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CAAP ATPL Knowledge Exam study materials
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Last-Minute Mail: Mass Limits and the Aft CG

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Last-Minute Mail: Mass Limits and the Aft CG

A twin-turbofan (MTOW 65 000 kg, MLW 59 500 kg, MZFW 58 000 kg) is being loaded for a sector of 2 h 10 min. The signed load sheet shows: DOW 41 500 kg, 160 passengers at 84 kg, 1 200 kg in forward hold 1, 1 400 kg in aft hold 4, ZFW 57 540 kg, block fuel 7 300 kg (200 kg taxi, 4 600 kg trip, 2 500 kg reserves), TOW 64 840 kg and CG 31.4 % MAC. MAC is 4.6 m, hold 4 is 6.5 m aft of the aircraft CG, and the CG envelope is 12 % to 32.0 % MAC for both take-off and landing. Ten minutes before pushback the freight agent loads another 600 kg of mail into hold 4 and hands over an amended manifest. The captain says: 'We are under MTOW and we will burn most of that fuel off, so just amend the figures.' Weather is fine, runway 2 400 m dry, no performance restriction, and no tankering issue.

1. Using the figures given, calculate the effect of the extra 600 kg of mail on ZFW, TOW, landing weight and CG, and state which limits are exceeded.

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The extra load is aft of the CG, so it changes both mass and balance.

Mass

  • ZFW: 57 540 + 600 = 58 140 kg, which is 140 kg above MZFW (58 000 kg).
  • TOW: 64 840 + 600 = 65 440 kg, which is 440 kg above MTOW (65 000 kg).
  • Landing weight = TOW minus trip fuel = 65 440 - 4 600 = 60 840 kg, which is 1 340 kg above MLW (59 500 kg).

Balance

  • Moment added = 600 x 6.5 = 3 900 kg m aft of the CG.
  • One percent MAC at the new mass = 65 440 x 4.6 / 100 = 3 010 kg m.
  • Change in CG = 3 900 / 3 010 = +1.30 % MAC, giving 31.4 + 1.3 = 32.7 % MAC, 0.7 % MAC beyond the aft limit.

So four limits are broken. The landing weight is the largest exceedance and the one the crew will miss if they only look at the take-off weight. The aft CG exceedance is easy to miss because the total mass still looks close to MTOW and the sheet has already been signed.

2. The captain proposes to depart as loaded and 'burn the excess off'. Assess that proposal, and state the minimum payload that must be removed and from where.

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The proposal is invalid, for two reasons.

  1. Fuel burn is already accounted for. The take-off weight of 65 440 kg already has the 4 600 kg trip fuel deducted to give the 60 840 kg landing weight. 'Burning it off' is already in the flight plan; the aircraft will still land 1 340 kg overweight.
  2. There is no fuel to give away. Block fuel is 7 300 kg = 200 taxi + 4 600 trip + 2 500 reserves. Any fuel removed to fix the 440 kg MTOW exceedance eats into reserves, and you would have to remove 1 340 kg of fuel to fix the landing weight, which is not available.

Minimum payload offload = 1 340 kg, driven by the MLW. Removing it also clears MZFW (56 800 kg) and MTOW (64 100 kg).

It must come from hold 4, so that the aft CG is pulled forward at the same time: 1 340 x 6.5 = 8 710 kg m of aft moment removed. In practice offload slightly more (for example 1 400 kg) so the aircraft does not land exactly at MLW with zero margin.

3. What is the CG after the minimum offload, and what must happen before the aircraft moves?

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New CG

  • Take-off mass = 65 440 - 1 340 = 64 100 kg.
  • Moment removed = 1 340 x 6.5 = 8 710 kg m.
  • One percent MAC at the new mass = 64 100 x 4.6 / 100 = 2 949 kg m.
  • Change = -8 710 / 2 949 = -2.95 % MAC, so the CG moves from 32.7 % to about 29.8 % MAC, inside the 12 % to 32.0 % envelope.
  • Landing weight becomes 59 500 kg, exactly MLW: legal but with no margin, which is why a little more should come off.

Note that the CG has moved a long way forward from the aft limit; if further cargo is removed from hold 4, the forward limit becomes the next thing to check.

Before pushback

  • The load sheet must be recalculated from the actual load and re-checked against the loading instruction, not simply annotated.
  • The trim sheet value must be re-entered, and the loading supervisor must confirm the offload physically took place.
  • Any further change is a last-minute change requiring the same process again.
  • The commander must be satisfied that the load sheet represents the actual load before accepting it and signing, and the flight crew should cross-check ZFW, TOW and CG against their own figures.