What is engine operability and how is it ensured across the flight envelope?

Operability ensures the engine operates safely and reliably at all flight conditions without surge, flame out, or overspeed. Key elements: Surge margin (distance from operating line to surge line, typically 15-25%), Stall margin at transient conditions, Stability at altitude relight conditions, Overspeed and overtemperature protection, and Foreign object tolerance. Ensuring operability: Variable geometry (VSV, VBV) scheduling, Control law design with protection features, Inlet compatibility across distortion levels, and Testing across the envelope including extreme corners. Analysis uses component maps and transient simulation validated by altitude testing. Military engines must handle rapid throttle movements, extreme maneuvers, and weapons firing effects.