An envelope protection method with autopilot systems was proposed based on the research of wing stall and tailplane stall under icing conditions. The angle of attack was identified as a critical parameter

and the pitch angle was identified as a command parameter. The detection of hinge moment coefficient could early warn aircraft stall and increase the margin of envelope protection. The flight longitudinal dynamics equation in pitch attitude holding (PAH) mode was built. Results show that

the envelope protection system works at the 2 degrees before the stall angle of attack through the detection of hinge moment under icing conditions

increasing the flight safety margin. With the control command limitation

the angle of attack of the automatic flight under icing conditions could be limited to the stall angle of attack.