Density altitude

The invisible performance thief: how pressure, temperature, and humidity combine into the number your airplane actually performs at.

12 min read · Weather

After this lesson you can

  • Compute pressure altitude from field elevation and altimeter setting.
  • Compute density altitude from pressure altitude and outside air temperature using a worked example.
  • Explain how pressure, temperature, and humidity each independently affect air density.
  • Describe, in general terms, what a high density altitude does to takeoff roll, climb rate, and engine power.

Density altitude is the single most underestimated killer in general aviation performance planning. A runway that is plenty long on a cool morning can be dangerously short on a hot, humid afternoon at the same field, even though the elevation on the airport diagram never changes. This lesson gives you the calculation, in full, with real FAA worked numbers, and connects it to what actually happens to the airplane.

You already met density altitude briefly in the previous lessons; here it gets the full worked treatment it deserves before your first cross-country planning session.

The rest of this lesson

  1. 1.Pressure altitude, precisely
  2. 2.From pressure altitude to density altitude
  3. 3.Pressure, temperature, and humidity: three independent effects
  4. 4.What density altitude costs on takeoff
  5. 5.Density altitude and the three places it bites
  6. 6.Deciding before you roll

These 6 sections, their figures, the checks along the way and the end-of-lesson quiz come with the PPL track. Paid once, yours for good.

See what the PPL track costsEvery module 1 is free to read, no account, no card.

Key terms

Try to recall each definition before turning the card.