What does "redatum" means? I was doing a question about a fully hydraulic elevator and it's position in relation to the trimmable horizontal stabiliser and the explanation of the answer contained the word &quot;redatum&quot;. What does it mean?
Full question, answer and explanation:

Q: What is the position of the elevator in relation to the trimmable horizontal stabiliser of an aeroplane with fully hydraulically operated flight controls that is in trim?
A: Elevator deflection is zero
A fully hydraulically operated flight controls is an irreversible system, and the trim is a redatum, as there is no physical connection between the stick and control. Therefore there will be no elevator deflection when trimmed.
I've never heard of the term, but from the context of the explanation it appears that they refer to a situation where the elevator neutral position is relative to the stabiliser, not to the aircraft. So stick moves elevator relative to aircraft and stabiliser, trim moves elevator relative to aircraft only because the elevator neutral position follows the stab position. Like the stabiliser/elevator arrangement on the B737.

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