Crabwise · Library
How should a removed load be represented without entering a negative final station weight?
Removing 120 of 200 units at arm 30 leaves an 80-unit row and moves the CG aft from 41.54 to 42.71, because the removed load sat forward of it.
Enter what remains, not a negative row
Crabwise's station list describes what is on board. To model an unloaded item, reduce the station's weight to what remains, or delete the row if nothing is left, and calculate again. A negative row would make the arithmetic work but describe a load that does not exist, and the Library worksheet rejects negative station weights outright.
Whether the CG moves forward or aft depends on where the removed weight sat relative to the current CG: removing weight forward of it moves the CG aft, removing weight aft of it moves the CG forward.
Worked example
With 1000 units at arm 40 and 50 remaining at arm 80, the model gives CG about 41.905. The original case with 100 at arm 80 had CG about 43.636.
- Remaining total weight
- 1050
- Remaining total moment
- 44000
- Remaining CG arm
- 41.904762
Record the change
- Save the full loading as it was before anything is removed.
- Change only the affected station to its remaining weight.
- Read the new total weight, total moment and CG.
- Keep a note of how much was removed and from which station, outside the station list.
Unloading part of a forward station
Start with 1000 at 40, 200 at 30 and 100 at 80: total weight 1300, moment 54000, CG ≈ 41.538462. Remove 120 units from the arm-30 station, leaving 80 there. Total weight becomes 1180 and total moment 40000 + 2400 + 8000 = 50400, so the CG ≈ 42.711864.
- Remaining weight at arm 30
- 80
- Total weight after removal
- 1180
- Total moment after removal
- 50400
- CG after removal
- 42.711864
The removed 120 units were at arm 30, forward of the original CG, so the CG moves aft even though the aircraft is lighter.
The moment drops by exactly 120 × 30 = 3600, from 54000 to 50400. Checking that difference against the removed weight times its arm confirms that only the intended station changed.
Common mistakes
- Adding a row of −120 at arm 30 instead of editing the existing station.
- Assuming a lighter aircraft always has a more forward CG.
- Deleting the original loading so the before-and-after comparison is lost.
- Changing the arm of the station instead of its weight when part of the load comes off.
Scope
A lighter total does not show the loading is acceptable. The FAA weight-and-balance chapter below covers removing weight; the aircraft's data sets the limits.
Educational calculation only. Check current aircraft limits, weather, performance and reserve requirements with approved information.