What was found
UPON TAKEOFF A POP WAS WAS HEARD FOLLOWED BY PERCEIVED SMOKE (DUST) IN THE CABIN. PACS WERE SELECTED TO OFF AND A IMMEDIATE LANDING MADE AT THE DEPARTURE AIRPORT. ALL COCKPIT INDICATIONS NORMAL. FOUND LT AIR CONDITIONING PAC MIXING CHAMBER AND HOT AIR SUPPLY DUCT SEPARATED/ DAMAGED ALLOWING HOT CONDITIONED AIR TO ENTER CABIN THROUGH RETURN GRILLS, THUS CREATING DUST. APPEARS TO HAVE EXPERIENCED INTERNAL DAMAGE RESULTING IN BACK PRESSURE WHICH CAUSED THE DUCT/FLANGE FAILURE. NO OTHER DAMAGE NOTED. REPLACED MIXING AND HOT AIR DUCTS.