Performance
Performance - what the exam tests
Performance questions combine POH chart reading with the physics behind it: what the aeroplane can actually do today, at this mass, on this runway, in this wind.
Core rules
- Take-off figures are normally published as ground roll and total distance to 50 ft; landing figures as distance from 50 ft and ground roll. Enter the chart with actual mass, pressure altitude and temperature.
- Density altitude = pressure altitude + 120 ft for each °C the OAT is above ISA. ISA is 15 °C at sea level, falling 2 °C per 1,000 ft.
- High density altitude reduces engine power and propeller thrust and reduces lift for a given IAS, so the ground roll lengthens and the climb gradient falls.
- Wind: apply only the headwind or tailwind component, taken from a crosswind chart - never the full wind speed.
- Slope, surface and contamination are corrections applied after the chart value, usually as percentages.
- Vx gives the maximum angle of climb (obstacle clearance); Vy gives the maximum rate of climb (time to altitude). They converge with altitude, and Vy falls as mass increases.
Worked example
Aerodrome elevation 2,000 ft, QNH 1013, OAT 30 °C. ISA temperature at 2,000 ft = 15 − (2 × 2) = 11 °C. Density altitude = 2,000 + 120 × (30 − 11) = 2,000 + 2,280 = 4,280 ft. Enter the POH at 4,280 ft and the actual take-off mass, read the ground roll, then apply the wet-runway allowance and any upslope correction. Using sea-level figures here would understate the run by well over a third.
Traps
- Using aerodrome elevation as pressure altitude with QNH still set on the subscale.
- Averaging wind instead of resolving the component.
- Reading the chart at maximum weight instead of actual weight.
- Forgetting that the published total distance already includes the airborne segment to 50 ft.
- Assuming a demonstrated crosswind is a limit - it is a demonstrated value, and the aircraft limit may be lower.