When an aircraft is weighed with full fuel in the tanks, the weight of the fuel must be accounted for by mathematically subtracting it from the scale readings. To subtract it, its weight, arm, and moment must be known. Although the standard weight for aviation gasoline is 6.0 lb/gal and jet fuel is 6.7 lb/gal, these values are not exact for all conditions. On a hot day versus a cold day, these values can vary dramatically. On a hot summer day in the state of Florida, aviation gasoline checked with a hydrometer typically weighs between 5.85 and 5.9 lb/gal. If 100 gallons of fuel were involved in a calculation, using the actual weight versus the standard weight would make a difference of 10 to 15 lb.
When an aircraft is weighed with fuel in the tanks, the weight of fuel per gallon should be checked with a hydrometer. A hydrometer consists of a weighted glass tube which is sealed, with a graduated set of markings on the side of the tube. The graduated markings and their corresponding number values represent units of pounds per gallon. When placed in a flask with fuel in it, the glass tube floats at a level dependent on the density of the fuel. Where the fuel intersects the markings on the side of the tube indicates the pounds per gallon.
Preparing an Aircraft for Weighing
Weighing an aircraft is a very important and exacting phase of aircraft maintenance, and must be carried out with accuracy and good workmanship. Thoughtful preparation saves time and prevents mistakes.
To begin, assemble all the necessary equipment, such as:
1. Scales, hoisting equipment, jacks, and leveling equipment.
2. Blocks, chocks, or sandbags for holding the airplane on the scales.
3. Straightedge, spirit level, plumb bobs, chalk line, and a measuring tape.
4. Applicable Aircraft Specifications and weight and balance computation forms.
If possible, aircraft should be weighed in a closed building where there are no air currents to cause incorrect scale readings. An outside weighing is permissible if wind and moisture are negligible.
When weighing an aircraft to determine its empty weight, only the weight of residual (unusable) fuel should be included. To ensure that only residual fuel is accounted for, the aircraft should be weighed in one of the following three conditions.
1. Weigh the aircraft with absolutely no fuel in the aircraft tanks or fuel lines. If an aircraft is weighed in this condition, the technician can mathematically add the proper amount of residual fuel to the aircraft, and account for its arm and moment. The proper amount of fuel can be determined by looking at the Aircraft Specifications or Type Certificate Data Sheet.
2. Weigh the aircraft with only residual fuel in the tanks and lines.
3. Weigh the aircraft with the fuel tanks completely full. If an aircraft is weighed in this condition, the technician can mathematically subtract the weight of usable fuel, and account for its arm and moment. A hydrometer can be used to determine the weight of each gallon of fuel, and the Aircraft Specifications or Type Certificate Data Sheet can be used to identify the fuel capacity. If an aircraft is to be weighed with load cells attached to jacks, the technician should check to make sure it is permissible to jack the aircraft with the fuel tanks full. It is possible that this may not be allowed because of stresses that would be placed on the aircraft.
Never weigh an aircraft with the fuel tanks partially full, because it will be impossible to determine exactly how much fuel to account for.
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