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2 Answers

Why does Atis give you Density Altitude?

Asked by: 707 views General Aviation

When I call the ATIS i receive a Density altitude of 1400 when the Airport is at 400MSL. Why does the ATIS provide that information and what is it being used for ? 

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2 Answers



  1. Russ Roslewski on Aug 31, 2016

    To your first question, why the ATIS provides the DA, it’s a simple answer. You could calculate it yourself if you wanted using the temperature and pressure altitude, but the ATIS just gives it to you so you don’t have to bother. Consider it a “service”.

    I will answer your second question “What is it used for” by way of some other questions. What effect does density altitude have on aircraft performance? Have you ever taken off at a significantly higher DA? What effect did it have? How about flying in the mountains? To determine the specific effects of high DA on your airplane, I refer you to your POH. A general discussion of the effects of DA is contained in virtually every Private Pilot textbook, though for a free discussion, download the FAA’s Pilot’s Handbook of Aeronautical Knowledge and refer to chapter 4.

    Your CFI should be able to answer any other questions you might have on DA.

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  2. sixzero on Aug 31, 2016

    Here is some additional info for you for your bag of tricks. In my opinion this is the easiest way to determine without utilizing an E6B or a computer to determine DA.

    The formula is: DA=PA + (120TV)

    120 = the change in DA for 1 degree of C deviation from standard, and TV = Temp Variation from standard which is 15 Degrees at sea level.

    A sample problem from the HAATS manual –

    PA = 6500′ and FAT = 35 degrees C

    At 6500′ using the standard lapse rate the temp should be around 2 degrees C. Subtract 2 from 35, which leaves you a deviation of 33 degrees. Multiply this by 120. The reason we use 120 is that 120 feet per 1 degree of from standard is a constant, therefor every 5 degree change in temp increases your DA by about 600 feet. 120X33 gives you 3960. Add this to your existing PA of 6500, now you have a DA of 10,460. Now your air is thinner, airfoils are not as effective, and engine performance is degraded.

    All the effects of High DA are well explained in chapter 4, as Mr. Roslewski mentioned. Most performance charts in operators manuals should have converted for you based off PA and temp, and the airport’s wx reporting service has already done it for you as well.

    The above method is made you to plan ahead for those areas where accurate information is not available, and performance planning is speculative at best.

    Not accounting for significant changes in DA, and poor performance planning has killed many aviators. Understanding its effects and compensating for it is extremely important.

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