The last Spirit Airlines flight — NK1833 from Detroit Metropolitan Wayne County Airport (DTW) to Dallas/Fort Worth International Airport (DFW) — touched down shortly after midnight in the early hours of Saturday, marking the end of more than four decades of operations for the ultra-low-cost carrier. The shutdown follows a tumultuous period for the Florida-based airline, which found itself squeezed by soaring jet fuel costs and the lingering grounding of its fleet due to Pratt & Whitney engine inspections.