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weight of the residual fuel is part of
the empty weight of the aircraft.
Unusable fuel (GAMA): Fuel
remaining after a runout test has
been completed in accordance with
governmental regulations.
3–4
Leveling the Aircraft
When an aircraft is weighed, it must be in its level flight
attitude so that all of the components will be at their correct
distance from the datum. This attitude is determined by
information in the TCDS. Some aircraft require a plumb line
to be dropped from a specified location so that the point of
the weight, the bob, hangs directly above an identifiable point.
Others specify that a spirit level be placed across two leveling
lugs, often special screws on the outside of the fuselage.
Other aircraft call for a spirit level to be placed on the upper
door sill.
Lateral level is not specified for all general aviation (GA)
airplanes, but provisions are normally made on transport
airplanes and helicopters for determining both longitudinal
and lateral level. This may be done by built-in leveling
indicators, or by a plumb bob that shows the conditions of
both longitudinal and lateral level.
The actual adjustments to level the aircraft using load cells
are made with the jacks. When weighing from the wheels,
leveling is normally done by adjusting the air pressure in the
nose wheel shock strut.
Determining the Center of Gravity
When the aircraft is in its level flight attitude, drop a plumb
line from the datum and make a mark on the hangar floor
below the tip of the bob. Draw a chalk line through this point
parallel to the longitudinal axis of the aircraft. Then draw
lateral lines between the actual weighing points for the main
wheels, and make a mark along the longitudinal line at the
weighing point for the nose wheel or the tail wheel. These
lines and marks on the floor allow you to make accurate
measurements between the datum and the weighing points
to determine their arms.
Two Ways to Express CG Location
The location of the CG may be expressed in terms of inches from a
datum specified by the aircraft manufacturer, or as a percentage of
the MAC. The location of the leading edge of the MAC, the LEMAC,
is a specified number of inches from the datum.
Safety Considerations
Special precautions must be taken when raising an aircraft on jacks.
1. Stress plates must be installed under the jack pads if the manufacturer
specifies them.
2. If anyone is required to be in the aircraft while it is being jacked,
there must be no movement.
3. The jacks must be straight under the jack pads before beginning
to raise the aircraft.
4. All jacks must be raised simultaneously and the safety collars
screwed down against the jack cylinder to prevent the aircraft tipping
if any jack should lose pressure.
Determine the CG by adding the weight and moment of each
weighing point to determine the total weight and total
moment. Then divide the total moment by the total weight to
determine the CG relative to the datum.
As an example of locating the CG with respect to the datum,
which in this case is the firewall, consider the tricycle landing
gear airplane in Figures 3-3 and 3-4.
When the airplane is on the scales with the parking brakes
off, place chocks around the wheels to keep the airplane from
rolling. Subtract the weight of the chocks, called tare weight ,
from the scale reading to determine the net weight at each
weighing point. Multiply each net weight by its arm to
determine its moment, then determine the total weight and
total moment. The CG is determined by dividing the total
moment by the total weight.
Tare weight: The weight of any
chocks or devices used to hold an
aircraft on the scales when it is
weighed. The tare weight must be
subtracted from the scale reading to
obtain the net weight of the aircraft.
Figure 3-3. The datum is located at the firewall.
The airplane in Figures 3-3 and 3-4 has a net weight of 2,006
pounds, and its CG is 32.8 inches behind the datum.
3–5
Empty-Weight Center
of Gravity Formulas
A chart like the one in Figure 3-4 helps visualize the weights,
arms, and moments when solving an EWCG problem, but it
is quicker to determine the EWCG by using formulas and an
electronic calculator. The use of a calculator for solving these
problems is described in Chapter 8.
There are four possible conditions and their formulas that
relate the location of the CG to the datum. Notice that the
formula for each condition first determines the moment
of the nose wheel or tail wheel and then
divides it by the total weight of the airplane. The arm thus
　
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