VINTAGE INSTRUCTOR
THE
BY DOUG STEWART
The “DA” The abbreviation “DA” means different things to different folks. For people who get involved with court proceedings, it brings to mind a state or county prosecutor. For those who grew up in the ’50s, it might evoke a hairstyle resembling the posterior of a waterfowl. But for pilots it should mean only one thing: density altitude. Unfortunately, I have found— not only as an examiner asking an applicant to describe what density altitude is during a practical test, but also as an interested pilot perusing the National Transportation Safety Board (NTSB) accident records—that many pilots really don’t understand what density altitude is. And without that understanding, many are getting themselves into bad situations because they fail to recognize the ramifications of highdensity altitudes. It’s true that many pilots can give the “official” definition of density altitude: pressure altitude adjusted for nonstandard temperature. But when asked how they might describe DA to a young child, they are at a loss. Before I offer my simple description of DA, let’s look at the definition first. We’ll begin with pressure altitude. The easiest way to explain pressure altitude is to say that it is indicated altitude on the altimeter, when the altimeter is set to the standard pressure of 29.92 inches Hg. Thus, the higher the atmospheric pressure, the lower the pressure altitude, and
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vice versa. We now adjust this altitude for nonstandard temperature. You will remember that standard temperature is 15°C (59°F). As the temperature rises above this standard, so will the density altitude.
And without the understanding, many are getting themselves into bad situations . . . But what if you are as numerically challenged as I am, or not of a scientific bent? We certainly don’t want to see our aircraft get bent, but it (your airplane) might very well end up rolled up in a ball if you fail to comprehend this important concept and its impact on aircraft performance. (You will, on the other hand, recognize its impact on the ground.) So I offer this simple explanation that even a young child could understand: Density altitude is the altitude that
your airplane “thinks” it’s at. If an aircraft were a sentient being capable of thoughts and feelings, it would factor in the mean sea altitude it was at, the barometric pressure, the temperature, and the humidity (which plays a major part in affecting aircraft performance, even though it is not factored into density-altitude calculations) and come up with a “feels like” altitude. The higher this “feels like” altitude, the more cautious we, as pilots, need to be. I hope we all know that the higher we go, the less dense the air gets, and the less dense the air gets, the poorer the aircraft performance gets, especially when it comes to takeoff, landing, and climb performance. This is really aeronautical knowledge 101. That being said, why is it that over the last five years (May 2003 through May 2008) the NTSB records show that there were 138 airplane accidents, including 79 fatalities, in which density altitude played a major part? (I’m sure if you asked an insurance underwriter whether he concurred with these numbers, he would come up with even more claims, as many accidents that might involve density altitude do not necessarily have to be reported to the NTSB.) As I looked at mentions of “probable cause” in the NTSB reports, I came up with the following statistics: • In 41 (29.7 percent) of these accidents, “poor pilot planning” (in many cases, no planning whatso-