Compartments, Fuel and the Moving CG - ATPL 031 — Mass & Balance

The loading manual is a map of everywhere mass can go: seat rows, cargo compartments, galleys, and the fuel tanks. This lesson covers the two halves of that map. First the cabin and holds: what the documents publish about seating schemes and compartment limits, and how to extract it. Then the fuel system, where the balance story gets dynamic: fuel is the one part of the load that moves and disappears in flight, so where the tanks sit decides how the CG wanders from take-off to landing, and one special tank family, the fin and tailplane trim tanks, exists purely to steer that wandering on purpose.

Cabin and compartment data

For the cabin, the documents publish the seating scheme: the seat rows, how many seats each row holds, and the arm of each row or cabin section from the datum, so that passengers can be turned into moments row by row or zone by zone. For the holds, each cargo compartment comes with its dimensions and volume, its arm, and its limitations: the maximum compartment load, and the floor-load and running-load limits taught in the mass-limits lesson. Extraction questions hand you such a table and ask for the right number: the discipline is to check three things before using any value, that it is the right compartment, the right kind of limit, and the right unit. A compartment's maximum load and its floor-load limit both cap loading, but one caps the total and the other caps the concentration, and the exam likes the difference.

The fuel system in the loading manual

The fuel side of the documents lists each tank's capacity and its position, as an arm from the datum. A typical transport aeroplane carries wing tanks, often split into inner and outer groups, plus a centre tank in the wing box under the fuselage; many long-haul types add a trim tank in the tailplane or fin. Capacities are published in volume and usable mass, and the arms let every kilogram of fuel enter the moment sum like any other load. Two habits matter for extraction: usable fuel is what the balance calculation loads (the unusable remainder is already inside the basic empty mass), and a tank's arm is the arm of its fuel's centre of gravity, which the documents give per tank or per filling level.

A loading manual states: 'Compartment 1: maximum load 2,268 kg; maximum floor load 732 kg/m². Centre tank: 8,000 kg usable, arm 656 in. Wing tanks: 2 × 4,000 kg usable, arm 650 in.' A 1,100 kg machine part arrives on a cradle with a 1.2 m² footprint, destined for compartment 1. May it be loaded, and what do the tank lines contribute to the balance sheet?

Run the three checks on the crate: right compartment, right limit, right unit. The total is fine, 1,100 kg against a 2,268 kg maximum, but the concentration is not: $\dfrac{1{,}100}{1.2} = 917\ \text{kg/m}^2 > 732\ \text{kg/m}^2$
The cure is a bigger footprint, and the limit itself says how big: $\dfrac{1{,}100}{732} = 1.503 \approx 1.51\ \text{m}^2$
On a cradle of at least 1.51 m² the same part loads legally. The tank lines read just as directly: 8,000 kg of usable centre-tank fuel acts at 656 in and each wing tank's 4,000 kg at 650 in, so every fuel state enters the moment sum like any other load. The full arithmetic of floors, spreaders and running loads belongs to the cargo handling lesson; what this table asked for was only the discipline of reading it.

How the CG moves as fuel burns

Burning fuel removes mass from wherever the feeding tank sits, and the CG moves away from the emptying tank: burn fuel from ahead of the CG and the CG drifts aft, burn it from behind and the CG drifts forward, and the further the tank from the CG, the larger the drift per kilogram. Three tank locations tell the whole story:

Centre tank — sits in the wing box, close under the CG, so its fuel has a short arm about the CG: burning it changes the mass a lot and the balance hardly at all.
Inner wing tanks — on a swept wing the inboard sections sit relatively far forward: their fuel is nearer the CG's fore-aft station, and burning it produces a modest CG movement.
Outer wing tanks — sweep carries the outboard wing aft, so outer-tank fuel sits behind the inner-tank fuel: burning it moves the CG forward, and filling or keeping it moves the loaded CG aft.

The burn sequence turns this geometry into a flight profile. A typical policy burns the centre tank first and the wing tanks last, partly for the wing-bending relief taught with the MZFM: keeping fuel in the wings as long as possible keeps the relieving download where the lift is. The CG consequence is that the early cruise sees little balance change while the centre tank drains, and the later flight sees the CG walk slowly as the wing tanks, with their fore-aft spread, empty in their scheduled order. The load sheet must therefore prove the CG stays inside limits not just at take-off but across the whole burn, which is why the envelope check of the coming module covers the flight, not a single moment.

Trim tanks in the fin and tailplane

The lesson on why mass and balance matters showed that an aft CG is cheaper to fly: less tail download, less induced and trim drag, lower fuel burn. A trim tank in the tailplane or fin exploits that deliberately. In cruise, fuel is pumped aft into the trim tank, walking the CG toward the aft limit and shaving drag for hours at a time; before descent and approach, the fuel comes forward again. The advantages are real: measurable fuel savings on long sectors, and extra tank volume that can stretch range. The risks are exactly what the mechanism suggests:

CG management failure — the aircraft is intentionally flown near the aft limit, so a miscalculated transfer, a pump failure or a stuck valve can leave the CG at or beyond the boundary where stability margins are thinnest.
Trapped fuel — fuel that cannot be transferred forward again is fuel the flight may not be able to burn late in the flight, and mass far aft exactly when the aircraft wants its CG forward for approach.
System dependence — the balance of the aircraft now rests on pumps, valves and transfer logic working in both directions; the crew must monitor the automatic CG control and know the manual procedures for its failures.

Extract seating-scheme, compartment-dimension and limitation data from sample documents

Extract fuel-tank capacities and positions from sample documents

Explain the CG movement as the flight progresses for fuel burned from centre, inner-wing and outer-wing tanks

Explain the advantages and risks of trim tanks in the fin or tailplane

Cabin and compartment data

The fuel system in the loading manual

Check you understood

How the CG moves as fuel burns

Trim tanks in the fin and tailplane