Highland takeoff with standing water: finding the governing limit and the real offload
You are computing takeoff data for a 41,000 kg MTOW twin-jet freighter at a highland runway: elevation 5,400 ft, QNH 1022, OAT 27 °C, runway 2,300 m, 0.5 % uphill, standing water reported in the touchdown zone. Wind 050/08, runway 05. Limiting weights from the performance manual, already corrected for flap 15, packs and anti-ice off: field length 32,600 kg dry / 31,900 kg with the standing water; obstacle 33,400 kg; brake energy 34,800 kg; tyre speed 33,900 kg; climb 33,400 kg. Planned TOW 32,400 kg with 2,100 kg trip fuel, giving a planned landing weight of 30,300 kg against a landing-limited weight at destination of 30,600 kg. Payload is 8,400 kg, of which a 500 kg pallet was booked late into the aft hold. Loading sheet: CG 31.2 % MAC, forward limit 18 %, aft limit 36 %; the aft hold moves CG aft by 0.12 % MAC per 100 kg. The dispatcher wants the numbers within 15 minutes.
1. Identify the governing limit and the maximum legal takeoff weight, and check what that does to the destination landing weight and to pressure altitude.
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First the pressure altitude, because it is the input everything else rests on: PA = elevation + 30 × (1013 − QNH) = 5,400 + 30 × (1013 − 1022) = 5,400 − 270 ≈ 5,130 ft. The high QNH gives back 270 ft, so the figures I have been quoted, if built on 5,400 ft, are conservative - but I would confirm which pressure altitude the manual figures were computed at rather than assume.
Comparing the corrected limits, the lowest is field length with standing water at 31,900 kg. Obstacle and climb are 33,400 kg, brake energy 34,800 kg, tyre speed 33,900 kg. So field length governs by a clear margin - and note that the 700 kg difference between the dry and wet figures is the whole cost of the standing water.
Planned TOW 32,400 kg against a limit of 31,900 kg means an offload of 500 kg. After that, landing weight at destination is 32,400 − 500 − 2,100 = 29,800 kg, inside the 30,600 kg landing limit with 800 kg to spare, so the destination does not constrain the takeoff.
With 15 minutes, the answer is: offload 500 kg, rebuild the load sheet, and do not sign until the wet-runway field-length figure in use is confirmed against the actual reported runway condition.
2. The captain proposes using the reciprocal runway 23 to get the downhill slope instead of the uphill, accepting the resulting tailwind. Evaluate that.
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Reject it. Runway 23 gives 2,300 m, a 0.5 % downslope and an 8 kt tailwind (050/08 against a 230° heading).
The tailwind is the dominant term and it works against you twice. It increases ground speed for the same airspeed, so the ground roll grows for the same rotation speed, and it increases the distance covered during the accelerate-stop segment. The 0.5 % downslope is a small help to acceleration, but the same slope reduces the deceleration available in a rejected takeoff, and on a runway already contaminated with standing water - where braking action is degraded and the AFM wet figures already assume a reduced friction coefficient - the effect is not one you want to trade into.
On a wet runway most performance manuals penalise tailwind heavily and some prohibit it outright; the correct course is to check the manual's tailwind allowance rather than assume one exists. Standing water also raises aquaplaning and directional-control concerns, which a tailwind component makes worse.
The real options are: offload the 500 kg and depart runway 05, or wait for the water to clear or the wind to shift and recompute. A slope advantage is never worth a tailwind on a contaminated runway.
3. Carry out the weight-and-balance check on the 500 kg offload and state what you would verify before signing.
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The 500 kg comes out of the aft hold, which moves the CG forward: 500/100 × 0.12 = 0.6 % MAC. New CG = 31.2 − 0.6 = 30.6 % MAC, against limits of 18 % forward and 36 % aft. Comfortably inside - 12.6 % MAC of forward margin and 5.4 % aft margin - so the offload does not create a balance problem and no ballast or reseating is needed.
What I would still verify before signing:
- That the 500 kg is actually removed from the aft hold and not redistributed forward or into the cabin, because taking it from the forward hold would push the CG aft to 31.8 % MAC - still legal, but a different aircraft and a different load sheet.
- Zero-fuel weight and the landing CG, not just the takeoff CG, since the trip fuel burns off a CG that has less margin at the end of the sector.
- That the final load sheet reflects the actual pallet count and that the loading supervisor's signature matches what is on the ramp.
- That the takeoff weight on the load sheet is the 31,900 kg figure, not the 32,400 kg the dispatcher first produced, and that the data card in the aircraft matches.