If an aircraft has two altimeters what is the allowed difference between them?
In flight, where there is no field elevation to compare to, tolerances are specified as &quot;between the 2 altimeters.&quot; On the ground, they also must both be within 75' of the known field elevation.
Different systems may specify different in-flight tolerances, which may also vary somewhat with altitude.
See also, What is the maximum allowed difference between two altimeters for flying in RVSM airspace?