Performance
Performance tests takeoff, landing, climb and obstacle clearance using aircraft data, atmospheric conditions and runway state. The exam expects correct interpretation of density altitude, wind, slope and contamination.
Core concepts
- Density altitude is pressure altitude corrected for temperature; high density altitude reduces engine/ propeller performance and lift.
- Headwind reduces ground roll but not the air distance required to accelerate to V speeds.
- Runway slope, contamination and braking action affect accelerate-stop and landing distances.
- Climb gradient requirements depend on obstacle clearance and engine-out configuration; WAT limits combine weight, altitude and temperature.
Worked example
A runway is 1,800 m available. The AFM gives a sea-level ISA takeoff distance of 1,500 m. At pressure altitude 4,000 ft and ISA+20, the performance factor is 1.25. Required distance = 1,500 × 1.25 = 1,875 m. This exceeds the 1,800 m available, so the takeoff is not legal unless weight, configuration or conditions change.
Traps
- Using IAS instead of TAS for climb gradient calculations.
- Applying a headwind to the air distance; wind affects ground distance.
- Ignoring the effect of contaminated runways on accelerate-stop distance.
- Treating a factored distance as unfactored, or using the wrong flap setting.