Oooh mmmmm, not as simple as that I'd say ( with regards to AIRBUS anyway )
On the ground with "Map not available" you cannot select Terrain.......( at least I think so )
Dual FMC failure ( Dual latch ) in the Air you see "Map not available" until backup Nav is selected.
Try it in the Sim and see if you can display Terrain in Backup NAV.
Last edited by ACMS; 18th May 2015 at 06:53.