The goal of this research was to improve the envelope protection capabilities of an aircraft in icing conditions. To accomplish this goal, open and closed loop envelope protection algorithms were developed to ensure the safe operation of an iced aircraft during both manual and autopilot modes of flight. The Iced Aircraft Envelope Protection system (IAEP), developed as a part of the Smart Icing Systems (SIS) research project at the University of Illinois, was based on data from wind tunnel tests, flight tests and iced aircraft simulations obtained from a six-degree-of- freedom computational flight dynamics model. The system consisted of estimative and predictive methods for approximating, and avoiding the envelope boundaries. Simulation results demonstrated that IAEP was capable of successfully avoiding incidents and accidents during flight in icing conditions. This paper includes a summary of the basic scheme of the longitudinal iced aircraft envelope protection system and a discussion of results obtained through simulation.