gravity.
Center of Gravity (CG): The Balance Point
The center of gravity is the single point at which the entire weight of the aircraft may be considered to act. It is the three-dimensional balance point of the aircraft, but in practical fixed-wing weight-and-balance work the CG is expressed as a location along the longitudinal axis, stated as a distance (in inches) aft of the datum.
The formula is straightforward: CG = Total Moment ÷ Total Weight. The result is an arm value—an inch station—that tells you exactly where balance is located. If the CG falls within the aircraft manufacturer's approved forward and aft limits (published in the TCDS and AFM), the aircraft is within balance. If it falls outside those limits, the aircraft may not be legally flown until the loading is corrected.
Forward CG limits exist because an excessively nose-heavy aircraft may require more up-elevator than is available to flare for landing, and will suffer increased trim drag. Aft CG limits exist because a tail-heavy aircraft becomes progressively more unstable and, beyond a critical point, uncontrollable. The approved CG envelope—often shown as a graph of CG range versus gross weight—represents the manufacturer's determination of where the aircraft remains both controllable and structurally safe.
Empty Weight: The Baseline
The empty weight is the weight of the airframe, powerplant, and all items of operating equipment that have fixed locations and are permanently installed. Under current FAA practice, most general aviation aircraft use the standard empty weight convention, under which empty weight includes the weight of unusable fuel and full operating fluids—including full engine oil, hydraulic fluid, and other trapped fluids. Usable fuel is not included in empty weight.
The empty weight is established during a formal aircraft weighing, where the aircraft is placed on calibrated scales under controlled conditions—level attitude, with all required equipment installed. The result, along with the empty-weight CG arm and moment, is documented on a weight-and-balance record that must be kept in the aircraft records at all times. Whenever maintenance adds, removes, or relocates equipment, this baseline document must be updated to reflect the new empty weight and CG.
How the Five Terms Work Together
In practice, a weight-and-balance calculation proceeds as follows. Start with the aircraft's empty weight and its corresponding moment (empty weight multiplied by the empty-weight CG arm, measured from the datum). Then, for each item of useful load—fuel, oil if applicable, pilot, passengers, and baggage—multiply the item's weight by its arm to get its moment. Add all weights to get total weight. Add all moments algebraically to get total moment. Divide total moment by total weight to get the loaded CG. Compare that CG arm to the manufacturer's approved limits.
Every step in that process depends on knowing precisely what each term means. A confusion between arm and moment, or between datum and CG, produces a calculation error that could place an aircraft outside its approved envelope—a serious airworthiness and legal concern.
Key Numbers and Rules
- Moment = Weight × Arm — always in pound-inches (lb-in).
- CG = Total Moment ÷ Total Weight — result is an arm in inches from the datum.
- Arms aft of datum are positive; arms forward of datum are negative.
- Empty weight includes unusable fuel and full operating fluids; it excludes usable fuel.
- Aviation gasoline weighs approximately 6 lb/gal; Jet-A approximately 6.7 lb/gal.
- The datum is defined by the manufacturer and published in the TCDS and AFM/POH weight-and-balance section.
- The current weight-and-balance data must be kept with the aircraft records and available as required by 14 CFR 91.9 and the aircraft's equipment list.
- Any alteration affecting empty weight or CG requires an updated weight-and-balance record signed by an authorized person.
Common Test Traps
- Confusing arm and moment. The arm is a distance (inches); the moment is a force-distance product (pound-inches). Test questions sometimes use the numbers interchangeably to see if you know the difference. Remember: you must multiply weight by arm to get a moment.
- Forgetting the sign convention. If the datum is not at the nose, some arms will be negative (forward of datum). Failing to apply negative signs will shift the calculated CG aft of its true location—a potentially unsafe error.
- Misidentifying what is included in empty weight. Unusable fuel IS part of empty weight. Usable fuel is NOT. Under the standard empty weight convention, full operating fluids including engine oil are included, so always check the specific aircraft's documentation.
- Assuming the datum is always at the firewall or nose. The manufacturer chooses the datum, and it varies. Never assume its location—always check the TCDS or approved weight-and-balance data for the specific aircraft.
- Confusing CG limits with CG location. The CG limit is the approved range (forward limit to aft limit). The CG is the calculated location of balance. A test question may give you a CG and ask whether it is within limits—you must compare the calculated arm to both the forward AND aft limits, not just one.