Mixed-phase Inflight Icing Conditions

Recent investigations with modern instrumentation suggest that mixed-phase icing conditions are more frequent and widespread than had previously been generally realized. A conservative estimate is that an aircraft may be in mixed-phase clouds as much as 20% of the time that it is operating in icing conditions. Substantially higher figures, 50-80%, may be appropriate in some geographic areas.

Characterization of ice crystal and droplet size distributions in these regions is limited and are needed to address questions of aircraft safety and remote sensing, and numerical and wind-tunnel simulations. The data available from research flights do not suggest that there is any difference in performance effects caused by structural icing resulting from flight in mixed-phase cloud as compared to purely liquid cloud. However, these data cover a restricted range of conditions and do not appear sufficient for generalization.

If there is a difference in the nature of icing in mixed-phase regions, such as the accretion rate, the texture and location of ice, this needs to be accounted for in numerical accretion and performance models and in wind tunnels used for icing research. Some methods have been used to create ice crystals in wind tunnels, but generally natural conditions are simulated only with limited fidelity. Furthermore, it is sometimes difficult to assess the degree of fidelity necessary for the investigation of various safety-related questions.

The prevalence of mixed phase clouds poses a challenge to remote sensing methods for icing detection. Radar reflectivity alone cannot clearly discriminate between ice crystals and water drops, even if temperature is known. In general, ice crystals are typically much larger than water drops, so that for short-wavelength cloud radars, non-Rayleigh scattering effects can be a problem. Polarimetric techniques which enable identification of crystal habit or detection of drizzle are in initial stages of testing for icing; their utility for general-use icing detection is not known at this time. Knowledge of typical ice crystal habits, sizes, and concentrations would help in development of techniques to deal with mixed-phase conditions. These and other remote sensing problems will be covered in the paper.

Data obtained in mixed-phase conditions, presented in a FAA-sponsored workshop in late 1998, will be summarized. In addition, calculations using representative ice and droplet concentrations and size distributions will be presented to show the temperatures and updrafts at which we can expect mixed phase clouds to persist. To date, these calculations have been oversimplified with unrealistic ice crystal concentrations and sizes. New calculations confirm that the conditions are indeed expected often.

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