If a SET OFFSIDE RNG/MODE message appears on the ND, what does it indicate?

The appearance of a "SET OFFSIDE RNG/MODE" message on the Navigation Display (ND) indicates that there is an issue related to the configuration and synchronization of the Flight Management and Guidance Computers (FMGCs). When this message is shown, it signifies that both Navigation Displays must be set to the same mode and range because there is a single FMGC failure affecting the data presented on the ND.

In normal operations, the FMGCs communicate and compare data to ensure both NDs display consistent information. If a fault occurs in one of the FMGCs, the system may not be able to verify the integrity of the data between the two systems, leading to the necessity for the pilot to manually confirm that both NDs are set identically. This is crucial for ensuring navigational accuracy and consistency between the displays, as relying on one failed FMGC could lead to incorrect navigation data being presented.

Understanding this message is vital for flight crews to ensure they can manage potential failures effectively and maintain situational awareness during their flight operations.