Climb-Limited Take-Off at Isfahan in High Temperature
You are computing take-off performance for a Boeing 737-800 at Isfahan (OIFM), runway 26, 3,600 m, elevation 5,050 ft, OAT 38°C, QNH 1011, wind 280/12G22, runway wet. Flap 5, assumed temperature derate 45°C. The airport obstacle chart shows a 120 ft building 2,500 m from the start of roll, 400 m left of centreline; the departure procedure requires a right turn at 400 ft AGL. The performance chart gives a field-length-limited weight of 69.2 t and a climb-limited weight of 67.8 t for these conditions. Actual take-off weight is 68.5 t. The crew has 15 minutes of extra fuel; destination weather is CAVOK. The runway has a 1% upslope in the first 1,000 m.
1. Which limit governs the take-off, and is the flight legal at 68.5 t?
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Field-length limit 69.2 t, climb limit 67.8 t, actual 68.5 t. The governing limit is climb, and 68.5 > 67.8 by 0.7 t, so the take-off is not legal as planned. Field length appears satisfactory, but the wet runway and 1% upslope may reduce the field-length limit below 69.2 t; must use wet/upslope corrections. Even if field length remains okay, the climb limit is breached. The 120 ft obstacle at 2,500 m and the 400 ft turn are additional considerations but do not override the weight limit.
2. What options do you have to achieve a legal take-off, and which is preferable?
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Options: (a) remove the assumed-temperature derate and use full/rated thrust, which increases climb gradient and raises the climb-limited weight; (b) offload at least 0.7 t of fuel or payload; (c) increase flap, if the chart shows improved climb; (d) use improved-climb procedure if approved. Prefer removing the derate first because it preserves payload and fuel, but verify the new climb-limited weight and obstacle clearance. If full thrust still leaves a breach, offload 0.7 t. Do not use extra fuel as ballast; the 15 minutes extra fuel can be offloaded if needed.
3. How do the gust, wet runway and upslope affect the decision?
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The gust 22 kt and steady 12 kt give a headwind component, but gust corrections increase V1/VR and can increase field length; do not use the gust as guaranteed performance. The wet runway reduces braking friction and increases stopping distance, lowering the field-length-limited weight. The 1% upslope also increases take-off distance. Therefore the field-length limit of 69.2 t is not valid for the wet/upslope case; recalculate with the wet chart and upslope correction. If the corrected field limit falls below 68.5 t, the take-off is illegal regardless of the climb fix.