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Fwd. cargo hold centroid ................................. station 352.1
MAC .......................................................................... 141.5 in
LEMAC .......................................................... station 549.13
CG Shift
When the CG moves aft, ? CG is positive; when it moves forward,
? CG is negative.
6– 7
1. Determine the CG location in inches from the datum
before the cargo is onloaded. Do this by first determining
the distance of the CG aft of the LEMAC:
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 added weight. Multiply
the amount of weight added (3,000 pounds) by the
centroid of the forward cargo hold (352.1 inches), and then
divide this by 1,000.
5. Add the onloaded cargo weight and its moment/1,000
to 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 of 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:
Onloading 3,000 pounds of cargo into the forward cargo hold
moves the CG forward 5.51 inches, from 27.1% MAC to
21.59% MAC.
6– 8
Effects of Shifting Cargo from
One Hold to Another
When cargo is shifted from one cargo hold to another, the CG
changes, but the total weight of the aircraft remains the same.
As an example, use this data:
Loaded weight ...................................................... 90,000 lbs
Loaded CG .............. station 580.97 (which is 22.5% MAC)
Fwd. cargo hold centroid .................................... station 352
Aft cargo hold centroid .................................... station 724.9
MAC .......................................................................... 141.5 in
LEMAC ............................................................... station 549
To determine the change in CG, or ? CG, caused by shifting
2,500 pounds of cargo from the forward cargo hold to the aft
cargo hold, use this formula:
The new CG in % MAC caused by shifting the cargo is the
sum of the old CG plus the change in CG:
Index point: A location specified by
the aircraft manufacturer from which
arms used in weight and balance
computations are measured. Arms
measured from the index point are
called index arms.
Since the weight was shifted aft, the CG moved aft, and the
CG change is positive. If the shift were forward, the CG
change would be negative.
Before the cargo was shifted, the CG was located at station
580.97, which is 22.5% MAC. The CG moved aft 10.36
inches, so the new CG is:
Convert the location of the CG in inches aft of the datum to
percent MAC by using this formula:
Some aircraft AFMs locate the CG relative to an index point
rather than the datum or the MAC. An index point is a location
specified by the aircraft manufacturer from which arms used in
weight and balance computations are measured. Arms measured
from the index point are called index arms, and objects ahead of
the index point have negative index arms, while those behind the
index point have positive index arms.
Use the same data as in the previous example, except for
these changes:
Loaded CG ..........index arm of 0.97, which is 22.5% MAC
Index point ......................................... fuselage station 580.0
Fwd. cargo hold centroid ........................ –227.9 index arm
Aft cargo hold centroid ........................... +144.9 index arm
MAC .......................................................................... 141.5 in
LEMAC ................................................... –30.87 index arm
The weight was shifted 372.8 inches (–227.9 to +144.9 =
372.8).
The change in CG can be calculated by using this formula:
Since the weight was shifted aft, the CG moved aft, and the
CG change is positive. If the shift were forward, the CG
change would be negative.
6– 9
Then determine the total weight of the loaded pallet:
Pallet 44.0 lbs
Tiedown devices 27.0 lbs
Cargo 786.5 lbs
857.5 lbs
Determine the load imposed on the floor by the loaded pallet:
The floor must have a minimum load limit of 76 pounds per
square foot.
Before the cargo was shifted, the CG was located at 0.97
index arm, which is 22.5% MAC. The CG moved aft 10.36
　
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