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Density altitude on ATIS

Asked by: 5321 views Commercial Pilot, FAA Regulations, Flight Instructor, General Aviation, Instrument Rating, Private Pilot, Student Pilot

When performing performance calculations, the charts start with pressure altitude and then have you interpolate for the current air temp. In the end you end up with performance figures based on density altitude. Why is it then that some ATIS report current density altitude, when the pressure altitude is the basis for performance calculations?

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2 Answers



  1. Russ Roslewski on Apr 28, 2016

    Three answers.

    1. “Pressure altitude” is NOT the basis for performance calculations. Density altitude is what ultimately affects aircraft performance. The POH you are using just integrates pressure altitude and temperature so that you don’t have to know the density altitude to use the chart. Not all POHs do it this way. For example, I have a POH for a Piper Comanche where the performance charts are referenced directly to density altitude (which is termed “standard altitude” in these POHs).

    2. Density altitude takes some calculation to arrive at, so the ATIS does it for you. However, you have an instrument right in front of you in the airplane that will tell you the pressure altitude – your altimeter. Just set the altimeter to 29.92 and whatever it shows is your pressure altitude. No reason to broadcast something that is so easy to determine.

    3. Pressure altitude changes very little. Rarely more than + or – 500 feet from your field elevation. So there is very little effect on performance due to pressure altitude. For all practical purposes, you could just assume pressure altitude is equal to your field elevation and have no problems. Density altitude, however, can vary by thousands of feet, so it has a very large effect on aircraft performance. It’s by far the most important number.

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  2. Mark Kolber on Apr 29, 2016

    Why is it then that some ATIS report current density altitude, when the pressure altitude is the basis for performance calculations?

    Russ’ explanation of how it works is spot on. But I’ll address the way you are asking the question.

    As Russ said, density altitude is ultimately the basis for all performance calculations. It’s the charts the aircraft manufacturers insert in their manuals that begin the calculation with pressure altitude. Whether graph (Piper) or table (Cessna), the POH starts with pressure altitude and then applies the temperature, the interim result at that point is density altitude.

    Why do they do that? Well, how does Cessna or Piper or Beech know whether you are flying out of an airport with an AWOS that broadcasts density altitude, a towered airport with an ATIS that just gives you temperature and says “check density altitude” or a quiet Class G with no weather reporting at all? (And of course, AWOS and reporting density altitude is relatively new in the history of aviation)

    They obviously don’t. So their charts give you the tools to calculate takeoff performance when you leave any airport (or even a landing site that is not an airport at all) – pressure altitude and outside air temperature – and leave the calculation for the table or graph.

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