How is aircraft service life defined and substantiated?

Service life defines operational limits for continued airworthiness. Life definitions: Design Service Goal (DSG) - Initial design target (typically 20+ years, 50,000+ flights for transport); Extended Service Goal (ESG) - Extended life target beyond DSG; Limit of Validity (LOV) - Certified period for maintenance program effectiveness. Substantiation: Fatigue testing - Full-scale test to 2+ lifetimes with damage; Damage tolerance analysis - Crack growth and inspection intervals; Corrosion prevention - Materials, coatings, drainage; and Environmental factors - Spectrum of loads, temperature, humidity. Life-limited parts: Landing gear, pressurized structure, engine mounts with specific life limits. Monitoring: Operational usage parameters, structural sampling, and Supplemental Structural Inspection Program. Life extension: Additional testing and analysis, Enhanced inspection programs, and Repair/modification as needed. LOV establishes period within which maintenance is effective; operations beyond LOV require additional substantiation.