Evaluating effectiveness of the FAA's CIT avoidance guidelines

Studies of historical data have shown that over 60% of turbulence-related aviation accidents are due to convectively-induced turbulence (CIT), many of them in the clear air outside of thunderstorms. The hazard presented by CIT has been a key motivation for the FAA's thunderstorm avoidance guidelines, which specify that aircraft should not fly within 20 nm of severe thunderstorms, within a specified distance above thunderstorms, or through areas having 6/10 or greater thunderstorm coverage, among other requirements. These restrictions affect significant volumes of airspace during convective events, resulting in costly limitations to airlines' flight operations. In order to determine whether they are adequate for the avoidance of CIT encounters or are overly restrictive, the FAA's Aviation Weather Research Program has directed NCAR to evaluate the guidelines and, if appropriate, recommend alternatives. This paper presents preliminary case studies and statistical analyses that exploit newly available quantitative turbulence data produced by a real-time in situ turbulence reporting algorithm running on United Airlines B-737 and B-757 aircraft. Data from over 35,000 summertime flights were used to identify incidents of moderate and severe turbulence encounters near thunderstorms, as identified by strong radar returns and the presence of lightning. A literature review and several case studies were performed in an attempt to identify readily-measurable thunderstorm characteristics that may be related to the generation and propagation of CIT. An algorithm for computing the horizontal and vertical proximity of aircraft to nearby thunderstorms and characterizing the local fraction of airspace covered by thunderstorms is described. This algorithm was applied to the in situ data and the results used to calculate probability of detection, false alarm rate, true skill statistic, and receiver operating characteristic curves for various interpretations and variations of the FAA guidelines. In future work, these measures of the FAA guidelines' skill in predicting the occurrence of turbulence will provide an objective basis for comparing them with alternatives suggested by case studies and simulations.