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True Altitude vs Indicated Altitude

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Aerodynamics

During my commercial training I am learning much more about how our Indicated Altitude can have errors due to non-standard atmospheric conditions.  We set our altimeter to nearest available setting which takes care of much of the variance between true altitude and indicated altitude.  But when we are flying along on our XC on a nonstandard day how do we make sure we are not going to bump into things up there? During my PPL training we talked very little about variances between true altitude and Indicated.   On my charts we would ensure that we were at least 500' above any obstacle along the way.  But is that really enough?   If we are 25C off of a standard day couldn't that 500' be reduced to 10'?  Also what about staying below/above airspaces?  Isnt ATC  going off of True Altitude?  Could I bust bravo or the SFRA(in the DC Area) due to these errors? 

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1 Answers



  1. John D Collins on Jul 13, 2017

    Yes there can be a significant difference between true altitude and MSL. MSL is the pressure altitude (29.92) corrected for the local barometric setting. Your transponder and any ADS-B system sends your uncorrected pressure altitude to ATC where the computer corrects it to show the controller your MSL altitude. So ATC will see the same value you have on your altimeter if it has the pressure set to the local value. True altitude is important if you don’t want to collide with the terrain or an obstacle, but if you don’t want to collide with another airplane or get in trouble with ATC, you will use the MSL altitude indicated on your altimeter. If you have a GPS altitude, it will indicate closer to true altitude as it is not affected by temperature.

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