Density altitude
Performance depends on air density. High field elevation, high temperature and high humidity all reduce density and therefore thrust and lift. Density altitude is the pressure altitude corrected for temperature — the altitude the aircraft 'thinks' it is at.
Takeoff and landing distance
Distance grows with weight, tailwind, upslope and contamination. A rough rule: 10% extra weight adds about 20% to takeoff distance. Wet or contaminated runways can add 15–60% to landing distance.
Climb and cruise
Best rate of climb speed gives the fastest gain of altitude; best angle gives the most altitude per unit distance for obstacle clearance. Optimum cruise altitude rises as fuel burns off and the aircraft becomes lighter — hence step climbs on long flights.
Key points
- Hot, high and heavy is the classic performance trap.
- Vx clears obstacles; Vy gains altitude fastest.
- Step climbs keep a long-haul flight near its optimum altitude.
Knowledge check