1 — Mass, Arm & Moment
Weight and balance is about two questions: is the aeroplane too heavy, and is the load in the right place? The second is pure leverage — think of the aeroplane as a seesaw.
Every item of load sits at an arm — a distance from a fixed reference, the datum. Multiply a mass by its arm and you get a moment (kg·m), the turning effect about the datum. Add up every mass and every moment, then:
The centre of gravity (CG) is where the whole aeroplane balances. Loading is just bookkeeping: list each mass × arm, total the column, divide.
2 — Forward vs Aft CG
The CG must stay between a forward and an aft limit. Both edges exist for a reason — and they fail in opposite ways.
- More stable, nose-heavy
- Heavier elevator · higher stall speed · longer takeoff
- Less stable, tail-light
- Lighter controls · risk of an unrecoverable stall / spin
3 — Load It Yourself
The forward and aft limits, plotted against mass, form the CG envelope — a box your loaded point must sit inside. Set the load and watch both your takeoff point and your zero-fuel point move:
| Item | Mass | Arm | Moment |
|---|
Illustrative SEP. Real figures always come from your aircraft’s mass & balance sheet.
4 — Fuel Burn Moves the CG
You don’t land with the mass you took off with — you burn fuel along the way. Because fuel has its own arm, burning it shifts the CG. On our example aeroplane the tank sits behind the CG, so as fuel burns the CG creeps forward.
Worked example — empty 770 kg @ 0.90 m, two front seats 160 kg @ 0.94 m, rear 80 kg @ 1.85 m, baggage 20 kg @ 2.40 m, fuel 100 L (72 kg) @ 1.20 m:
- Moments: 693 + 150.4 + 148 + 48 + 86.4 = 1 125.8 kg·m.
- Mass: 770 + 160 + 80 + 20 + 72 = 1 102 kg (under MTOM 1 150 ✓).
- CG: 1 125.8 ÷ 1 102 = 1.022 m (inside 0.89–1.20 ✓).
- At landing (zero fuel): 1 030 kg, CG 1.009 m — still inside. Both ends of the flight are legal.
5 — The Two Checks & Common Traps
Every load must pass two independent checks: mass ≤ MTOM, and CG within the envelope. One can pass while the other fails.