Questions from the Cockpit: The lower the better?

Jasmine, a private pilot from Maine, asks: What’s the lowest density altitude ever recorded? And just how well would a plane perform in those conditions? 

What is density altitude? It’s slang for how thin the air is. As we go up in altitude, air molecules socially distance, reducing the air pressure. This has the practical effect that there’s “less” air to breathe in Denver than in Boca Raton.

This thin air affects more than just the pilot. The airplane’s engine needs to breathe, too, so when the air is thinner it develops less power. At the same time, the prop generates less thrust because there is less air to move, and the wings produce less lift.

A takeoff chart for a Cessna 172R.

A glance at any aircraft’s takeoff chart shows that performance drops with altitude — and the reason for this is the air density. The higher you go, the more runway you need to takeoff and the slower you climb. That’s why airports at higher altitudes have longer runways.

But here’s the cool thing: The air can also get thinner without leaving Boca Raton. Just add heat.

Actually, I guess I should have said, here’s a hot tip, rather than “here’s the cool thing.”

Moving on… Increasing air temperature also increases the social distancing of the air molecules. Hot air expands. It thins. Being warmer in any given place is the same as being at a higher altitude, at least when it comes to aircraft performance.

And that’s what density altitude is: It’s a way of quantifying temperature-induced air thinness in an easy to understand way. We simply report density altitude to a comparable real-world altitude on a standard day.

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