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Difference between True Altitude and Pressure Altitude

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Private Pilot

True altitude is height above MSL and barometric pressure of MSL is 2992. I am not sure how true altitude is different from pressure altitude which also requires kolesman window setting 2992. They both required 2992 and sound same to me. If two are not always the same, when true altitude and pressure altitude could be different? I am struggling to figure out this for hours. please help.

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  1. John D Collins on Apr 09, 2015

    By your terminology, true altitude is what I would call MSL altitude. MSL altitude is pressure altitude corrected for non standard pressure. True altitude is what you would obtain if you had a sea level reference point and used a tape measure to see how high you were above this point. Pressure altitude is based on a model of the atmosphere where the pressure and temperature gradient are standard and referenced to a sea level standard pressure of 29.92 inches of mercury. But pressure varies with weather and temperature. If you correct the altimeter setting to reflect a non standard pressure, this will give you MSL altitude and be what your altimeter reads if you set the appropriate altimeter setting into the altimeter. But it still won’t match the true altitude, because it is also based on a standard temperature lapse rate. There is hardly a day that goes by that one could find the atmosphere cooperate and exactly match the model. The MSL altitude is useful for separating aircraft from each other vertically, and fairly accurate when the altitude is close to the location and altitude where the altimeter setting is determined. It is not so good for avoiding obstacles or mountains that are well above the location where the setting is established. It also has more error as one increases in altitude. That is one reason why above 18,000 feet MSL in the US we all set out altimeters to 29.92. At this setting, the altitude is called a flight level because it is a level with a constant pressure and may be far off the true altitude.

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