Density Altitude: Know Your Enemy

It’s best to crunch the numbers before density altitude crunches you.

Meg Godlewski

Tuesday, May 16, 2023 at 11:26 AM ET

Verified

Preparing for density altitude when you fly this summer means always crunching the numbers. [Credit: Shutterstock]

Key Takeaways:

Density altitude (DA) is often misunderstood by pilots as an automatic "no-go" indicator, when it actually signifies reduced aircraft performance that requires careful calculation and planning.
DA is pressure altitude corrected for non-standard temperature and humidity ("high, hot, humid") and can be determined using an altimeter, E6-B flight computer, POH charts, or web-based applications.
High density altitude degrades aircraft performance, resulting in increased takeoff and landing distances, reduced climb rates, and higher true airspeeds on approach, emphasizing the need for pilots to consult performance charts and plan conservatively.

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It was a warm summer day, and the learner called to cancel his flight about an hour before he was scheduled to show up. His CFI noted the learner canceled two more times that week— this was unusual. During the work week, the learner usually flies in late afternoon. On Saturday, he flew in the morning. The Saturday appointment he kept, saying he was glad to fly and how bummed he was to have missed so many days because of high density altitude.

I was perplexed, as the DA never climbed above 1,600 feet that week, a value easily handled by the school’s fleet of Cessna 172s on the 3,600-foot runway at the airport with the field elevation of 534 feet msl. I asked the learner about it. He replied that he had been calling the airport’s automated weather a few hours before his flight. When he heard the words “density altitude,” he hung up, thinking it was a no-go day. He based this on a video clip he’d seen online that showed a very long takeoff roll of a Stinson 108 in Idaho, followed by a labored takeoff, followed by a stall and impact. His CFI sent him the clip as an illustration of density altitude. The learner took that to mean DA made airplanes go down. Therefore, when it was reported, it was a no-go situation.