Performance
Performance
Performance tests take-off, landing, climb, cruise and obstacle clearance under given mass, altitude, temperature, wind and runway conditions. The CPL exam emphasises density altitude, chart use, V-speeds and the difference between pressure altitude and true altitude.
Core rules
- Pressure altitude is indicated altitude with the altimeter set to 1013 hPa. Density altitude is pressure altitude corrected for non-standard temperature; high temperature, high altitude and high humidity reduce air density and increase take-off and landing distances.
- Take-off distance depends on mass, density altitude, wind, runway slope and surface. A tailwind or upslope increases ground roll; a headwind or downslope reduces it.
- V1, VR and V2 apply to multi-engine transport aircraft; light aircraft use rotation and best-angle/best-rate speeds. Best angle of climb (VX) clears obstacles; best rate (VY) gives maximum height gain per time.
- Climb performance is limited by engine power, propeller efficiency and cooling. Cruise charts give fuel flow and true airspeed for power settings.
Worked example
Pressure altitude is 5,000 ft; OAT is 25°C. ISA temperature at 5,000 ft is 15 − 2 × 5 = 5°C. The temperature is 20°C above ISA, so density altitude ≈ 5,000 + 120 × 20 = 7,400 ft. The pilot must use the 7,400 ft line, not 5,000 ft, for take-off distance; the penalty can be several hundred feet.
Traps
- Using QNH altitude instead of pressure altitude.
- Ignoring the effect of a wet or sloping runway.
- Reading the chart at the aerodrome elevation rather than density altitude.
- Assuming published distances include obstacle clearance when they do not.