Loading the 79-tonne jet: which limit bites first?
You are the loading officer for a departure at 06:20 UTC. Aircraft data: DOW 41 500 kg, MTOW 79 000 kg, MLW 66 000 kg, MZFW 62 000 kg. Planned block fuel 12 200 kg, taxi fuel 200 kg, trip fuel 5 600 kg, alternate fuel 1 400 kg, final reserve 1 100 kg, contingency 300 kg. Traffic load: 175 passengers at 84 kg standard mass and 4 200 kg of cargo; one 1 200 kg pallet in that cargo has not yet been assigned to a hold. The mass and balance manual gives the datum at the nose, MAC leading edge at 14.60 m, MAC length 5.20 m, forward CG limit 12% MAC, aft limit 33% MAC. The load sheet, with the pallet provisionally in hold 1, gives a take-off CG of 16.06 m. Hold 1 has an arm of 5.00 m, hold 4 an arm of 22.00 m.
1. Compute ZFW, TOW and LDW, identify which limit is exceeded and by how much, and state the smallest change that makes the flight legal. Which option do you choose, and why?
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Masses
- Payload = 175 × 84 + 4 200 = 14 700 + 4 200 = 18 900 kg
- ZFW = 41 500 + 18 900 = 60 400 kg (MZFW 62 000, margin 1 600 kg)
- Take-off fuel = 12 200 − 200 = 12 000 kg, so TOW = 72 400 kg (MTOW 79 000, margin 6 600 kg)
- LDW = 72 400 − 5 600 = 66 800 kg → MLW exceeded by 800 kg. This is the only exceedance.
Minimum fuel at destination = alternate 1 400 + final reserve 1 100 + contingency 300 = 2 800 kg. Planned landing fuel is 12 000 − 5 600 = 6 400 kg, an excess of 3 600 kg.
Smallest change: cut take-off fuel by 800 kg to 11 200 kg. LDW then equals 66 000 kg exactly and landing fuel is 5 600 kg, still 2 800 kg above the minimum. Offloading 800 kg of payload is equally legal but costs revenue. Take the fuel cut unless the commander judges the destination forecast needs the extra holding fuel, in which case offload the payload instead.
2. Ground staff want to move the 1 200 kg pallet from hold 1 to hold 4. Is that acceptable, and what is the maximum mass that may be moved aft?
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Convert the limits to metres
- Aft limit = 14.60 + 0.33 × 5.20 = 16.316 m
- Forward limit = 14.60 + 0.12 × 5.20 = 15.224 m
- Current CG 16.06 m = (16.06 − 14.60) / 5.20 = 28.1% MAC - inside.
Effect of the move (arm change 22.00 − 5.00 = 17.00 m aft): ΔCG = m × d / W = 1 200 × 17.00 / 72 400 = +0.282 m → CG 16.342 m = 33.5% MAC.
That is 0.5% MAC (26 mm) beyond the aft limit - not acceptable.
Maximum permissible shift = 16.316 − 16.06 = 0.256 m, so m = 0.256 × 72 400 / 17.00 ≈ 1 090 kg. Put about 1 090 kg in hold 4 and leave 110 kg in hold 1, then re-issue the load sheet.
Recalculate after the 800 kg fuel cut from the first task: W falls to 71 600 kg, so the same 1 090 kg would now overshoot and the cap drops to about 1 078 kg.
3. After the fuel cut, the destination temperature rises and landing performance now limits the landing mass at destination to 65 200 kg. What are the options and what do you recommend?
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LDW is 66 000 kg against a performance limit of 65 200 kg - 800 kg too heavy.
Options:
- Cut fuel by a further 800 kg to 10 400 kg take-off fuel. Landing fuel becomes 4 800 kg, still above the 2 800 kg minimum, so this is legal. ZFW is unchanged.
- Offload 800 kg of payload (roughly nine passengers or equivalent freight). ZFW falls to 59 600 kg and the CG moves forward - re-check the 15.224 m forward limit, which is now the risk.
- Split the difference between the two.
Recommendation: the destination forecast carries TEMPO thunderstorm activity, so landing fuel is worth more than 800 kg of payload on this sector. Offload the payload if the commercial side agrees; if it does not, cut the fuel and accept a thinner buffer, but record the decision. Do not simply accept the higher landing mass - the performance limit is a hard limit, not a planning target.