Abstract: Icing will lead to shrinkage of flight safety envelope and thus a serious threat to flight safety. The study on the changes of flight safety envelope after icing is of great significance for designing maneuvering coping strategy and improving flight safety. In this paper, a dynamic model for the longitudinal channel of the iced NASA Generic Transport Model(GTM) is established based on polynomial fitting of the aerodynamic parameters of the icing aircraft. To obtain the safety envelope that can vary with the degree of icing, the reachability analysis theory is applied to the safety analysis of the icing aircraft during the landing phase. The intersection of the forward reachable set and backward reachable set is proposed as the flight safety envelope. The reachable set is obtained via computing the optimal solution for the Hamilton-Jacobi partial differential equation via the level set method. Finally, the time domain verification is carried out for different degrees of icing conditions, and the corresponding maneuvering coping strategies are proposed. The results show that mild icing condition has little effect on the safety envelope, and the flight state can be maintained within the safety envelope all the time under the guidance of the optimal control during the landing phase. However, for the severe icing condition, the safety envelope shrinks severely and regular manipulation cannot meet the landing requirements. At this time, the behavior of the aircraft should be changed. The research results lay the foundation for flight control and real-time envelope protection.

Key words: flight safety envelope, icing aircraft, level set method, Hamilton-Jacobi equation, flight manipulation