Figure 3-5. Locating the CG of an airplane relative to the datum.
Weighing point Net weight (lb)TARE weight (lb)Scale reading (lb) Arm (in) Moment (lb-in) CG
46.0
46.0
−32.0
32.8
Right side
Left side
Nose
Total
16
16
8
830
836
340
2,006
38,180
38,456
−10,880
65,756
846
852
348
Configuration of the Aircraf
Consult the aircraft service manual regarding position of
the landing gear shock struts and the control surfaces for
weighing. When weighing a helicopter, the main rotor must
be in its correct position.
Jacking the Aircraft
Aircraft are often weighed by rolling them onto ramps in
which load cells are embedded. This eliminates the problems
associated with jacking the aircraft off the ground. However,
many aircraft are weighed by jacking the aircraft up and then
lowering them onto scales or load cells.
Extra care must be used when raising an aircraft on jacks for
weighing. If the aircraft has spring steel landing gear and it
is jacked at the wheel, the landing gear will slide inward as
the weight is taken off of the tire, and care must be taken to
prevent the jack from tipping over.
For some aircraft, stress panels or plates must be installed before
the aircraft is raised with wing jacks to distribute the weight
over the jack pad. Be sure to follow the recommendations
of the aircraft manufacturer in detail anytime an aircraft is
jacked. When using two wing jacks, take special care to raise
them simultaneously, keeping the aircraft so it does not slip
off the jacks. As the jacks are raised, keep the safety collars
screwed down against the 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 fligh
attitude so that all of the components are 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
nosewheel 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.
Determining the CG
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 nosewheel
or the tailwheel. These lines and marks on the floor allow
accurate measurements between the datum and the weighting
points to determine their arms.
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 as
detailed in the Figure 3-5 table and illustrated in Figure 3-6.
When the airplane is on the scales with the parking brakes
Figure 3-9. Determining the CG with datum forward of an airplane
with nosewheel landing gear.
CG F × L
W= D −
= 128 −
114.8=
( )
340 × 78
2,006( )
Figure 3-8. The datum is 100 inches forward of the wing root
leading edge.
D = 128.0
100.0 13.2
50
114.8
Datum
L = 78.0
Figure 3-10. The datum is aft of the main wheels at the wing
trailing edge.
153
D = 75
88.2
Datum
L = 78.0
Figure 3-7. Determining the CG.
CG Total moment
Total weight=
65,756
2,006=
32.8 inches behind the datum=
Figure 3-6. The datum is located at the firewall.
46.0
32
32.8
Datum
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. [Figure 3-7]
The airplane illustrated in Figures 3-5 and 3-6 has a net
weight of 2,006 pounds, and its CG is 32.8 inches behind
the datum.
EWCG Formulas
A chart such as the one in Figure 3-5 helps the pilot 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, Use of
Computers in Weight and Balance Computations.
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 nosewheel or tailwheel 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.
Datum Forward of the Airplane—Nosewheel
Landing Gear
The datum of the airplane in Figure 3-8 is 100 inches forward
of the leading edge of the wing root or 128 inches forward
of the main-wheel weighing points. This is distance (D). The
weight of the nosewheel (F) is 340 pounds, and the distance
between main wheels and nosewheel (L) is 78 inches. The
total weight of the airplane (W) is 2,006 pounds. Determine
the CG by using the formula in Figure 3-9.
The CG is 114.8 inches aft of the datum. This is 13.2 inches
forward of the main-wheel weighing points, which proves
the location of the datum has no effect on the location of the
CG if all measurements are made from the same location.
Datum Aft of the Main Wheels—Nosewheel
Landing Gear
The datum of some aircraft may be located aft of the main
wheels. The airplane in this example is the same one just
discussed, but the datum is at the intersection of the trailing
edge of the wing with the fuselage. The distance (D) between
the datum of the airplane in Figure 3-10 and the main-wheel
Figure 3-13. Determining the CG with datum forward of the main
wheels in an airplane with tailwheel landing gear.
CG R × L
W= D +
= 7.5 +
19.7=
( )
67 × 222
1,218( )
Figure 3-11. Determining the CG with datum aft of the main wheels
of an airplane with nosewheel landing gear.
CG F × L
W= − D +
= − 75 +
−88.2=
( )
340 × 78
2,006( )
Figure 3-12. The datum of this tailwheel airplane is the wing root
leading edge.
D = 7.5
Datum
229.5
L = 222.0
19.7
Datum
L = 222.0
D = −80
142
67.8
Figure 3-14. The datum is aft of the main wheels, at the intersection
of the wing trailing edge and the fuselage.
CG R × L
W= −D +
= −80 +
= −67.8
( )
67 × 222
1,218( )
Figure 3-16. Determining the CG with datum aft of the main wheels
in an airplane with tailwheel landing gear.
weighing points is 75 inches, the weight of the nosewheel
(F) is 340 pounds, and the distance between main wheels
and nosewheel (L) is 78 inches. The total net weight of the
airplane (W) is 2,006 pounds.
The location of the CG may be determined by using the
formula in Figure 3-11.
The CG location is a negative value, which means it is 88.2
inches forward of the datum. This places it 13.2 inches
forward of the main wheels, exactly the same location as
when it was measured from other datum locations.
Location of Datum
The location of the datum is not important, but all
measurements must be made from the same location.
Datum Forward of the Main Wheels—Tailwheel
Landing Gear
Locating the CG of a tailwheel airplane is done in the same
way as locating it for a nosewheel airplane except the formula
is .
The distance (D) between the datum of the airplane in
Figure 3-12 and the main-gear weighing points is 7.5 inches,
the weight of the tailwheel (R) is 67 pounds, and the distance
(L) between the main-wheel and the tailwheel weighing points
is 222 inches. The total weight of the airplane (W) is 1,218
pounds. Determine the CG by using the formula in Figure 3-13.
R × L
W
The CG is 19.7 inches behind the datum.
Datum Aft of the Main Wheels—Tailwheel
Landing Gear
The datum of the airplane in Figure 3-14 is located at the
intersection of the wing root trailing edge and the fuselage.
This places the arm of the main gear (D) at –80 inches. The
net weight of the tailwheel (R) is 67 pounds, the distance
between the main wheels and the tailwheel (L) is 222 inches,
and the total net weight (W) of the airplane is 1,218 pounds.
Since the datum is aft of the main wheels, use the formula
found in Figure 3-15.
The CG is 67.8 inches forward of the datum or 12.2 inches
aft of the main-gear weighing points. The CG is in exactly
the same location relative to the main wheels, regardless of