Runway-limited take-off with contaminated surface
A light jet is dispatched from a 2 200 m asphalt runway, elevation 1 200 ft, OAT 28 degrees C, QNH 1003 hPa. The AFM gives a dry take-off distance of 1 610 m at sea level ISA and a 12 percent penalty for a wet runway. The runway has 150 m of displaced threshold and a 60 m stopway. Obstacle clearance requires a net take-off flight path clearing a 15 m obstacle 4 500 m from the start of roll. The aircraft is loaded to 95 percent of maximum take-off mass. The crew notes that the runway is wet with standing water in patches and that the reported braking action is medium.
1. Determine whether the take-off is legally permissible and what performance factor applies.
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First correct for pressure altitude: 1 200 ft elevation plus (1013 - 1003) x 30 ft = 300 ft, giving about 1 500 ft pressure altitude. Temperature ISA deviation at 1 500 ft is 28 - (15 - 2 x 1.5) = 28 - 12 = +16 degrees C. Applying typical AFM corrections (about 1 percent per degree C above ISA and about 10 percent per 1 000 ft) gives a substantial increase over the 1 610 m sea-level figure, well beyond 2 200 m once the wet penalty of 12 percent is added.
Standing water and medium braking action mean the wet-runway factor is not sufficient; a contaminated-runway correction applies. The available distance is the take-off run available, not the full runway: the 150 m displaced threshold reduces the usable length to 2 050 m, and the 60 m stopway may be used only for accelerate-stop, not for take-off distance. The take-off is not permissible at 95 percent mass; a reduction in mass or a change of runway is required.
2. Describe how the net take-off flight path and obstacle clearance change the decision.
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The net take-off flight path is the gross path reduced by 0.8 percent for a twin. At high OAT and elevation, the second-segment climb gradient is degraded, so the aircraft may not reach 15 m at 4 500 m from the start of roll. The crew must check the net path against the obstacle using the actual mass and conditions, not the sea-level book figures. If the net path clears the obstacle only by a small margin, the take-off is not acceptable. Reducing mass improves both the field length and the climb gradient; a bleeds-off or reduced-flap configuration may also be required. The decision must be made before taxi, with the performance calculation recorded and cross-checked.