Spirit AeroSystems is one of the world's largest independent manufacturers of aerostructures — the structural components that make up an airplane's body, wings, and engine housings. Spirit's core product is fuselages, wings, nacelles, and pylons for commercial jetliners. Spirit's most important program is the fuselage for Boeing's 737, which represents roughly 52% of Spirit's total backlog. Spirit is Boeing's sole-source supplier for the 737 fuselage, meaning Boeing has no qualified alternative. Spirit also produces structural components for the Boeing 787 and wing systems for the Airbus A220 and A320 family. Boeing represents approximately 58% of Spirit's revenue, and Airbus approximately 21%. Spirit operates through three segments: Commercial (~78% of revenue), Defense & Space (~15%), and Aftermarket (~7%). Spirit sells under long-term, sole-source supply agreements tied to the life of each aircraft program. Revenue is driven by the number of shipsets delivered and the negotiated price per shipset. Spirit's economics are highly sensitive to production rates, given its large fixed cost base; the 737 program is the primary driver of profitability. A key structural challenge is Spirit's use of long-term fixed-price contracts on programs where costs exceed contract prices, generating forward losses — particularly on the Airbus A220 and A350. Spirit's Defense segment operates under cost-reimbursable or fixed-price government contracts, generating more stable margins than the Commercial segment.