Everything Explained: Weight and Balance | Flying Magazine

Everything Explained: Weight and Balance

Here's what these common terms mean and why they're important.

everything explained weight and balance

FAA regulations could change at any time. Please refer to current FARs to ensure you are legal.

Illustration by Tim Barker

Empty Weight Empty weight is defined as the total weight of an aircraft including all fixed ballast, unusable fuel, undrainable oil, total quantity of engine coolant and total quantity of hydraulic fluid, and excluding crew, payload, usable fuel and drainable oil.

Basic Operating Weight (BOW) Total weight of the aircraft, including crew, ready for flight, but without payload or fuel (sometimes excludes the crew). Includes all fixed ballast, unusable fuel, normal operating level of oil and total quantity of hydraulic fluid (transport aircraft only).

Fuel Load Usable fuel only, not the fuel in the lines or trapped in the sumps.

Maximum Allowable Zero Fuel Weight (MZFW) 1. The wings flex up and down with turbulence, applying severe bending loads at the wing roots. These loads increase dramatically as weight in the fuselage increases. Zero fuel weight sets a maximum allowable weight that can be carried in the fuselage in order to eliminate the possibility of destructive wing-bending stress. 2. The maximum permissible weight of an aircraft with no disposable fuel or oil. 3. Zero fuel weight for a particular flight is the basic operating weight plus payload. 4. Maximum weight that can be concentrated in the fuselage (pilots plus payload). 5. Based on the wings’ ability to tolerate bending stresses (wing-bending limit). 6. All weight over and above MZFW must be in the form of fuel in the wings.

The zero fuel weight figure may be found in either the aircraft Type Certificate Data Sheet or the approved Aircraft Flight Manual.

Useful Load Useful load is the weight of the pilots, passengers, baggage, usable fuel and drainable oil. The empty weight subtracted from the max allowable takeoff weight gives us this number.

Datum
An imaginary line from which all measurements of arm are taken. Location of the datum is established by the manufacturer. The datum is station zero.

Station A location in an aircraft identified by a number designating its distance in inches from the datum. The datum is station zero. The station and arm are usually identical. An object located at station +103, for example, would therefore have an arm of +103 inches.

**Arm (Moment Arm) ** Distance in inches from the reference datum line to the center of gravity of an item — (+) if measured aft of the datum or (–) if measured forward of the datum.

Moment Product of the weight of an item multiplied by its arm expressed in pound-inches.

Moment Index A moment divided by a constant such as 100, 1,000 or 10,000. Used to simplify the W&B computations for larger aircraft (heavy weights x long arms = huge numbers).

Center of Gravity The point about which an aircraft would balance if it were possible to suspend it at that point, expressed in inches from the datum. It is the theoretical “mass center” of the aircraft.

> Weight x arm = moment
> Total moment ÷ total weight = CG

Center of Gravity Limits The forward and aft locations beyond which the CG must not be located during takeoff, flight or landing. CG range is the distance between the forward and aft limits.

Chord of an Airfoil An imaginary line drawn through an airfoil from its leading edge to its trailing edge.

Mean Aerodynamic Chord (MAC) Average distance from the leading edge to the trailing edge of the wing. An imaginary airfoil that has the same aerodynamic characteristics as the actual airfoil. CG is often expressed as a percentage of MAC for larger aircraft. Normally, an aircraft will yield acceptable flight characteristics if the CG is located somewhere near 25 percent of the average chord — which is one-fourth of the distance back from the leading edge of the mean aerodynamic chord (average wing section).

LEMAC/TEMAC Leading edge (or trailing edge) of the mean aerodynamic chord.

Maximum Takeoff Weight Max allowable weight for takeoff. Some aircraft are approved for a greater maximum ramp weight (MRW) to allow for fuel burn during taxi. Takeoff weight may be limited to a lesser weight due to runway length, density altitude or other variables.

Max Landing Weight You can guess what this is!

Somewhat interchangeably with “Regulated Landing Weight,” Max landing weight is dependent on field conditions (altitude, temp, pressure, wind and slope).

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