See how fuel quantity, density, tank position and fuel burn change aircraft weight, centre of gravity, lateral balance and legal limits.
In aviation and real-world flying, fuel increases an aircraft’s total weight and changes its balance according to where it sits relative to the datum. As fuel burns, weight falls and the centre of gravity may move forward, aft or sideways. Loading calculations must account for fuel quantity, density, tank location and burn sequence.
Fuel therefore changes two separate values: total aircraft weight and the moments used to calculate centre of gravity (CG). Detailed flight simulators model the same relationship, although the accuracy of their fuel systems and loading pages varies.
How is fuel included in aircraft weight?
For loading purposes, fuel weight is calculated as fuel weight = volume × density. Litres, Imperial gallons and US gallons are volume units, not weight units, so they must be converted using the approved fuel density and the correct unit system.
Density varies with fuel type and temperature. Generic approximations may be suitable for rough planning, but the aircraft flight manual, pilot’s operating handbook or operator data takes precedence.
Fuel aboard before engine start forms part of ramp weight. Expected start and taxi consumption is deducted to obtain take-off weight, while planned flight consumption is deducted to estimate landing weight. Reserve and diversion fuel remains aboard at landing rather than disappearing from the calculation.
Usable fuel is normally added as a separate load, while unusable fuel is commonly included in the aircraft’s empty-weight figure. Definitions can differ, so verify which fuel counts as usable and how unusable fuel is treated before adding anything.
How does tank position change centre of gravity?
Fuel moves the CG because each tank has an arm: its distance from the aircraft datum. Its contribution is calculated as moment = fuel weight × arm, with the aircraft CG obtained by dividing total moment by total weight.
| Tank position | Effect when fuel is added | Effect as fuel burns |
|---|---|---|
| Ahead of the existing CG | CG moves forward | CG tends to move aft |
| Near the existing CG | Little longitudinal movement | Little longitudinal movement |
| Behind the existing CG | CG moves aft | CG tends to move forward |
A tank’s effective arm may change as its level falls because tank shape is rarely a perfect rectangular box. If the approved loading data provides fuel moments or index values for specific quantities, use those instead of assuming one fixed arm.
Wing tanks are often close to the longitudinal CG, limiting CG movement while also affecting lateral balance and structural loading. Our explanation of why fuel tanks are commonly located in the wings covers those design reasons.
Does burning fuel always improve weight and balance?
No. Burning fuel reduces gross weight, but it can move the CG towards or beyond a limit depending on which tanks supply the engines.
An aircraft may be within its take-off envelope yet approach a different CG boundary at landing because the allowable envelope can change with weight. Aircraft with several tanks may use a prescribed burn or transfer sequence to control this movement; some systems handle it automatically, while others require crew action.
Fuel burn also does not change zero-fuel weight because passengers, baggage and cargo remain aboard. If an aircraft exceeds its maximum zero-fuel weight, adding more fuel does not cure the loading error, and planning to burn fuel after departure cannot make an illegal take-off condition acceptable.
Even within certified limits, carrying extra fuel increases take-off distance and stall speed while reducing climb performance. CG position also affects stability, control authority and trim drag; these control and performance consequences of aircraft loading explain why merely staying below maximum weight is not enough.
How is fuel imbalance different from longitudinal CG?
Unequal fuel quantities in left and right wing tanks create a lateral imbalance and a rolling moment towards the heavier wing. Aircraft publish maximum imbalance limits and approved correction procedures.
Opening a crossfeed valve does not necessarily transfer fuel directly from one tank to another. Depending on the system, it may instead allow one or both engines to consume fuel from the fuller tank. Pumps, selectors and transfer valves must be used only as prescribed for that aircraft.
How should fuel be entered in a weight-and-balance calculation?
Fuel should be entered by tank and by flight phase, using the aircraft’s approved loading data.
- Confirm the empty condition. Check the current empty weight, moment or CG, and whether unusable fuel is already included.
- Identify the actual fuel quantity. Separate usable fuel by tank rather than treating the entire load as one central mass.
- Convert volume to weight. Use the correct units and approved density for the fuel being carried.
- Apply each tank’s loading data. Use its arm, moment or index table. The full weight, arm and moment calculation method explains the arithmetic.
- Check every relevant phase. Confirm ramp, take-off and estimated landing weights, longitudinal CG, lateral imbalance and any structural fuel limits.
What mistakes cause incorrect fuel balance results?
The most common errors come from wrong units, double-counted fuel and unrealistic tank distribution.
- Confusing volume with weight: 100 litres of fuel does not weigh 100 kilograms.
- Mixing gallon types: an Imperial gallon and a US gallon are different volumes.
- Adding unusable fuel twice: it may already be included in the published empty weight.
- Using one fuel arm: separate tanks can have substantially different effects on CG.
- Checking only maximum take-off weight: an aircraft can be under that limit but outside its CG envelope or maximum landing weight.
- Assuming equal fuel burn: selectors, crossfeed operation, transfer systems or an abnormal condition may change which tank empties first.
- Trusting a simulator percentage blindly: a 50% setting may fill every defined tank halfway rather than represent one central fuel load. Inspect the per-tank quantities where the aircraft provides them.
What should be changed if the aircraft is outside limits?
The correction depends on which limit has been exceeded. An overweight aircraft needs payload removed or fuel reduced without compromising required reserves; a longitudinal CG error usually requires passengers, baggage or cargo to be repositioned.
Fuel distribution should be changed only through approved tank-loading or transfer procedures. A lateral imbalance requires the aircraft-specific balancing procedure, while a maximum zero-fuel-weight exceedance requires payload removal rather than extra fuel.