Unless the specific weight for an item is known, the standard weights used in aircraft weight and balance are as follows:
- Aviation gasoline 6 lb/gal
- Turbine fuel 6.7 lb/gal
- Lubricating oil 7.5 lb/gal
- Water 8.35 lb/gal
- Crew and passengers 170 lb per person
Example Weighing of an Airplane
In Figure 4-20, a tricycle gear airplane is being weighed by using three floor scales. The specifications on the airplane and the weighing specific data are as follows:
- Aircraft Datum: Leading edge of the wing
- Leveling Means: Two screws, left side of fuselage below window
- Wheelbase: 100″
- Fuel Capacity: 30 gal aviation gasoline at +95″
- Unusable Fuel: 6 lb at +98″
- Oil Capacity: 8 qt at –38″
- Note 1: Empty weight includes unusable fuel and full oil
- Left Main Scale Reading: 650 lb
- Right Main Scale Reading: 640 lb
- Nose Scale Reading: 225 lb
- Tare Weight: 5 lb chocks on left main 5 lb chocks on right main 2.5 lb chock on nose
- During Weighing: Fuel tanks full and oil full Hydrometer check on fuel shows 5.9 lb/gal
By analyzing the data identified for the airplane being weighed in Figure 4-20, the following needed information is determined.
- Because the airplane was weighed with the fuel tanks full, the full weight of the fuel must be subtracted and the unusable fuel added back in. The weight of the fuel being subtracted is based on the pounds per gallon determined by the hydrometer check (5.9 lb/gal).
- Because wheel chocks are used to keep the airplane from rolling off the scales, their weight must be subtracted from the scale readings as tare weight.
- Because the main wheel centerline is 70″ behind the datum, its arm is a +70″.
- The arm for the nosewheel is the difference between the wheelbase (100″) and the distance from the datum to the main wheel centerline (70″). Therefore, the arm for the nosewheel is −30″.
To calculate the airplane’s empty weight and empty weight center of gravity, a six column chart is used. Figure 4-21 shows the calculation for the airplane in Figure 4-20.
Based on the calculation shown in the chart, the center of gravity is at +50.1″, which means it is 50.1″ aft of the datum. This places the center of gravity forward of the main landing gear, which must be the case for a tricycle gear airplane. This number is the result of dividing the total moment of 66,698 in-lb by the total weight of 1,331.5 lb.