Crabwise · Library

Why must all station arms use the same datum?

Measured from a datum 10 units further aft, arms 40 and 80 become 30 and 70 and the CG reads 33.64 instead of 43.64; mixing the two gives 42.73.

One reference for every arm

Crabwise multiplies each weight by the arm you enter and divides the total moment by the total weight. It has no datum field: the datum is whatever reference all your arms were measured from. If every arm shifts by the same amount, the CG shifts by that amount too and describes the same physical point. If only some arms are measured from a different reference, the result is a CG that matches no real position.

The app's unit note asks for pounds with inches or kilograms with millimetres, so the arm unit must also be the same across rows.

Worked example

Two educational stations, 1000 units at arm 40 and 100 units at arm 80, give 48000 moment units and a centre of gravity of about 43.636 arm units. No aircraft limits are supplied by these numbers.

Total weight (chosen common unit): 1100
Total moment (weight unit × arm unit): 48000
Centre of gravity from the common datum (arm unit): 43.636364

Check the station definitions

Find the datum stated in the aircraft's weight-and-balance data.
Confirm that every arm you copy is measured from that datum, with the same sign convention.
Enter the stations and check each moment line.
Use an envelope only if its CG axis uses the same datum.

Shifting and mixing references

With 1000 at arm 40 and 100 at arm 80, the CG is 48000 ÷ 1100 ≈ 43.636364. Measured from a datum 10 units further aft, the arms are 30 and 70, total moment 37000 and CG ≈ 33.636364: exactly 10 less, the same point. If only the second station is re-measured, giving 1000 at 40 and 100 at 70, the moment is 47000 and the CG ≈ 42.727273, which belongs to neither reference.

CG, original datum: 43.636364
CG, datum 10 units aft: 33.636364
CG, mixed references: 42.727273

Common mistakes

Copying a seat arm from one document and a baggage arm from another.
Entering an arm aft of the datum with the wrong sign when the datum is inside the aircraft.
Comparing a CG with an envelope drawn for a different datum.

Scope

A consistent calculation can still rest on a wrong datum. The FAA weight-and-balance chapter and the NIST unit table below explain datums and units; the aircraft's records define them.

Educational calculation only. Check current aircraft limits, weather, performance and reserve requirements with approved information.