Just curious. What if you put a (general aviation or airliner) plane into a hard vacuum chamber? Would its altimeter simply read the highest possible altitude it could? That's rather impossible for general aviation planes as their altimeter can theoretically go on infinitely. Or would the altimeter simply fail?
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3$\begingroup$ "their altimeter can theoretically go on infinitely" [citation needed] $\endgroup$– FedericoOct 27, 2021 at 9:42
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$\begingroup$ @Federico General aviation aircraft usually have analog pointers, not digital screens showing the altitude. You wouldn't know whether the pointer shows 1,000 ft or 101,000 ft as it would look the same for each 100,000 ft. $\endgroup$– GiovanniOct 27, 2021 at 10:30
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1$\begingroup$ From linked post, for example for 1 unit: The actual mechanical range is -15,000 to +50,000 feet. $\endgroup$– user14897Oct 27, 2021 at 10:41
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$\begingroup$ the fact that the arms seem to be able to rotate indefinitely, does not mean that they can, theoretically or practically. $\endgroup$– FedericoOct 27, 2021 at 10:44
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$\begingroup$ @Federico Alright, but that's what I meant by "theoretically go on infinitely" and GdD's answer explains why it cannot do so practically. $\endgroup$– GiovanniOct 27, 2021 at 10:46
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1 Answer
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Analog altimeters are mechanical devices which have a set of air filled discs called aneroid wafers inside which expand or contract with air pressure changes. The expansion or contraction of the wafers is translated to rotational movement which turns the pointers. As air pressure decreases the wafers will expand to their maximum, then stop as they can't move any farther, further decreases in pressure will not increase the altitude readout. A full vacuum may rupture the wafers, which would make the instrument inoperable.
Electronic instruments will behave as they are programmed, so the answers are likely to differ by manufacturer. Although I have no direct experience with this I would expect that when the sensor's pressure data gets below a certain threshold that the computer is more likely to show it as an error rather than show the airplane being at an altitude that's impossible. Ladies and gentlemen, we are not cruising at 150,000ft, I know that much!
