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Basic operating weight (BOW):
The empty weight of the aircraft plus
the weight of the required crew, their
baggage and other standard items
such as meals and potable water.
6– 4
Figure 6-2. Loading schedule for determining weight and CG.
6– 5
Determining the Correct Stabilizer
Trim Setting
It is important before takeoff to set the stabilizer trim for the
existing CG location. There are two ways the stabilizer trim
setting systems may be calibrated: in % MAC, and in Units
ANU (Airplane Nose Up).
Stabilizer Trim Setting in % MAC
If the stabilizer trim is calibrated in units of % MAC, determine
the CG location in % MAC as has just been described, then
set the stabilizer trim on the percentage figure thus determined.
Stabilizer Trim Setting in Units ANU
(Airplane Nose Up)
Some aircraft give the stabilizer trim setting in Units ANU
(Airplane Nose Up) that correspond with the location of the
CG in % MAC. When preparing for takeoff in an aircraft
equipped with this system, first determine the CG in % MAC
in the way described above, then refer to the Stabilizer Trim
Setting Chart on the Takeoff Performance page of the AFM.
Figure 6-3 is an excerpt from such a page from the AFM of a
Boeing 737.
Consider an airplane with these specifications:
CG location ....................................................... station 635.7
LEMAC ............................................................... station 625
MAC .......................................................................... 134.0 in
First determine the distance from the CG to the LEMAC by
using this formula:
Figure 6-3. Stabilizer trim setting in ANU units.
Determining CG Changes Caused
by Modifying the Cargo
Large aircraft carry so much cargo that adding, subtracting,
or moving any of it from one hold to another can cause large
shifts in the CG.
Effects of Loading or Offloading Cargo
Both the weight and CG of an aircraft are changed when cargo
is offloaded or onloaded. This example shows the way to
determine the new weight and CG after 2,500 pounds of cargo
is offloaded from the forward cargo hold.
Consider these specifications:
Loaded weight ...................................................... 90,000 lbs
Loaded CG ........................................................ 22.5% MAC
Weight change ..................................................... –2,500 lbs
Fwd. cargo hold centroid ................................ station 352.1
MAC .......................................................................... 141.5 in
LEMAC .......................................................... station 549.13
Then determine the location of the CG in percent of MAC
by using this formula:
Refer to Figure 6-3. For all flap settings and a CG located at
8% MAC, the stabilizer setting is 73/4 Units ANU.
Centroid: The distance in inches aft
of the datum of the center of a
compartment or a fuel tank for
weight and balance purposes.
6– 6
1. Determine the CG location in inches from the datum
before the cargo is removed. Do this by first determining
the distance of the CG aft of the LEMAC:
6. Determine the location of the new CG by dividing the
total moment/1,000 by the total weight and multiplying
this by the reduction factor of 1,000.
2. Determine the distance between the CG and the datum
by adding the CG in inches aft of LEMAC to the distance
from the datum to LEMAC:
3. Determine the moment/1,000 for the original weight:
4. Determine the new weight and new CG by first
determining the moment/1,000 of the removed weight.
Multiply the amount of weight removed (–2,500 pounds)
by the centroid of the forward cargo hold (352.1 inches),
and then divide this by 1,000.
5. Subtract the removed weight and its moment/1,000 from
the original weight and moment/1,000.
7. Convert the new CG location to % MAC. First, determine
the distance between the CG location and LEMAC:
8. Then, determine new CG in % MAC:
Offloading 2,500 pounds of cargo from the forward cargo
hold moves the CG from 22.5% MAC to 27.1% MAC.
Effects of Onloading Cargo
The previous example showed the way the weight and CG
changed when cargo was offloaded. This example shows the
way both parameters change when cargo is onloaded.
The same basic airplane is used in the example, but 3,000
pounds of cargo is onloaded in the forward cargo hold.
Weight before cargo is loaded ............................. 87,500 lbs
CG before cargo is loaded ............................... 27.1% MAC
Weight change ..................................................... + 3,000 lbs
　
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