Performance
What the exam tests
Performance questions test takeoff and landing distance, density altitude, wind, runway slope and contamination, climb performance, obstacle clearance, V speeds and use of performance charts. The exam expects interpolation and correct application of safety factors.
Core rules
- Takeoff and landing distances increase with altitude, temperature, mass, tailwind, uphill slope and contaminated runway.
- Density altitude is pressure altitude corrected for temperature. High density altitude reduces engine power, propeller thrust and lift.
- V1 is decision speed; VR is rotation speed; V2 is takeoff safety speed. They change with mass and configuration.
- Climb gradient requirements depend on obstacle clearance and engine configuration; a missed approach may require a specified gradient.
- Use the chart units, configuration, bleed setting and safety factor; do not mix chart conditions.
Worked example
Pressure altitude is 2,000 ft and OAT is 30 C. ISA temperature at 2,000 ft is about 15 - 3.96 = 11.0 C. The ISA deviation is +19 C. Using the approximation of 120 ft per degree C, density altitude is about 2,000 + (120 x 19) = 4,280 ft. The takeoff run must be read at about 4,300 ft density altitude, not 2,000 ft. That higher density altitude reduces performance and may make the runway limiting.
Traps
Do not use pressure altitude as density altitude. Do not ignore a tailwind component or runway slope. Do not apply a dry-runway chart to a wet or contaminated runway. Do not interpolate outside the chart range; use the conservative envelope.