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.
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.
Things You'll Need
- Pilot operating handbook
- Weight and balance report
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.
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.
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