Although calculating weight and balance for an aircraft may seem like a boring and tedious task for some pilots, ensuring that an airplane is within limits is crucial to ensuring safe flight. Because of this, pilots should be extra diligent when making weight and balance calculations and use the correct method to ensure that the airplane is within weight limits and is balanced properly for flight.
Weight and Balance AIRCRAFT DATA “ The Empty Weight, Arm and Moment information for the aircraft below were extracted. From each aircrafts’ POH. This information is contained in Section 6 of each aircraft’s POH.” Download Weights and Balances PDF. The center of gravity (CG) of an aircraft is the point over which the aircraft would balance. Its position is calculated after supporting the aircraft on at least two sets of weighing scales or load cells and noting the weight shown on each set of scales or load cells. The center of gravity affects the stability of the aircraft. To ensure the aircraft is safe to fly, the center of gravity must.
Look at the aircraft's weight and balance report. This report, which is a legally required issue with each and every aircraft in the United States, is specific to a certain aircraft and contains its empty weight, including oil and other necessary fluids but not fuel, and center of gravity limits.
Calculate the aircraft's weight. Begin with the empty weight listed in the weight and balance report and add weights for front-seat passengers (including the pilot), back-seat passengers, baggage in the baggage compartment and fuel. Note that most aviation gas weighs six pounds per gallon, while jet fuel weighs 6.6 pounds per gallon and kerosene weighs seven pounds per gallon. This is the aircraft's gross weight.
Check the pilot operating handbook for the aircraft's maximum gross weight and ensure that the loaded airplane does not exceed it. If it is too heavy, remove some baggage or consider transporting fewer passengers.
Look in the pilot operating handbook to determine the rate of fuel burn for the specific aircraft. This can generally be found in a graph and can fluctuate by the amount of throttle being used.
Plan for the amount of fuel that will burn off during taxi and engine run-up activities. Subtract the weight of this fuel from the gross weight to determine takeoff weight. Ensure that takeoff weight does not exceed the maximum gross weight in the pilot operating handbook.
Calculate the amount of fuel that will be burned in flight. Subtract the weight of that fuel from the takeoff weight to get the aircraft's landing weight.
Use the weight and balance sheet to determine the moment arm for each portion of the aircraft. The moment arm is the distance an area of the aircraft is from its datum -- an arbitrarily selected point by aircraft engineers that acts as the focus of the center of gravity.
Multiply the empty weight of the aircraft by its moment arm to obtain the empty weight moment index. For example, if the aircraft is 1,205 pounds empty and the moment arm is listed on the weight and balance sheet as 37 inches, the moment index would be 1,205 x 37 = 44,585.
Determine the moment index for the front-seat passengers, rear-seat passengers, baggage compartment and fuel on board the aircraft. For instance, multiply the weight of the fuel by the moment arm of the fuel tanks, which is located on the weight and balance sheet. Add each moment index on to the empty weight moment index. This will yield the gross moment index.
Determine the moment index of the fuel to be burned during taxi to get the takeoff moment index, and of the fuel to be burned during flight to get the landing moment index.
Divide the gross moment index by gross weight, the takeoff moment index by takeoff weight and the landing moment index by landing weight. This will determine the aircraft's overall center of gravity position.
Ensure the center of gravity is within the limits outlined on the weight and balance sheet. For example, the sheet may list acceptable center of gravity limits between 33.5 and 42.5 inches. All of the center of gravity calculations, from gross to takeoff to landing, should all fall within these limits.
Tip
Some aircraft have graphs in the pilot operating handbook that allow for quick calculation of moment indexes and acceptable weight and balance limits without the pain of calculations. Use these if available, but try to be equally as accurate.
Warning
Never fly an aircraft that is too heavy or out of limits. An aircraft that is too heavy will struggle to become airborne, while one that is out of center of gravity limits is very difficult to control. If the aircraft is out of balance, try shifting passengers or baggage around or consider adding ballast to add more weight in an area and even things out.
Always calculate the center of gravity for takeoffs and landings. As fuel burns, the aircraft gets lighter and the center of gravity slowly moves forward. An aircraft that may have been within limits at takeoff could be dangerously out of limits come landing time. Always ensure that weight and balance during each stage of flight is within limits.
The reference datum is an imaginary vertical plane from which horizontal distances are measured for aircraft weight and balance purposes. The reference datum is at location '0' and measurements for other reference points, like the baggage area or the passenger seats, are made in relation to the reference datum. The datum is determined by the manufacturer, and in small aircraft, the reference datum is often located along the firewall or at the leading edge of the wing.
Station
Regarding weight and balance on an airplane, the station is a location along the airplane fuselage given in terms of distance from the reference datum.
Arm
The arm is the horizontal distance from the reference datum to the center of gravity (CG) of an item.
CG Arm
The CG arm (where CG stands for center of gravity) is the arm obtained by adding the aircraft’s individual moments and dividing the sum by the total weight of the unit.
Moment
A moment is the product of the weight of an item multiplied by its arm. (Moment/1000 is used to simplify digits in some cases.)
Center of Gravity (CG)
And aircraft's center of gravity is the point at which it would balance if it were suspended in air. Its distance from the datum is found by dividing the total moment by the total weight of the airplane. The center of gravity can be thought of as where all of the aircraft's mass is concentrated, or the 'heaviest' part of the airplane.
Center of Lift
The center of lift is the point along the chord line of an aircraft wing or airfoil at which the force of lift is concentrated.
CG Limits
The forward and aft center of gravity locations within which the airplane must be operated are referred to as CG limits. CG limits are based on a given weight.
Chord
The chord, or chord line, of a wing isan imaginary line representing a straight-line distance from the leading edge to the trailing edge of an airfoil.
Standard Empty Weight
The empty weight of an aircraft is the weight of the aircraft without including passengers, baggage, or fuel. Standard empty weight usually includes unusable fuel, full operating fluids, and full engine oil.
Basic Empty Weight
The basic empty weight of an airplane is the standard empty weight of the airplane plus optional equipment installed.
Maximum Landing Weight
The maximum landing weight is, as you might imagine, the maximum aircraft weight limit approved for an aircraft to land. Landing above this weight can cause structural damage.
Maximum Ramp Weight
The maximum weight for maneuvering on the ground is called the max ramp weight. Max ramp weight includes the weight of fuel used for start, taxi, and aircraft run-up procedures.
Maximum Takeoff Weight
The maximum weight limits for an aircraft to begin its takeoff roll is called the max takeoff weight.
Useful Load
The useful load is the difference between ramp weight or max allowable weight and basic empty weight. Useful load is the weight of the useful items on board, such as passengers and baggage.
Payload
An aircraft's cargo, baggage, and passengers (including pilots) make up its payload.
Load Factor
The ratio of the amount of load and aircraft can withstand its maximum weight is called the load factor.
Tare
Tare is the weight of chocks, blocks, stands, etc. used when weighing an airplane. Tare weight is included in the scale readings and deducted from the scale reading to obtain the actual (net) airplane weight.
Standard Weights of Fluids
- Fuel: 6 lbs/gal
- Oil: 7.5 lbs/gal
- Water: 8.35 lbs/gal
Source: FAA Aircraft Weight & Balance Handbook, FAA-H-8083-1A