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Density altitude - the secret killer

Updated: Mar 9, 2022

It's no secret that airframe icing kills pilots. It's no secret that turbulence kills pilots. It's no secret that deep, moist convection and thunderstorms kill pilots. But what about density altitude? Density altitude is perhaps just as hazardous as airframe icing and turbulence. In an older NTSB study, density altitude contributed to just as many accidents as airframe icing and as many accidents as turbulence as shown in the pie chart below.

It is true that some of the density altitude accidents in this NTSB study were caused by pilots departing in an over-gross weight aircraft or using improper procedures (e.g., improper flap usage). However, pilots need to be aware that gross mistakes such as this are not forgiving when the density altitude is high. 

What is density altitude? In simple terms, density altitude (DA) is pressure altitude corrected for non-standard temperature. Therefore, if the pressure and temperature throughout the atmosphere matches the standard, then pressure altitude and density altitude are the same. Of course, during the warm season, the temperature is generally above standard in most airports throughout the U.S.

While a pilot can determine pressure altitude in the cockpit (by setting the altimeter to 29.92"), there's no instrument that you can use to directly measure the density altitude. It must be calculated based on the pressure, temperature and dewpoint temperature.  

The best way to think about density altitude is to first calculate the density of air (without moisture) based on a standard atmosphere. The table below is that calculation up to 4,000 feet for brevity. The column on the right is the resulting density of air based on the standard temperature and pressure while disregarding moisture (for now).