How do you certify an engine for ice crystal icing conditions?

Ice crystal icing (ICI) occurs in high-altitude convective weather where ice crystals enter the engine, partially melt on warm surfaces, then accrete. Certification per FAR 33.68: Analysis of ice crystal ingestion at FAA Appendix D conditions, Altitude chamber testing with ice crystal simulation, Engine test with ice injection demonstrating safe operation and recovery, and Computational modeling validated against test data. Key considerations: Ice accretion on fan blades, stator vanes, and compressor stages, Shedding effects on downstream components, Power loss and recovery characteristics, and Pilot procedure development. Several A380, 787 incidents drove regulatory updates. Modern engines include ice detection and anti-ice/de-ice systems on vulnerable surfaces.