Welcome Guest. Sign in or Signup

2 Answers

Why does Atis give you Density Altitude?

Asked by: 792 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 ? 

Ace Any FAA Written Test!
Actual FAA Questions / Free Lifetime Updates
The best explanations in the business
Fast, efficient study.
Pass Your Checkride With Confidence!
FAA Practical Test prep that reflects actual checkrides.
Any checkride: Airplane, Helicopter, Glider, etc.
Written and maintained by actual pilot examiners and master CFIs.
The World's Most Trusted eLogbook
Be Organized, Current, Professional, and Safe.
Highly customizable - for student pilots through pros.
Free Transition Service for users of other eLogs.
Our sincere thanks to pilots such as yourself who support AskACFI while helping themselves by using the awesome PC, Mac, iPhone/iPad, and Android aviation apps of our sponsors.

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.

    +1 Votes Thumb up 1 Votes Thumb down 0 Votes

  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.

    0 Votes Thumb up 0 Votes Thumb down 0 Votes

The following terms have been auto-detected the question above and any answers or discussion provided. Click on a term to see its definition from the Dauntless Aviation JargonBuster Glossary.

Answer Question

Our sincere thanks to all who contribute constructively to this forum in answering flight training questions. If you are a flight instructor or represent a flight school / FBO offering flight instruction, you are welcome to include links to your site and related contact information as it pertains to offering local flight instruction in a specific geographic area. Additionally, direct links to FAA and related official government sources of information are welcome. However we thank you for your understanding that links to other sites or text that may be construed as explicit or implicit advertising of other business, sites, or goods/services are not permitted even if such links nominally are relevant to the question asked.