Flight
The Atmosphere & Altitude
13 min · 2 worked examplesFree
Air gets thinner with height, and almost every aerospace consequence follows from that one fact.
After this lesson you can
- Say how density changes with altitude
- Tell indicated from true airspeed
- Explain why a hot day lengthens a takeoff run
Density falls with height
Atmospheric pressure and density fall roughly exponentially with altitude. At about 5.5 km both are half their sea-level value; at 11 km, around a quarter.
Temperature falls too, at about 6.5 °C per kilometre through the troposphere, until the tropopause at roughly 11 km, above which it stops falling.
The standard atmosphere is an agreed model of all this, 15 °C and 101.3 kPa at sea level, so that aircraft performance can be quoted on a common basis. Real conditions differ, which is why performance charts are corrected for temperature and pressure on the day.
Every term in the lift equation that involves ρ is affected, so thinner air means less lift, less drag and less engine power, all at once.
In the full lesson
- Density falls with height
- Indicated and true airspeed
- Density altitude
Free with an account. Read it, work the examples, answer the questions, then build it in the simulator.