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CG location in percent of MAC, and an indicator of the
ground attitude of the aircraft.
The strain sensors measure the amount each axle deflects
and sends this data into the computer, where signals from
all of the transducers and the ground attitude sensor are
integrated. The results are displayed on the indicators for
the flightcrew.
7–
Figure 7-2. Loading schedule for determining weight and CG.
7–
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 Percent of 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 Percent of ANU
(Airplane Nose Up)
Some aircraft give the stabilizer trim setting in Units ANU
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 7-3 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:
Then determine the location of the CG in percent of MAC
by using this formula:
Refer to Figure 7-3. For all flap settings and a CG located
at 8% MAC, the stabilizer setting is 73/4 Units ANU.
Figure 7-3. 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..................................................141.1 in
LEMAC................................... station 549.13
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:
7–
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.
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 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 following 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
Fwd. cargo hold centroid........... station 352.1
MAC..................................................141.5 in
LEMAC................................... station 549.13
1. Determine the CG location in inches from the
datum before the cargo is onloaded. Do this by first
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Aircraft Weight and Balance Handbook(25)
