Why is aircraft weight and balance important?
Aircraft weight and balance are important because an overloaded aircraft may not take off, climb, land or withstand manoeuvres safely, while an out-of-limits centre of gravity can make pitch control ineffective or unstable. Pilots must keep both total weight and centre of gravity within approved limits for every flight.
In our Aviation & Real-World Flying coverage, we treat weight and balance as two separate tests. An aircraft can be below its maximum weight but dangerously out of balance; it can also have a perfectly acceptable centre of gravity while exceeding a structural or operational weight limit.
What do aircraft weight and balance mean?
Weight is the aircraft's total loaded weight, including its empty weight, occupants, baggage, cargo and usable fuel. It must remain below every applicable limit, which may include maximum ramp, take-off, landing and zero-fuel weights.
Balance describes where that weight acts through the aircraft's centre of gravity, or CG. The CG is usually expressed as a distance from a reference datum or, on many larger aircraft, as a percentage of mean aerodynamic chord.
The datum is an arbitrary reference selected by the manufacturer; it is not necessarily the aircraft's nose or firewall. Arms, moments and CG limits from one aircraft model must never be substituted for those of another.
The permitted CG range may also change with weight, producing a loading envelope rather than one universal forward and aft limit. Some aircraft, particularly helicopters and aircraft with unusual loading arrangements, also have lateral CG limits.
What happens if aircraft weight or CG is outside limits?
An overweight or out-of-balance aircraft may lack the performance, control authority or structural margin required for safe flight.
| Loading problem | Likely effects |
|---|---|
| Above maximum weight | Higher stall and reference speeds, longer take-off and landing distances, poorer climb, greater brake energy and increased structural loads. |
| CG too far forward | Heavy pitch forces, more tail-down force on a conventional tailplane, longer take-off run and insufficient elevator authority to rotate or flare. |
| CG too far aft | Reduced longitudinal stability, excessive pitch sensitivity and more difficult stall or spin recovery. The aircraft may rotate readily but be harder to control. |
| Lateral imbalance or shifted cargo | Persistent rolling tendency, uneven control authority and an unexpected CG change during flight. |
A forward CG is not automatically safe merely because it makes the aircraft feel stable. If the elevator cannot raise the nose at take-off or arrest the descent during landing, that stability has become a control problem. Excessive weight and forward CG are among the loading faults covered in our guide to diagnosing an aircraft that will not rotate in a simulator.
Maximum certified weight is not a promise that the aircraft can use every runway. Temperature, pressure altitude, wind, runway surface, obstacles and aircraft condition still require a separate performance calculation. A heavier aircraft's stall speed increases approximately with the square root of the weight ratio when configuration and load factor remain the same.
How is aircraft weight and balance calculated?
Aircraft weight and balance is calculated by adding every load and its moment, dividing total moment by total weight, and checking the result against the aircraft-specific loading envelope.
- Use the correct aircraft data. Start with the latest approved empty weight, empty moment, station arms and loading envelope for that aircraft. Equipment changes can make an old weighing record inaccurate.
- List every load. Include occupants, baggage, cargo and usable fuel at their assigned stations. Use actual weights or authorised standard weights according to the applicable procedure.
- Calculate each moment. The basic formula is
moment = weight × arm. Keep pounds and kilograms, inches and metres, and any published moment index consistent throughout. - Find the loaded CG. Use
CG arm = total moment ÷ total weight. If the aircraft uses percentage of mean aerodynamic chord, apply the conversion specified in its loading data. - Check every applicable limit. Plot the total weight and CG within the approved envelope, then verify ramp, take-off, landing and zero-fuel conditions where those limits are published. Our explanation of the relationship between zero-fuel weight, payload and fuel covers this transport-aircraft limit in more detail.
- Run the performance calculation. Recalculate take-off speeds, runway distance, climb capability and landing performance for the accepted load.
Common weight-and-balance calculation mistakes
Most loading errors come from using correct arithmetic with incorrect inputs.
- Checking only maximum gross weight and ignoring CG position.
- Mixing pounds with kilograms or treating gallons and litres as units of weight.
- Using the wrong fuel density or failing to account for fuel at its assigned tank arm.
- Entering baggage at the wrong station or overlooking an individual compartment limit.
- Using an obsolete empty weight after equipment has been installed or removed.
- Misreading a moment index that has been divided by a factor specified in the loading data.
- Calculating a safe load but failing to secure baggage or cargo, allowing it to move in flight.
Does fuel burn change aircraft balance?
Fuel burn reduces total weight and can move the centre of gravity forward or aft, depending on tank locations and the order in which fuel is used.
The landing CG therefore cannot always be found by subtracting fuel weight alone; its moment must also be removed. Aircraft with fuel transfer systems or several tanks may have a non-linear CG movement, so crews use the type's approved loading tables or onboard calculation rather than assuming all fuel acts at one average arm.
Passengers moving, cargo shifting, stores being released and water or chemicals being discharged can also change balance. The aircraft must remain within its envelope throughout the planned flight, not merely while parked before departure.
Correcting an unsafe aircraft load
An out-of-limits load must be redistributed or reduced before flight; trim cannot make an invalid loading condition safe.
- Move payload to another approved seat, cargo station or baggage compartment without exceeding that station's limit.
- Remove payload when total weight or a compartment limit is exceeded.
- Adjust fuel only when range, reserve, tank and performance requirements remain satisfied.
- Use approved ballast when the aircraft's procedures permit it, remembering that ballast also increases total weight and must be secured.
- Recalculate everything after any change rather than estimating that the correction is sufficient.
Take-off trim is set for the valid CG calculated from the load; it is not a cure for a CG beyond the approved envelope. If no practical loading change satisfies every limit, the aircraft must not depart in that condition.
Weight and balance in flight simulators
Weight and balance still matters in a flight simulator because higher-fidelity aircraft use loaded weight and CG to determine acceleration, rotation, climb, pitch response, stall behaviour and landing energy.
A mistake we see constantly is entering payload in both the simulator's general loading screen and an aircraft-specific tablet or load manager, then assuming the two systems have synchronised. Confirm the resulting gross weight, fuel quantity and CG in the aircraft's own displays or documentation, and check whether the interface expects pounds, kilograms, gallons or litres.
Some simulators and add-ons will allow an impossible load without displaying structural damage, but the handling and performance can still be badly affected. An overweight arrival demands higher speeds and more runway; our practical example of managing an overweight A320 landing shows how that changes the approach and stopping problem.
After changing payload or fuel, recalculate take-off trim, speeds and performance. If the aircraft remains difficult to rotate or control, verify the loading entries before compensating with extreme trim or blaming the flight model.