Performance
Performance questions test whether you can predict what the aircraft will actually do on a given day, then apply the correct correction factors in the correct order.
Density altitude and the chart
Pressure altitude is indicated altitude with 1013 hPa set; density altitude is pressure altitude corrected for temperature. High temperature, high humidity and low pressure all raise density altitude and degrade takeoff, climb and landing performance. Use the AFM/POH chart: enter with pressure altitude, move to temperature, read density altitude or takeoff distance, then apply wind and slope corrections as the chart specifies.
Speeds and gradients
Stall speed varies with the square root of weight: 21% more weight gives about 10% higher Vs. Climb gradient (%) = rate of climb (ft/min) ÷ ground speed (ft/min) × 100, with 1 kt = 101.3 ft/min. Obstacle clearance uses ground speed and gradient, not rate of climb alone.
Worked example
Rate of climb 500 ft/min, still-air TAS 100 kt. Ground speed in ft/min = 100 × 101.3 = 10,130 ft/min. Gradient = 500 ÷ 10,130 × 100 = 4.94%. To clear a 500 ft obstacle: 500 ÷ 0.0494 = 10,120 ft, about 1.67 NM. A headwind lowers ground speed and shortens the still-air distance to the obstacle, but the gradient relative to the airmass is unchanged.
Traps
Mixing pressure altitude and density altitude; applying wind correction before slope; using TAS where the chart wants ground speed; assuming a tailwind improves climb gradient; ignoring the effect of a wet or contaminated runway.