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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:
7–
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.
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:
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:
7–
Convert the location of the CG in inches aft of the datum
to % MAC by using this formula:
The new CG in % MAC caused by shifting the cargo is the
sum of the old CG plus the change in CG:
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.
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
inches, and the new CG is:
The change in the CG in % MAC is determined by using
this formula:
The new CG in % MAC is the sum of the old CG plus the
change in CG:
Notice that the new CG is in the same location whether the
distances are measured from the datum or from the index
point.
Determining Cargo Pallet Loads with Regard
to Floor Loading Limits
Each cargo hold has a structural floor loading limit based
on the weight of the load and the area over which this
weight is distributed. To determine the maximum weight
of a loaded cargo pallet that can be carried in a cargo hold,
divide its total weight, which includes the weight of the
empty pallet and its tiedown devices, by its area in square
feet. This load per square foot must be equal to or less than
the floor load limit.
In this example, determine the maximum load that can be
placed on this pallet without exceeding the floor load limit.
Pallet dimensions.......................................36 by 48 in
Empty pallet weight........................................... 47 lbs
　
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