Obstacle-Limited Take-off and the Clearway Trap
Runway 09 at a 500 ft elevation airport has TORA 3,000 m, TODA 3,300 m and ASDA 3,000 m. A 15 m obstacle lies 4,500 m from the start of roll, on the extended centreline. OAT is 35°C, wind calm, QNH 1010 hPa. Take-off mass is 70,200 kg. The AFM/QRH limiting masses for the planned flap and thrust are: field length 72,000 kg, obstacle 69,500 kg, tyre speed 71,000 kg, brake energy 70,500 kg. The net take-off flight path must clear the obstacle by 35 ft; the scheduled V2 is 155 kt. The operator proposes to use the 300 m clearway to raise the field-length limit, and the first officer asks whether an improved-climb procedure would solve the problem. No assumed temperature thrust is used.
1. Identify the controlling limit and determine whether the take-off at 70,200 kg is legal. Explain the comparison.
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Actual mass is 70,200 kg. Compare each limit: field length 72,000 kg, obstacle 69,500 kg, tyre speed 71,000 kg, brake energy 70,500 kg. The obstacle limit is the lowest and governs. Therefore the take-off at 70,200 kg is not legal: the net take-off flight path would not assure 35 ft obstacle clearance. The fact that field length and brake energy are satisfactory does not help. Reduce mass to 69,500 kg or less, change flap/thrust to improve obstacle performance, or use another runway. Do not dispatch on the field-length limit alone.
2. The operator proposes using the 300 m clearway to raise the field-length limit. Will that make the take-off legal? Explain.
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The 300 m clearway raises TODA from 3,000 m to 3,300 m and can increase the field-length-limited mass. It does not remove the obstacle: the net flight path still has to clear the 15 m obstacle at 4,500 m by 35 ft. Here the obstacle limit, 69,500 kg, is already below the actual mass, while the field limit is 72,000 kg. Since field length is not the controlling limit, adding clearway does not make the take-off legal. Clearway also cannot be used as stopway; ASDA remains 3,000 m. The crew must treat the obstacle as the limiting case.
3. What is the effect of an improved-climb procedure, and which limits must be rechecked?
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Improved climb increases V2 and the climb speed after rotation, producing a steeper net flight path and usually a higher obstacle-limited mass. It also increases take-off distance and tyre/brake energy, so the field, tyre and brake limits must be recalculated. If improved climb raises the obstacle limit above 70,200 kg while field remains above 70,200 kg, tyre above 70,200 kg and brake energy above 70,200 kg, the take-off could become legal. If any of those become limiting below 70,200 kg, it is not. The procedure must be the approved one, with revised V1/VR/V2 and obstacle accountability; no assumed temperature should be used unless the full performance is rechecked.