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The Perlan II glider has two altitude indicators and an altimeter on the navi screen, each showing different altitudes: Perlan II cockpit inside

How is each of the altimeters different? One indicator certainly shows MSL and the navi screen shows AGL but what does the other indicator with a yet different value show?

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  • $\begingroup$ It's possible one indicator shows pressure altitude (flight level) while the other one shows absolute altitude above a geoid regardless of pressure (e.g. exactly 29,032 ft at Everest's altitude). But which is which? $\endgroup$
    – Giovanni
    Nov 13, 2021 at 15:43

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The upper altitude is 66,560ft MSL. The middle altitude is 65,958ft AGL. Presumably, the terrain rises to 602ft MSL in that location.

The bottom altitude is 19,494m MSL, which converts to 63,956ft MSL. Note that this is GPS altitude, whereas the other two should be pressure altitude when up this high, so it is natural that they disagree by quite a bit.

Also, the two displays on the right show 66,569ft and 66,430ft. The former agrees with the MSL at the top of the main display, as I’d expect. The latter does not appear to be within tolerance and should be tested/calibrated, though perhaps the pilot just forgot to change it to pressure altitude.

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  • $\begingroup$ I think so too, the 66569 ft number is probably the sea level pressure altitude as it's on the navi, while the 66430 ft number is an auxiliary altimeter to be trimmed to another pressure? $\endgroup$
    – Giovanni
    Nov 13, 2021 at 17:38

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