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jack cylinder to prevent the aircraft from tilting if one of
the jacks should lose hydraulic pressure.
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 light aircraft,
but provisions are normally made on 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.
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
devices are against the jack cylinder to prevent the
aircraft tipping if any jack should lose pressure. Not all
jacks have screw down collars, some use drop pins or
friction locks.
3–
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 weighting
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 weighting points to determine their arms.
Figure 3-3. The datum is located at the firewall.
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, and then determine the
total weight and total moment. The CG is determined by
dividing the total moment by the total weight.
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.
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 leading edge mean aerodynamic
cord (LEMAC), is a specified number of inches from the
datum.
Figure 3-4. Locating the CG of an airplane relative to the datum, which is located at the firewall. See Figure 3-3.
3–
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 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
determined is then added to or subtracted from the distance
between the main wheels and the datum, distance D.
　
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