Estimating Maximum Aircraft Icing Environments Using a Large Database of In-Situ Observations

Abstract

A large data base of in-situ aircraft icing observations collected during five field campaigns with two research aircraft is used to assess the 99 and 99.9% liquid water content (LWC) values associated with icing environments. Icing environments assessed include those with drops smaller than 100 μm and those with supercooled large drops (SLD) larger than 100 μm. The low probability LWC values were calculated using an extreme value analysis technique, and the results were compared to those obtained by assuming that the LWC observations could be fitted to exponential, gamma or Weibull distributions. Extreme value analysis allows quantification of the nature of distributions in the tails of the distributions, and hence provides a more accurate method for determining extreme values and their associated confidence limits. The results are compared to the icing envelopes from the Federal Aviation Administration Regulation 25 Appendix C and with other icing envelopes. Scale factors for computation of 99 and 99.9% LWC values for icing environments at horizontal length scales larger than 3 km are also determined.