EMS Base Question  

The secondary is, indeed, a backup that's just not the same aiframe type.  For most regions, the spare aircraft is the same aircraft type (although may not have the same cockpit setup so they make sure you have differences training to know how to use the SAS or other system across all cockpits of that aircraft type). 

You'll usually get a spare pretty quickly if your bird goes down hard for mx.  Just depends on if the spare is taken.  If not; you come in for your usual shift and hang out (knock out online training, clean the base, etc.).