It is well known that exposure to supercooled large drops (SLD - subfreezing water droplets with diameters greater than ~50 microns) can pose a significant threat to the safety of some aircraft. Although, by definition, SLD includes both freezing drizzle (FZDZ) and freezing rain (FZRA), much of the SLD research and development of operations SLD forecast tools has focussed on FZDZ and ignored FZRA, regarding it as "less of a hazard to aviation." This mindset is primarily based on a few published FZRA encounters by one research aircraft where the resulting ice accretion was rather smooth, conformed to the airfoil and did not appear to cause a significant degredation in aircraft performance.
During the winters of 1997 and 1998, the NASA-Lewis Research Center Twin Otter made several flights into FZRA. Along with the collection of standard meteorological state-parameter and microphysical probe data, NASA engineers and pilots obtained detailed records of the ice accretions and performed maneuvers with the iced aircraft to assess performance degradation. During one case, the Twin Otter experienced a prolonged exposure to freezing rain. In this paper, the following will be discussed: 1) the meteorological setup for the freezing rain, including the synoptic-scale weather pattern, horizontal and vertical extent, temperature and microphysical characteristics, 2) ice which accreted on the aircraft, including its shape, location and the effects of deicing equipment, 3) performance degradation and safety margins, 4) on-board SLD icing cues, and 5) weather clues which are useful for the identification of FZRA in automated icing diagnosis algorithms. Finally, scenarios where FZRA may or may not present a hazard to aircraft will be described, and past research aircraft encounters with FZRA will be placed into this context.
The 8th Conference on Aviation, Range, and Aerospace Meteorology