Density Altitude

Every summer writers warn about the effects of density altitude on aircraft performance; and every year pilots mush off runways into trees and power lines. Why is that? For one thing, density altitude is a nondenominational, equal opportunity killer. It doesn’t matter what kind of airplane you’re flying or what rating you might have.

Conceptually I think everyone who completes ground school in pursuit of a private pilot certificate understands that with higher altitude or hotter temperature, aircraft performance decreases. But even some experienced pilots don’t appreciate the significant impact of high temperature and/ or high altitude on an aircraft’s ability to fly. In part, the performance degradation is masked in many POH tables; it really doesn’t jump out at you.

A friend of mine who has been a private pilot for some time had his “Aha” moment when he was shown a Koch Chart, which you will find below. To determine the additional runway distance— and corresponding decrease in rate of climb—you will require at any given temperature, take a straight edge and line it up with the temperature on the left scale and with the airport’s pressure altitude, as shown below, on the right scale. You may determine airport pressure altitude by setting the Kollsman window at 29.92 and noting the altimeter reading. Next, check the effect on your aircraft performance on the scales in the middle of the chart.

In this example, with a pressure altitude of 6,000 feet (Big Bear’s field elevation is 6,752 feet) and a temperature of 100°F, you will need to increase your standard day normal takeoff distance by at least 200%; and you should expect 75% decrease to standard day climb performance.

After seeing this chart during a Wings seminar, my friend commented that he now understood why he had such a hard time taking off and climbing out from Albuquerque one summer day several years before. To his dismay, he realized he had been an accident waiting to happen for all those years because he didn’t appreciate the drastic affect higher altitudes and higher temperatures have on every airplane’s performance. My son and I had a similar experience one summer day taking off from a high altitude airport. But because we were aware of the density altitude, we readily accepted a little crosswind to use a longer runway rather than the one more aligned with the wind.

You can see from the chart above that rising temperature profoundly affects aircraft performance, specifically takeoff distance. That’s because there are fewer molecules of air going over your wing to generate lift; through your propeller to push and generate thrust; and into your engine’s fuel/air mixture to generate power—a triple threat. A good solution: Takeoff early in the morning before it gets hot, if your schedule allows. If not, check your takeoff data carefully; make sure you have plenty of runway available, and use it all. Density altitude is so critical many airports now include it in their ATIS. So, if you’re taking off from an airport with an elevation of 4,000 feet and the ATIS is reporting the density altitude at 7,000 or 8,000 feet, an alarm needs to off in your head: Did I calculate my takeoff data accurately and with the most current weather information?

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About Rich Martindell

Instrument flight instructor (CFII), rated airline transport pilot (ATP), former military instructor pilot in F-4s and F-15s. Aircraft accident investigator and flight safety consultant. FAA Safety Team Lead Representative.
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