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3 Answers

Pressure Altitude and Density Altitude

Asked by: 8455 views General Aviation

define in detail what is pressure altitude , density altitude and how do they affect each other please

 (not a text book definition! more like your undestanding of them)

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3 Answers

  1. John D. Collins on May 09, 2012

    Pressure altitude is the altitude that the altimeter reads if the altimeter is set to the standard sea level pressure value of 29.92.  It is not corrected for local pressure variations. This is the value that is transmitted by your transponder in mode C. It is also the value used when aircraft are operating at flight levels which usually start at 18,000 MSL in the USA.
    Density altitude is the one used for performance calculations, particularly for take off.  It takes into consideration the differences in pressure and temperature and how they affect performance. For all practical purposes, it is the equivalent altitude from a performance point of view that the aircraft and engine experience if one was to operate at that altitude on a standard pressure, standard temperature day.

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  2. Lucas on May 09, 2012

    Lets say that you have a house in the Rocky mountains and one in Florida, and lets say I would bring you blindfolded to one of these locations. If it was cold and you could barely breathe you would say I brought you to the mountains, right?
    Well you are in Florida on a cold day with low pressure.
    As your body interprets cold as being at high altitude, so does the altimeter, and try running a marathon at 10,000 feet, ahhhhh. Where is my oxygen?
    The higher you climb the colder it gets, so if it gets cold all of a sudden, as your body would, the altimeter would believe you climbed indicating a higher altitude (indicated higher than true).
    If the pressure of the air drops the altimeter, again as your body would, thinks it has climbed reading a higher altitude (indicated higher than true).

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  3. Pete Kemble on May 14, 2012

    Pressure altitude is more of a theoretical value – it only exists in reality on a day with standard atmospheric conditions and a standard lapse rate (29.92″, 15C and -2 deg C per 1000 feet). Obviously, the lapse rate isn’t that uniform, and the surface temperature and pressure change as well. Thinking only of theoretical numbers for a minute (pressure altitude), Aircraft performance charts are based off pressure altitude, because standard atmospheric conditions are a nice steady constant. You remember, though, that the first variable is usually the temperature – enter density altitude. Pressure altitude is a theoretical constant, given that the temperature will be standard at any altitude, and the surface pressure at sea level is 29.92. Density altitude is the same thing, only it takes into account non-standard temperature (i.e., it’s not always 13C at 2000′ PRESSURE altitude). I had a really hard time understanding temperature, volume (density) and pressure when I was working on my PPL, so forgive me if this doesn’t help!

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