Flight Performance: Power, Density Altitude and Spray Load
The performance problem
Helicopter performance is limited by power available versus power required. Power required rises with weight, density altitude, and speed (through induced and parasite drag). Hover, climb and translational lift all depend on air density. Agricultural work adds a heavy, variable load and demands low-level flight, both of which push the margins.
Concepts to command
Know the hover ceiling in ground effect (HIGE) and out of ground effect (HOGE), and why HOGE is lower. Understand density altitude: high elevation, high temperature, high humidity and low pressure all raise it and reduce performance. Know translational lift and effective translational lift speed, and how wind affects them. Understand the height-velocity diagram (dead man's curve) and why low-level spraying sits inside the avoid region. Know the effects of gross weight on rate of climb, and the concept of max performance takeoff and confined-area departure.
Traps
Questions often present a takeoff at high density altitude with a full hopper and ask for the limiting factor; the answer is usually power or tail rotor authority, not airspeed. Another trap: assuming a headwind always helps; a tailwind during low-level work reduces translational lift and margin. Also watch for the confusion between HIGE and HOGE limits and between indicated and true airspeed at altitude.
Rule of thumb
Before each spray load, do the density-altitude calculation: pressure altitude plus temperature correction (roughly 120 ft per °C above ISA). If the result is high, reduce the load, not the safety margin.
Worked example: Field elevation 900 m, QNH 1013, temperature 30 °C. Pressure altitude is about 900 m; ISA temperature at that level is about 15 − 6.5×0.9 ≈ 9 °C. The 21 °C excess adds roughly 21 × 120 ft ≈ 2,520 ft (about 770 m) of density altitude. Performance must be read at roughly 1,670 m, not 900 m.