Aft Load Shift and Short-Runway Takeoff at a Hot, High Airport
An IFR charter is loaded at a 1,500 ft-elevation airport. Runway 22 is 2,400 m long, dry, with a 1% uphill slope and a 60 m stopway but no clearway. Aircraft limits: MTOW 67,000 kg, MLW 63,000 kg, MZFW 58,000 kg, OEW 41,000 kg. Planned takeoff mass is 64,000 kg with 9,000 kg fuel and 14,000 kg payload. Before a last-minute change, the load sheet shows CG at 27.8% MAC; the aft limit is 32.0% MAC and MAC length is 4.5 m. A 250 kg pallet is then moved from the forward hold, arm 14 m forward of datum, to the aft hold, arm 36 m aft of datum. OAT is 30°C, QNH 1000 hPa, wind 220°/12 kt. The AFM gives a sea-level ISA dry takeoff distance of 1,950 m at 64,000 kg. Apply +15% for 1,500 ft pressure altitude, +1.5% per °C above ISA, -1% per 5 kt headwind, and +5% for 1% uphill. The AFM figures assume the CG is within limits. The crew has 20 minutes before slot expiry.
1. Calculate the CG after the pallet move and state whether the aircraft is within the aft CG limit. Show the moment calculation.
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The pallet moves from 14 m forward of datum to 36 m aft of datum, a movement of 36 - (-14) = 50 m aft.
Moment change = 250 kg × 50 m = 12,500 kg·m aft.
CG shift = moment change / total mass = 12,500 / 64,000 = 0.195 m aft = 19.5 cm aft.
As a percentage of MAC: 0.195 / 4.5 × 100 = 4.34% MAC.
New CG = 27.8 + 4.34 = 32.14% MAC.
The aft limit is 32.0% MAC, so the aircraft is 0.14% MAC beyond the limit. That is a small amount, but it is still outside the certified envelope. The AFM takeoff and handling data assume a CG within limits, so the load sheet cannot be signed as presented. The pallet should be moved forward again, or an equivalent load adjustment made, and the load sheet recalculated before departure. Do not accept the argument that 0.14% is 'close enough'; the limit is a certification boundary, not a target.
2. Compute the corrected takeoff distance and compare it with TODA and ASDA. Is the runway limiting?
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Start with the AFM sea-level ISA dry figure: 1,950 m at 64,000 kg.
- Pressure altitude 1,500 ft: +15% → 1,950 × 1.15 = 2,242.5 m.
- Temperature: ISA at 1,500 ft is about 15 - 2 × 1.5 = 12°C. OAT 30°C is ISA+18. Correction = 18 × 1.5% = +27%. → 2,242.5 × 1.27 = 2,848 m.
- Wind: runway 22 is 220°; wind 220°/12 kt is a 12 kt headwind. Correction = 12/5 × -1% = -2.4%. → 2,848 × 0.976 = 2,780 m.
- Slope: +5% for 1% uphill → 2,780 × 1.05 = 2,919 m.
TODA = runway length + clearway = 2,400 + 0 = 2,400 m. ASDA = runway + stopway = 2,400 + 60 = 2,460 m.
The corrected takeoff distance is about 2,919 m, which exceeds TODA by about 519 m and ASDA by about 459 m. The runway is limiting. The calculation also assumes the CG is within limits; the actual aft CG would invalidate the AFM figures anyway.
3. Recommend a safe action before departure, considering the slot pressure.
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Do not depart as loaded. There are two independent problems: the CG is outside the aft limit, and the corrected takeoff distance exceeds TODA/ASDA.
First restore a valid CG by moving the 250 kg pallet forward or removing it. Then reduce takeoff mass or improve the performance condition. Practical options are offloading payload, reducing fuel to the minimum safe figure, waiting for a lower OAT, or using a longer/downhill runway if one is available. The exact maximum mass must be read from the AFM after the CG is corrected; a rough estimate is that several thousand kilograms must come off, so a small trim adjustment will not solve the runway problem.
Slot pressure is commercial, not operational. If the slot expires, the crew should request a new slot or delay. Departing with an out-of-limit CG and a negative takeoff margin is not a defensible judgement call. The correct decision is to stop the loading process, correct the load sheet, and recompute performance before accepting the aircraft.