"Aircraft Ice Detection Method"

Abstract

A method of ice detection is discussed which utilizes a combination of impedance, thermal conductivity and temperature measurements to detect the presence of glaze ice and to determine its thickness. Two supplemental means are described by which in-flight impedance data may be checked to assure that it is free of significant error. I Introduction To be alerted in real time that ice is forming on an aircraft or UAV and to be provided with the ice thickness variation with time remains a problem of interest, Ref. 1 and Ref. 2. Ice principally deposits from supercooled water droplets in the atmosphere that freeze on the vehicle at atmospheric/aircraft surface temperatures in the range from 0C to -25C and at altitudes between sea level and 22,000 ft, Ref. 3 and Ref. 4, though on occasion ice occurs outside these ranges. Two forms of ice may be encountered: glaze or rime, Ref. 3. Glaze ice is clear in appearance and weighs 0.98-0.99 g/cubic cm, while rime is white and opaque in appearance and weighs between 0.35-0.55 g/cubic cm depending on aircraft Mach number. Prior methods proposed for ice detection have included, but have not been limited to, visual identification, ice effects on vibrating structures, light reflection using fiber optics, Ref. 5, capacitive measurements, Ref. 6, and time domain reflectometry, Ref. 7. This paper discusses a method of ice detection which utilizes a combination of impedance, thermal conductivity and temperature measurements to detect the presence of ice and to provide its thickness. The method also includes supplementary ways for confirming the type of ice that is collecting and the fact that a correct measurement is being made. There are a number of general considerations that confront any aircraft ice detection approach. Not only must it operate properly during icing conditions, but it must demonstrate certain measurement values and characteristics that are unique to icing and not replicated when flying in fog or rain to avoid confusion. In addition, its use should not interfere significantly with airflow or add significant drag. Any electrical method, of which the impedance measurement approach discussed here is one, may be challenged by the dramatic variation with temperature of the electrical properties of pure glaze ice as well as rime ice in the temperature range of interest (taken here to be 0C to -40C), Ref. 8. Electrical measurements may be affected by cracks, flaws and/or voids in the ice formed over the sensor being used to make the measurement, Ref. 9 ; a situation associated more with rime ice though it may also occur with glaze ice. Acid rain present in the atmosphere and other contaminents may also have some effect on ice electrical properties of the ice that is formed. Finally, it is important