Crabwise · Library
How can two fuel-load states be compared for their effect on centre of gravity?
Compare full and half fuel as two loadings: 30 US gal of avgas is 180 lb at arm 48, and burning to 15 gal moves the CG from 39.96 to 39.51.
Fuel as one more station
Crabwise has no separate fuel-burn model in weight and balance. Fuel is entered as a station: a weight and an arm. To see the effect of burning fuel, you calculate two complete loadings, one with the starting fuel weight and one with the remaining weight, and compare their CG values.
Fuel quantities are usually known as volume. The unit screen converts volume to weight with its planning densities, 6.0 lb per US gallon for avgas and 6.7 for Jet-A, which are assumptions rather than measured values.
Worked example
The first teaching case uses 200 weight units at the fixed example arm 60.
- Total with 200 example fuel-weight units
- 1200
- CG arm in the first stated case
- 43.333333
Worked example
The second case uses 100 remaining units at that same assumed arm. It does not establish that a real tank has a constant arm or that its intermediate states have been checked.
- Total with 100 example fuel-weight units
- 1100
- CG arm in the second stated case
- 41.818182
Build the two cases
- Convert each fuel quantity to weight on the unit screen and note the density used.
- Enter the fixed stations and the starting fuel weight at the tank arm.
- Record total weight and CG.
- Change only the fuel row to the remaining weight and record the second result.
- Check both points against the envelope if you entered one.
Full and half tanks
Take a fixed load of 1500 lb at arm 39 and a tank at arm 48. With 30 US gal of avgas the fuel row is 30 × 6 = 180 lb; total weight is 1680 lb, total moment 58500 + 8640 = 67140 and CG ≈ 39.964286. With 15 gal the fuel row is 90 lb; total weight 1590 lb, moment 62820 and CG ≈ 39.509434.
- Total weight with 30 US gal (lb)
- 1680
- CG with 30 US gal
- 39.964286
- Total weight with 15 US gal (lb)
- 1590
- CG with 15 US gal
- 39.509434
Because the tank arm is aft of the fixed load's CG, burning fuel here moves the CG forward.
Common mistakes
- Entering fuel in gallons in a station list that is otherwise in pounds.
- Checking only the take-off loading and assuming the landing loading follows.
- Using one arm for tanks that sit at different positions.
Scope
Two endpoints do not show the path between them, and the app does not model tank sequencing or an arm that shifts with quantity. The FAA weight-and-balance chapter below covers fuel as a loading item; the aircraft's data gives the tank arms.
Educational calculation only. Check current aircraft limits, weather, performance and reserve requirements with approved information.