The C-5 is the only ally in Arcade mode. A single aircraft appears in Arcade Mission 3, and must be escorted until it passes the southern boundary. Generally, staying in formation with it isn't too important, as it can take large amounts of damage before the mission is failed due to its destruction. In addition, it is rarely attacked, and few enemies attack at a time due to the spawn pattern.
This is the C-5's only appearance in the entire game.