British Young Pilots · Principles of Flight · Module POF-1.2INSTRUCTOR
Gravity pulls everything vertically towards the centre of the Earth. The aeroplane's mass times that gravitational force is its weight, and in flying we always mean the all-up weight: the whole loaded aeroplane, people, fuel and bags included.
All of it can be treated as one force acting through one point: the centre of gravity (CG), the aeroplane's point of balance. Hang the aeroplane from a rope at the CG and it would sit level.
The quartered circle is the centroid symbol, which is how the CG is marked on diagrams. Source: Pooleys Vol 4 (10th ed.) p.7.
How much: maximum weights such as the Maximum Take-Off Mass (MTOM) are set for every aircraft, because of the structural strength of its components and what the design is required to do.
Where: the CG's position affects the aeroplane's stability and performance, so it must stay within the manufacturer's limits at all stages of flight. You can be under MTOM and still dangerously out of balance; they are separate checks.
Source: Pooleys Vol 4 (10th ed.) p.7.
The CG sits wherever the load puts it, so it moves when the load moves: passengers shifting seats, fuel transferring between tanks. Meanwhile the all-up weight normally decreases through the flight as fuel burns off. That's why weight and balance are checked before flight; exceeding any limit at any point compromises safety.
FOUNDATIONSBalance a ruler on your finger, then slide a coin along it, and your finger has to chase the balance point. Loading an aeroplane is the same game.
STRETCHWhere would you put the fuel tanks so the CG barely moves as fuel burns away?
Source: Pooleys Vol 4 (10th ed.) p.7–8.
How hard are the wings working? In straight and level flight the wings support the whole weight, and wing loading spreads that over the wing area:
Wing loading = weight of the aeroplane ÷ wing area
Example (our own figures): 900 kg on 12 m² of wing → 900 ÷ 12 = 75 kg/m².
FOUNDATIONSImagine the aeroplane's weight shared out evenly across every square of the wings, like students spreading out evenly across a trampoline.
Source: Pooleys Vol 4 (10th ed.) p.8 (formula; example figures are ours).
Misconception 1: "weight acts through the wheels or the middle of the aeroplane." It acts through the CG, wherever the CG currently is, and the CG moves with the load.
Misconception 2: weight and balance are the same check. They are two separate limits, total mass (MTOM) and CG position, and you can bust either one alone. The load-sheet game is built exactly on that distinction: the flight-bag load reaches exactly MTOM yet goes tail-heavy, and a seats-only load goes nose-heavy.
Prop: a ruler and two coins. Same total weight, different coin positions, different balance point. That is a load sheet in miniature.
Delivery: let them fail the game once before explaining; the nose-heavy empty-bay result surprises most groups and opens the "two limits" discussion naturally.
Instructor copy only. Sources: Vol 4 (10th ed.) p.7–8.
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Aeroplane + pilot: 600 kg, arm 1.0 m. MTOM 800 kg. CG limits 1.06–1.10 m. Both passengers must fly; passengers sit in seats. Tap an item, then tap where it goes. Tap a filled station to unload it. (Training figures, not a real type.)
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