The content about airplane critical ice shapes in Amendment No. 25-121 of 14 CFR Parts 25 is analyzed. We presented the determination method of the critical ice shapes on large airplanes used in icing airworthiness validation. The study is conducted on the ice accretion characteristics within the different flight phases. The critical analysis about icing parameters is conducted based on the different parts of aircraft to determine the critical icing condition. The critical ice shapes is determined using the numerical simulation and icing wind tunnel test. The method to determine the disabled ice and delayed turn on(DTO) ice accretion is also presented. According to the different needs about icing criticality of the flight performance, maneuver and stabilization and engine design, the effects to the aircraft of all ice shapes are compared and evaluated. Then the critical ice shapes used in airworthiness validation is determined. The contents of this paper can be be consulted in large aircraft airworthiness validation.