Observations of Ice Ridge Formation on the SJ30-2 Leading Edge Slat In Freezing Drizzle Conditions 2007-01-3365

Significant freezing drizzle or supercooled large droplet (SLD) conditions were encountered during an FAA certification flight test of the Sino Swearingen Model SJ30-2 airplane. Flaps and slats were extended to simulate an approach in icing conditions. The freezing drizzle was embedded in a relatively small portion of a stratiform cloud in which the airplane was orbiting to maintain the required icing conditions for the test. Although efforts were made to avoid this portion of the cloud once recognized, the test was completed with a total of about six minutes of SLD exposure. A ridge accreted on the thermally anti-iced slat, a result which had not been observed in a prior encounter at similar liquid water content and temperature. Post-encounter handling qualities evaluations, including an approach to stall warning, revealed normal flight characteristics. When slats were retracted, the slat ridge shed quickly. The aircraft was equipped with a 2D imaging probe, with 25 micron resolution (no gray-shading) capable of recording particle images up to 1500 microns. An FSSP droplet probe and hot-wire LWC probe were also installed. The freezing drizzle cloud was found to contain within a 30 second segment an MVD of 180 microns with a LWC of .30 g/m³, a sample of fairly severe SLD conditions. The test results, particularly the ridge formation, caused us to perform analyses of the accretion mechanism. Droplet trajectories, collection efficiencies, and water catch were computed for the test conditions, and also for two icing encounters within the FAR 25 Appendix C envelope utilizing a Langmuir D droplet distribution. The results indicated that for a ridge of this size to accrete, much of the recorded liquid water must have been on the order of 200 microns diameter, a conclusion entirely consistent with the cloud physics data recorded by the imaging probe. The observations of normal flight characteristics in spite of the ridge accretion also stimulated a review of the aerodynamics of the SJ30-2 wing and leading edge slats. It is concluded that the SJ30-2 leading edge slats in these cloud conditions appear to afford an appreciable tolerance of SLD ice contamination on wing lift characteristics. Further research on the possible benefits of leading edge slats to ameliorate the effects of SLD icing, as indicated by this freezing drizzle encounter, is suggested.