Abstract: Icing is an important environmental factor threatening flight safety. It is extremely important to study the safe flight envelope of aircraft in complex icing environment for flight envelope protection and improvement of flight safety. Taking Research Civil Aircraft Model (RCAM) as an example, the aircraft with high nonlinear system is decoupled into two subsystems based on the time scale separation principle, establishing the longitudinal motion model which considers the transverse motion of aircraft and the robust aerodynamic derivative model in a complex icing environment. Based on the optimal control theory of reachability, the backward reachable set is used as the safe flight envelope. Then, the effects of icing and rolling motion on the safe flight envelope are studied with reachability analysis. On the above basis, the robust safe flight envelope is further analyzed under uncertain icing conditions such as the icing position, icing type and distribution at different icing degrees. The results show that with the increase of icing degree, the safe flight envelope is narrowed. When considering the uncertainty factors, the robust safe flight envelope under mild icing case condition shrinks more significantly than that under severe icing condition. Therefore, even if the aircraft is mildly frozen, the flight risk is still large when considering the uncertainty factors, and the pilot should remain vigilant. The results can provide some reference for the flight envelope protection of the aircraft in complex icing environment.

Key words: complex icing condition, flight safe envelope protection, time scale separation, reachability analysis, robust flight safe envelope