Two aeroplanes at the exact same weight can handle very differently. Total mass tells you whether the aeroplane can take off; centre of gravity tells you how it will fly once it's up. This is a weight and balance calculation in the term most pilots search for, but mass and balance is the correct EASA wording, and both halves of it must be within limits. Passing one does not excuse the other.
The idea: mass × arm = moment
Every load on board (pilot, passengers, fuel, baggage) exerts a turning effect around a fixed reference point on the aeroplane, called the datum.
- The arm is the distance between the datum and where the load sits. It comes straight from the flight manual.
- The moment is
mass × arm.
The aircraft's overall centre of gravity is found by adding everything up:
CG = total moment ÷ total mass
That's the whole formula. The difficulty is never the arithmetic — it's forgetting a line item, or mixing units halfway through.
A worked example
| Item | Mass (kg) | Arm (m) | Moment |
|---|---|---|---|
| Basic empty mass | 700 | 2.20 | 1,540 |
| Pilot + front passenger | 150 | 2.05 | 307.5 |
| Rear passengers | 70 | 3.00 | 210 |
| Fuel | 90 | 2.40 | 216 |
| Baggage | 15 | 3.60 | 54 |
| Total | 1,025 | 2,327.5 |
CG = 2,327.5 ÷ 1,025 = 2.27 m
Two checks follow, never just one:
- Is the total mass (1,025 kg) below the maximum take-off mass (MTOM)?
- Does the CG (2.27 m) fall inside the CG envelope for the flight manual, at that mass?
American flight manuals run this same arithmetic in pounds and inches rather than kilograms and metres; the method doesn't change, only the units do.
The envelope is an area, not a line
The CG envelope is a mass/balance graph. The forward and aft limits shift with mass — they often tighten at the higher end. A centre of gravity acceptable at 900 kg can sit outside the envelope at 1,050 kg on the same aeroplane.
Check the CG at take-off and at landing. Fuel burns off, and its position moves the CG through the flight. Depending on the aeroplane's geometry, a flight that starts well inside the envelope can finish at the aft limit.
Remember: a balance calculation done only for take-off is half a calculation.
Forward CG: heavy nose, stable, thirsty
With the CG near the forward limit, the aeroplane is very stable in pitch — it returns to trimmed attitude on its own. The cost:
- lost lift: the tailplane must push down harder, and the wing has to compensate for it;
- higher stall speed;
- longer take-off roll and a heavier rotation;
- worse cruise performance and higher fuel burn;
- a flare that demands a lot of elevator effort on landing, up to the risk of running out of aft travel.
Aft CG: light, unstable, dangerous
With the CG near the aft limit, or beyond it, the aeroplane actually performs better: less drag, better cruise, a lower stall speed. That is exactly what makes this case so easy to walk into.
The trade-offs are severe:
- reduced longitudinal stability: the aeroplane no longer returns to trim by itself, it has to be flown continuously;
- very light control forces, so overcontrolling is easy;
- recovery from a stall or spin can become impossible — this is the critical point, not a comfort issue;
- a markedly sharper stall break.
An out-of-limits aft CG says nothing during taxi. It shows up at rotation, when the nose pitches up faster than expected, at the exact moment there's no option left.
The mistakes that keep coming back
- Using a generic empty mass instead of the specific aircraft's own figure: the weighing schedule is unique to each airframe and changes after every modification.
- Confusing litres with kilograms of fuel. Avgas weighs roughly 0.72 kg/l: 100 litres is not 100 kg, it's about 72 kg.
- Forgetting last-minute baggage, the bag loaded after the calculation was already done.
- Not redoing the sums when the load changes: one extra passenger in the back moves the CG far more than the same passenger up front, because their arm is longer.
What this changes in flight
A mass and balance calculation isn't paperwork before a flight — it's the one point where you find out, on the ground, that the planned load doesn't work. Once airborne, there's no fixing it.
Two habits cover most of the risk: redo the sums whenever a load changes, and check the envelope at take-off and at landing. If the margin is thin, fuel is usually the easiest variable to adjust — and that trade-off, reserves included, needs to be made deliberately on the ground, not discovered in the air.