Wet Grass, High Elevation and Obstacle-Limited Take-off
You are planning a departure from a 700 m grass runway at elevation 4,000 ft. Temperature is 28°C, QNH 1013 hPa, wind calm. The runway is wet after rain. TORA is 700 m and TODA is 780 m; there is a 12 m obstacle at the upwind end. The aircraft POH, at MTOW and zero wind, gives a take-off ground roll of 420 m and a 50 ft distance of 760 m at 4,000 ft and 30°C. The POH notes: wet grass increases ground roll by 25% and 50 ft distance by 10%. You are at MTOW with two passengers and full tanks. The next maintenance is due in 20 hours. The forecast says temperature will remain 28°C for two hours, then fall to 22°C. You have a 2-hour flight planned with 45 minutes reserve.
1. Calculate the required take-off ground roll and 50 ft distance, and compare them with the available distances.
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At 4,000 ft and 30°C the POH 50 ft distance is 760 m. Wet grass adds 10%: 760 × 1.10 = 836 m. The available TODA is 780 m, so the required distance exceeds available by 56 m. Ground roll is 420 × 1.25 = 525 m, which is within the 700 m TORA, but ground roll is not the limiting factor; obstacle clearance is. With wind calm, no headwind credit is available. The take-off as loaded is not acceptable.
2. Can you depart legally and safely at MTOW? Explain which limit controls.
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No. Even though TORA is sufficient, the aircraft cannot meet the 50 ft obstacle requirement within TODA. The 12 m obstacle at the upwind end makes the TODA limit the controlling one. Because you are at MTOW and the POH figures are already at MTOW, no reduction is available unless weight is reduced. Temperature 28°C is close to the 30°C POH column; waiting for 22°C may improve performance, but you do not have a POH figure for 22°C here, so you cannot assume the improvement.
3. What practical options remain if the flight is important, and what must not be assumed?
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Options are to reduce weight by offloading fuel or passengers, use a longer runway or one with a lower obstacle, or wait for a cooler temperature and recalculate with the POH. Do not use a gut-feel margin. If the flight is necessary, recalculate with actual weight and temperature and ensure required TODA is at least 780 m plus a personal margin. Otherwise cancel or delay. The 45-minute reserve is irrelevant to the take-off problem.