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5 Answers

Pressure and Temperature Changes

Asked by: 2074 views Private Pilot

Studying for this written exam and some of these questions have me good and confused.  Going through the pressure and temperature changes and the program I'm using talks about "high to low, look out below" but that doesn't always seem to work with the questions.  Dealing with some of them that have you changing altitude without adjusting your alitimeter and the opposite appears to be true (unless the course is giving me the wrong answers).  Can anyone provide some guidance with this one?

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5 Answers

  1. Best Answer


    Lucas on Jul 08, 2012

    Dear Greg
    This is how we teach it at pilottrainingsolutions.com, see if this works for you. We have a completely different way of teaching altimeters.
    Here is the video, enjoy and let me know if this works for you
    http://www.youtube.com/watch?v=dLdsdQa6cmo
     
    lucas
    http://passfaaexams.com/

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  2. John D. Collins on Jul 08, 2012

    A few definitions might help.  The barometric altimeter reads pressure calibrated in altitude. So, the lower the pressure, the higher the indicated altitude will be. As a rough measure, the pressure at sea level is approximately 29.92 inches of mercury on a standard day and decrease approximately 1 inch per 1000 feet.  If the altimeter doesn’t have a barometric setting, it will indicate what is known as pressure altitude that is altitude based on a sea level standard pressure of 29.92 inches.  The altimeter in our aircraft has an adjustment that can compensate for higher or lower pressure by adjusting the altimeter setting in the Kollsman window.  We get this value from ATIS/ASOS/ATC and adjust our altimeters accordingly. According to the FAR’s, we must do this at a minimum of every 100 NM traveled.

     

    If you travel from a high pressure area, say 30.12 to a lower pressure area, say 29.82, and you forget to obtain and adjust your altimeter setting, the altimeter will indicate 300 feet higher than you actually are.  This is where the memory aid “look out below when going from high to low” comes from.

     

    Temperature also affects the altimeter, but there isn’t a correction for it on our altimeters.  A colder temperature will decrease the air pressure, making the altimeter indicate higher than you actually are. This is because the pressure and the temperature are related by the equation P=NRT/V, where P is the pressure of a gas, V is the volume of the gas and essentially a constant, N is the quantity of the gas and R is a constant, and T is the absolute temperature. This can be approximated as P=kT where k is a constant, because in the atmosphere, the quantity and volume don’t change. As the temperature rises, the pressure rises and conversely as the temperature falls the pressure falls.  So the memory aid works equally well when flying from a high temperature area to a low temperature area “look out below when going from high to low”.

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  3. Nathan Parker on Jul 08, 2012

    “Dealing with some of them that have you changing altitude without adjusting your alitimeter and the opposite appears to be true (unless the course is giving me the wrong answers).  Can anyone provide some guidance with this one?”
     
    Keep in mind that these questions generally assume that you will maintain a constant indicated altitude.  So if you go into a low pressure area, you will descend in order to keep your indicated altitude the same.  Your height above ground would decrease, hence the “look about below”.

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  4. Frank Baard on Jul 08, 2012

    try this video http://www.youtube.com/watch?v=dLdsdQa6cmo
    its a different way of looking at altimeters
    it worked great for me.

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  5. Greg Adler on Jul 09, 2012

    Frank and Lucas – thanks!  Working as a geologist for 15 years, I consider myself a pretty smart guy, even if I am from L.A. (Lower Alabama) but I just couldn’t make sense of that thing.  Don’t want to be a flippin meteorologist!  The video definitely made this very easy to grasp. 
    Appreciate all the helpful answers!

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