Copied from the TA Q&A

No other carrier, regional, or otherwise can do Spirit’s flying, except if Spirit contracts out to have it performed. Such contracting would be subject to the time limits and circumstances in Section 1.B.2. Spirit could, perhaps, code share with a regional carrier using the MRJ. However, in a typical code share agreement the carrier doing the flying keeps 99% of the revenue and, as with contracting out, there is no reason for Spirit to give up revenue from a profitable route just to have a regional carrier perform it using equipment with less seating capacity. Fee for service arrangements would be even more impractical because Spirit would not only be losing much of the revenue from the flying, they would be paying to have another carrier do flying Spirit that could do profitably, or for passenger feed – something Spirit has never done and which would make no sense.

Finally, Spirit could acquire a regional carrier that used Mitsubishi aircraft. However, the Mitsubishi (at 88,000 pounds) is above the weight limits for regional aircraft as defined in Section 1.BL of the TA and, therefore, any growth in flying performed by a carrier using the Mitsubishi would have to be accompanied by comparable growth in Spirit flying.