Despite past research programs focusing on tropical convection, the explicit studies of high ice water content (IWC) regions in Mesoscale Convective Systems (MCS) are rare, although high IWC conditions are potentially encountered by commercial aircraft during multiple in-service engine powerloss and airdata probe events.To gather quantitative data in high IWC regions, a multi-year international HAIC/HIWC (High Altitude Ice Crystals / High Ice Water Content) field project has been designed including a first field campaign conducted out of Darwin (Australia) in 2014. The airborne instrumentation included a new reference bulk water content measurement probe and optical array probes (OAP) recording 2D images of encountered ice crystals.The study herein focuses on ice crystal size properties in high IWC regions, analyzing in detail the 2D image data from the particle measuring probes. Various geometrical parameters were extracted from the images in order to calculate particle size distributions (PSDs) and finally deduce median mass diameters with additional information on the ice density.The preliminary analysis of all HAIC/HIWC flights performed during this first flight campaign out of Darwin, demonstrates that various flights include high IWC regions mostly produced by high concentrations of small crystals while other flights with similar peak IWCs indicates that high IWC regions could be nevertheless composed primarily of larger particles. This interesting result indicates that high IWC can be produced and maintained in various environments, preferentially high concentrations of small crystals, however sometimes by smaller concentrations of larger sized crystal populations.