Electrical anti-icing of aircraft windshields

PREVENTION of ice formation on the aircraft windshield presents difficult problems. In addition to optical quality, the thermal and structural characteristics of suitable transparent materials, such as glass and certain plastics, must be given careful consideration. Further, an important design criterion for an aircraft windshield is its ability to withstand impact loads such as those which result when a large bird strikes the windshield. Contemporary windshield design practice usually calls for a laminated structure, commonly consisting of a layer of polyvinyl butyral, encased between two plates of tempered glass. The impact resistance of such a construction has been shown to be dependent upon the vinyl temperature, the optimum temperature being of the order of 110 degrees Fahrenheit. Direct correlation thus exists between impact resistance and anti-icing ability in windshield design. The regulations of the Civil Aeronautics Board specify that the windshield and supporting structure of a transport airplane shall be designed to resist the impact of a 4-pound bird when the airplane is flying at sea level cruising speed.¹ It is apparent, therefore, that it may be necessary to supply heat to a laminated windshield continuously, regardless of the prevalence of icing conditions, and further that the amount of heat must be controlled within the limits specified by impact resistance as well as deicing.