High-Density Altitude Departure from a Grass Strip
Pilot plans a departure from a 1,050 m grass strip, elevation 4,200 ft, QNH 1005 hPa, OAT 30°C. Aircraft is a C172 at MTOW 1,043 kg. Wind is 300° at 10 kt, runway 27, giving a 5 kt tailwind component. The strip slopes up 1.5%. The POH gives a takeoff distance to 50 ft of 780 m at 4,000 ft and 30°C, paved, no wind, MTOW. The POH notes: grass add 15%; 5 kt tailwind add 10%; 1.5% upslope add 10%. The pilot has 1,050 m available and no obstacle at the end. The pilot is experienced on type but has not operated from this strip before. The aircraft is loaded to MTOW, and the density altitude is high. The pilot considers using a soft-field technique and rotating early.
1. Calculate the pressure altitude, approximate density altitude, and the corrected takeoff distance. Does the flight have a safe margin?
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Pressure altitude = 4,200 + 30(1013-1005) = 4,200 + 240 = 4,440 ft. ISA temperature at 4,440 ft is about 15 - 2(4.44) = 6.1°C. The OAT is 30°C, so ISA deviation is about +24°C. A rough density altitude is 4,440 + 120(24) = 7,320 ft. Corrected takeoff distance: 780 × 1.15 (grass) = 897 m; × 1.10 (tailwind) = 987 m; × 1.10 (upslope) = 1,086 m. That exceeds the 1,050 m available by about 36 m, before any allowance for pilot technique, soft grass, or obstacle. The margin is negative, so the flight is not safe at MTOW under these conditions.
2. What operational changes would restore an acceptable margin, and which is most effective?
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Reduce weight: offload fuel or payload to lower the takeoff distance and improve climb gradient. Wait for a cooler time of day, which lowers density altitude. Use a runway with an into-wind component if available. Avoid the upslope runway if another strip is available. Use the correct soft-field technique, but do not rely on technique to overcome a negative book margin. Of these, weight reduction and cooler temperature are usually the most effective; a 5 kt tailwind is already costing about 10%, and reversing it to a headwind would help significantly. The pilot should recalculate with the POH figures after any change and require a clear margin, not just equality with the runway length.
3. If the pilot decides to depart after reducing fuel, what should be the abort criteria?
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Before departure, nominate an abort point and a decision speed. If the aircraft has not reached about 70% of the required takeoff speed by the abort point, or if acceleration is sluggish, reject the takeoff. If the engine fails before rotation, close the throttle, maintain directional control, and brake. After rotation, do not attempt to reject unless the aircraft is still low and there is adequate runway remaining; otherwise, fly the aircraft and land ahead or in a suitable area. With a positive but small margin, the pilot should use the full runway, avoid early rotation, and be prepared to lower the nose if the airspeed decays. Do not use a soft-field technique that keeps the aircraft in ground effect beyond the point where terrain or obstacles require climb.