Landfast sea ice (fast ice) attached to the Antarctic coast is a key component of the regional atmosphere-ice-ocean- and eco-systems. Through its direct coupling with the atmosphere and ocean, changes of fast ice and its snow cover are also potential indicators of climate change. Some Antarctic fast ice survives several summers and, therefore, contributes to reductions of iceberg calving and stabilization of glaciers. Also, large amounts of platelet ice can accumulate under fast ice, indicating the presence of Ice Shelf Water (ISW) and therefore of nearby ice shelf bottom melt. During the R/V Polarstern expedition FROST (Filchner Ronne Outflow System Tomorrow, PS111) in February 2018 to the southern Weddell Sea we investigated the thickness of the Berkner Island fast-ice tongue and the platelet ice accumulation beneath. We carried out five helicopter electromagnetic induction (HEM) surveys to measure sea-ice and sub-ice platelet layer thicknesses of this prominent fast ice region off the Filcher and Ronne Ice Shelves. Results showed that the Berkner fast ice tongue consisted primarily of very thick, highly deformed first-year ice. The central part of the ice tongue was strongly ridged with ice thicknesses of up to 18 meters. However, there was much thinner ice between 1 and 2 meters thick immediately adjacent and north of the Filchner Ice Shelf, indicating the presence of open water and polynyas earlier in the winter, and a highly dynamic ice regime leading to the formation of the fast ice tongue. In contrast to fast ice along the eastern Weddell Sea shelf and elsewhere in Antarctica, we hardly found any evidence of the presence of platelet ice below the fast ice. This result has important implications for oceanographic conditions in the region off the Filchner Ronne Ice Shelves as it indicates the absence of Ice Shelf water near the ocean surface.