The aux tank was connected directly to the engine after the aircraft fuel
system, Not to the wing and the provided documentation and system
description mentioned nothing about the fuel return to the main tanks.

How would the fuel get to the main tanks in the first place? Is the
engine the only connection? (if so, with the fuel selector OFF that
should block fuel flow to the main tanks). Is there a vent line that
connects them?

Jose
--
"Never trust anything that can think for itself, if you can't see where
it keeps its brain." (chapter 10 of book 3 - Harry Potter).
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