The Braniff Convair was on operating on the flight Dallas-Oklahoma City-Wichita-Kansas City-Chicago. Departure time a Kansas City was 04:35 as the aircraft started VFR for Chicago. IFR was requested at 05:19. At 05:56 the crew received a weather forecast: thin (1000 feet) obscuration; visibility 1/2 mile. The crew then reported over Napierville at 06:18 and were radar vectored to the runway 13R outer marker for an ILS approach. On final approach the Convair descended too low and collided with a commercial sign. The aircraft then crashed through the airport boundary fence in flames and came to rest inverted. PROBABLE CAUSE: “Momentary disorientation caused by the loss of visual reference during the final visual phase of the approach resulting in an increased rate of descent an altitude too low to effect recovery.”