Number of altimeters
I am interested in medical instrumentation and am keen to find out about aviation instrumentation as a comparitive study.
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I am curious about the number of altimeters in the big jets. I am getting conflicting information. I have been told by some that airliners have three altimeters so that if two don't agree, you have a third to compare to find out which of the two is correct. However, i have been told by others (quoting all sorts of rules) that this is not true, and that jet liners need to have only two (unless two are electronic, you need a standby one).
SO who is right ? And if only two are required, then how would one know which is the correct one?