How do you develop a fatigue loading spectrum for aircraft structural analysis?

Fatigue spectrum development involves: Define mission profiles (typical flight segments - climb, cruise, descent, maneuvers), Ground-Air-Ground (GAG) cycles for pressurization, gust loads from atmospheric turbulence models (continuous gust or discrete), maneuver loads from V-n diagram and usage data, landing/taxi loads from statistical surveys, and system/component loads. Loads are combined into sequences representing design service life (DSG - typically 60,000-120,000 flights for commercial). Rainflow counting reduces to equivalent cycles. Truncation of low-damage cycles and omission reduces test time while conservatism is maintained. Industry standard spectra (e.g., TWIST, MINITWIST for transport wings) provide benchmarks.