Late Aft Pallet: CG Out of Envelope on an A320
You are the loading officer for an A320-200 departing OKBK. Basic operating weight is 42,500 kg with the centre of gravity at 17.85 m aft of the datum. The certified envelope is 15% to 33% MAC, with the MAC leading edge at 16.60 m aft of the datum and a MAC length of 4.19 m. The load sheet shows: 150 passengers and carry-on at an average arm of 18.60 m, 84 kg each; 2,000 kg in the forward cargo hold at 15.20 m; 1,200 kg in the aft hold at 21.40 m; fuel 9,000 kg at 17.90 m. Ten minutes before departure a late pallet of 1,500 kg is accepted and loaded in the aft hold at 21.40 m, and the trim sheet is not recalculated. MTOW is 77,000 kg, MZFW 62,500 kg, MLW 66,000 kg, and planned trip fuel is 5,200 kg.
1. Calculate the CG in percent MAC with the late pallet on board and state whether the aircraft is inside the certified envelope.
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Step 1 - limits in metres.
- Forward limit = 16.60 + (0.15 × 4.19) = 16.60 + 0.629 = 17.229 m
- Aft limit = 16.60 + (0.33 × 4.19) = 16.60 + 1.383 = 17.983 m
Step 2 - moments about the datum (kg·m).
- BOW: 42,500 × 17.85 = 758,625
- Passengers: 150 × 84 = 12,600 kg at 18.60 = 234,360
- Forward hold: 2,000 × 15.20 = 30,400
- Aft hold: 1,200 × 21.40 = 25,680
- Late pallet: 1,500 × 21.40 = 32,100
- Fuel: 9,000 × 17.90 = 161,100
Total mass = 68,800 kg; total moment = 1,242,265 kg·m
Step 3 - CG. 1,242,265 ÷ 68,800 = 18.056 m aft of datum.
Step 4 - convert. (18.056 − 16.60) ÷ 4.19 × 100 = 34.75% MAC.
Conclusion: the aft limit is 33.0% MAC (17.983 m); the aircraft is 1.75% MAC (0.073 m) aft of the aft limit and must not depart in this configuration. Note the mass limits are all satisfied - TOW 68,800 kg, ZFW 59,800 kg, landing mass 63,600 kg - so the problem is purely balance.
2. Propose a corrective loading action, verify it by calculation, and state why simply offloading the pallet is not the best answer.
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Shift load forward; do not simply delete the pallet.
Option A - offload the 1,500 kg pallet. Mass 67,300 kg, moment 1,210,165 kg·m, CG = 17.982 m = 32.97% MAC. That is inside the envelope by less than 1 mm - mathematically legal, operationally unacceptable: it leaves no margin for passenger seating variation, baggage weighing error or a last-minute fuel change. It also throws away 1,500 kg of revenue.
Option B - transfer 1,200 kg from the aft hold to the forward hold. Moment change = 1,200 × (15.20 − 21.40) = −7,440 kg·m New moment = 1,242,265 − 7,440 = 1,234,825 kg·m; mass unchanged at 68,800 kg. New CG = 1,234,825 ÷ 68,800 = 17.948 m = 32.17% MAC - inside the aft limit with about 0.035 m (0.83% MAC) margin, and the forward limit is nowhere near.
Option C - offload the pallet and add 400 kg forward as ballast if the pallet cannot be broken down: mass 67,700 kg, moment 1,210,165 + (400 × 15.20) = 1,216,245; CG = 17.965 m = 32.57% MAC.
Then verify the whole picture: recalculate ZFW and landing CG (fuel at 17.90 m is close to the CG, so burn-off moves the CG only slightly forward), re-issue the trim sheet, and have the commander sign the final figures - the error here was a loading change accepted without a recalculation, not the pallet itself.
3. Explain why the aft limit exists, and what handling characteristics would have signalled the problem in flight.
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The aft limit is a stability and recovery limit, not a structural one in the ordinary sense.
- Reduced static margin. As the CG moves aft the distance between the CG and the aerodynamic centre shrinks, so the nose-up pitching moment from a disturbance is resisted by a smaller restoring moment. The aircraft becomes progressively less stable in pitch.
- Light control forces and over-controlling. Stick force per g falls, the aircraft is sensitive in pitch, and a small input produces a large attitude change - dangerous on a gusty Kuwait departure or in a windshear recovery.
- Stall and spin. Elevator authority to break a stall and to recover from a spin or a deep stall is reduced; the stick-shaker/stick-pusher margin is reached at a lower angle of attack because the tailplane is already working hard to hold the aircraft in trim.
- Rotation and tail strike. Aft CG raises the tail during rotation, and the aircraft tends to over-rotate; on a heavy, hot take-off the tail-clearance margin is eaten from both ends.
- In flight the cues would be: very light pitch forces, a nose-up tendency needing continuous nose-down trim, difficulty holding a stable attitude in turbulence, and speed instability (the aircraft wanders off the target speed rather than settling). Any of those reports should trigger a suspicion of a loading or trim-sheet error, and a recheck of the figures after landing.