Inflight icing data gathering during routine flight operations - a case study

For the purpose of the design and certification of inflight icing protectio n systems for transport and general aviation aircraft, the eventual re-defi nition/expansion of the icing environment of FAR 25/JAR 25, Appendix C is u nder consideration. Such a re-definition will be aided by gathering as much inflight icing event data as reasonably possible, from widely-different ge ographic locations. The results of a 12-month pilot programme of icing even t data gathering are presented. Using non-instrumented turboprop aircraft f lying upon mid-altitude routine air transport operations, the programme has gathered observational data from across the British Isles and central Fran ce. By observing a number of metrics, notably windscreen lower-corner ice i mpingement limits, against an opposing corner vortex-flow, supported by win g leading edge impingement limits, the observed icing events have been clas sified as 'small', 'medium' or 'large' droplet. Using the guidance of dropl et trajectory modelling, MVD values for the three droplet size bins have be en conjectured to be 15, 40 and 80mm. Hence, the 'large' droplet category w ould be in exceedance of FAR/JAR 25, Appendix C. Data sets of 117 winter-season and 55 summer-season icing events have been statistically analysed. As defined above, the data sets include 11 winter a nd five summer large droplet icing encounters. Icing events included 'sandp aper' icing from short-duration 'large' droplets, and a singular ridge form ation icing event in 'large' droplet. The frequency of 'large' droplet icin g events amounted to 1 in 20 flight hours in winter and 1 in 35 flight hour s in summer. These figures reflect 'large' droplet icing encounter probabil ities perhaps substantially greater than previously considered. The 'large' droplet events were quite localised, mean scale-size being about 6nm.