Originally Posted by Rightseat Ballast

Shuttle does united flying only out of Denver. However, on occasion you may see a Shuttle America spare a/c at a Frontier gate doing a Frontier flight. However, these spares are flown by Republic crews using the Brickyard callsign. RAH finally found a way to convince the FSDO that a Shuttle America 170 is the same as a Republic 170, and we now have a legal means to swap a few airframes between certificates. So yes, you may see "operated by Shuttle America" on the side of a Frontier flight on occasion.