HC wrote
The FMS is not "Client's operational equipment", it is part of the basic aircraft and thus must be serviceable unless allowed to be U/S in the MMEL/MEL. Which it is, but with the caveat "provided procedures do not require their use". And for the GPS "as required" (by operational regulations).
My company (one of those ignorant interfering clients you like so much) requires 2x FMS. I think we can depart with 1 unserviceable in certain weather conditions.