Density altitude in the mountains

Performance math at high elevation, with a worked hot-day example.

17 min read · Mountain and high-altitude flying

After this lesson you can

  • Compute density altitude from field elevation, altimeter setting, and outside air temperature.
  • Explain why an airplane's indicated rotation speed stays the same at altitude while its true airspeed and ground roll do not.
  • Read a takeoff or landing distance chart by interpolating instead of rounding to a convenient line.
  • Identify the climb-performance limits, Vx, Vy, service ceiling, and absolute ceiling, that density altitude erodes.

Every student pilot learns density altitude as a formula to plug numbers into. At a 7,000-foot mountain airport on a warm afternoon, it stops being a formula and becomes the difference between an airplane that climbs away from rising terrain and one that does not. The runway looks the same length it always did. The altimeter reads the same field elevation it always has. Nothing on the panel announces that the engine, the propeller, and the wing have all just quietly lost a large share of what they can do.

This lesson builds the calculation from the ground up, walks through a real hot-day departure at a well-known high-elevation airport, and then follows the number through to what it actually changes: the ground roll, the climb, and the ceiling the airplane can reach that day. The goal is not to memorize a formula. It is to be able to look at a temperature and an elevation and know, before the throttle goes forward, whether the airplane and the runway are still a match.

The rest of this lesson

  1. 1.Pressure altitude and the standard atmosphere
  2. 2.A hot afternoon at Aspen: a worked example
  3. 3.What thin air does to the engine and the propeller
  4. 4.The airspeed indicator's lie
  5. 5.Climb performance: Vx, Vy, and the two ceilings
  6. 6.Reading the performance chart without lying to yourself
  7. 7.Deciding on the ground

These 7 sections, their figures, the checks along the way and the end-of-lesson quiz come with the CPL track. Paid once, yours for good.

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

Key terms

Try to recall each definition before turning the card.