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If an Altimeter must be within +/- 75 of known field elevation, and the aircraft has two altimeters, one could be +74 while the other could be -74 and both would be within 75 of known field elevation, but 148ft apart from each other.

I've heard some people say the two altimeters must be within 75 feet of each other, but if this is the case, then the first rule can't be correct.

If ALT1 was -74ft from field elevation and ALT2 was -148ft, then they would be within 75ft of each other, but it would violate the first rule as ALT2 would be -148ft from known field elevation.

It has to be one or the other, it can't be both rules. I think the second rule of within 75 feet of each other is just made up, but I wanted to check to see if anyone has a source for that rule.

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In flight, where there is no field elevation to compare to, tolerances are specified as "between the 2 altimeters." 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?

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