Density altitude explained
What density altitude is, why hot-and-high days hurt aircraft performance, and how to estimate it.
Density altitude is pressure altitude corrected for non-standard temperature — in plain terms, the altitude the air "feels like" to your wings, propeller and engine. It is the single best predictor of aircraft performance, and on a hot day at a high-elevation airport it can be a killer.
Air gets less dense as temperature rises, pressure falls, and humidity increases. Thin air means:
- The wing produces less lift, so you need more speed (and runway) to fly.
- The propeller bites less air, producing less thrust.
- A normally aspirated engine makes less power.
The result is a longer takeoff roll, shallower climb, and higher true airspeed for the same indicated speed. "Hot, high, and humid" is the classic danger combination — a field at 6,000 ft elevation on a 35°C afternoon can have a density altitude near 9,000–10,000 ft, robbing a light trainer of much of its climb.
To estimate it: start with pressure altitude (set 29.92 in the altimeter and read it, or correct field elevation for the altimeter setting), then add roughly 120 ft for every degree Celsius above standard for that altitude. Many E6Bs, apps, and the Koch chart do this for you. Always compare the result against your aircraft's performance charts — and add a healthy margin. When in doubt, wait for the cool of the morning, reduce weight, or choose a longer runway.
*Reference and training only. Use your aircraft's performance data.*