Many Spirit flights appear scheduled with 757s and 737s. They don’t have those in the fleet. is this incorrect filing from the airline?
I’m not quite sure what’s up with that. Sometimes during the summer smaller airlines like spirit need more capacity but don’t have the fleet abilities. When this happens they usually charter or lease some spare aircraft. The charter option would make sense because 737 classics and 757s are very common on the charter market.
Hope that helps
theplanegeek is correct in his assessment. The need for sub service is usually caused by an aircraft down for unplanned maintenance, and no spare aircraft available to fill in. That happened several times during my tenure at Spirit. ( I retired from flying at Spirit in July 2014.) Spirit wet leased from Miami Air from time to time. Keeping the aircraft on the same route ie; FLL-LGA-FLL. Miami Air retained operational control of the aircraft.
Hopes this helps clarify.