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Guest bfindlay

High atitude BARO setting - Confused and unclear....

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I am currently in the FFX 733 DELTA heading to KMCO in heavy'ish t-storm weather. I'm at FL330 and the cockpit BARO is 29.92. But the MSFS GPS readout at the top left of the screen says I am at MSL 32289. I've observed this kind of diff on other flights using Weather Themes that I sometimes tweak in the Advanced Weather windows.Excuse my stupidity but I am still confused about pressure, baro, and ALT. Can somebody explain to me why I have this difference in ALT readings and are they normal?Thanks!JS

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Ignore the GPS, it is reading the altimeter setting that would be used below FL180 (i.e, the "local" setting). In the US, all aircraft operating above the transition altitude (FL180) are required to use the Standard Altimeter setting of 29.92. So all aircraft will maintain proper altitude separation, no matter what the actual altitude above sea level is. ATC expects your altimeter to be set to 29.92, when you reach the assigned altitude per your altimeter, you level off. You are now at the proper altitude as asigned by ATC. Set a "local" setting, and ATC will show you as not being at your assigned altitude. On descent, reset your altimeter to local settings as you pass down through FL180. HTHPaul

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The other poster is correct, in that altimeters are set to 29.92 above FL180 (and for transoceanic flight at any altitude in the absence of reporting stations). However your GPS is giving you good info - that is your absolute altitude (ie how high you truly are) as it triangulates it off satellite signals. The difference in the two indicates how much absolute error exists in your pressure altimeter. Also, he is quite correct in saying that you should ignore the gps - at least as far as flying at assigned altitudes. Since all aircraft will be experiencing the same pressure inaccuracies, separation will still be maintained. An example of precision vs accuracy.

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