What was found
(CAN) DURING TROUBLESHOOTING OF A DEFERRED DEFECT RECIRCULATING FAN FAULT MESSAGE. DEBRIS WAS DISCOVERED BLOCKING A SCREEN IN A UNDERFLOOR SUPPLY DUCT AT FS 677-682, ABOUT MID CABIN AND RESTRICTING AIRFLOW. THE FOD WAS REMOVED AND THE SYS WAS FUNCTION CHECKED SERVICEABLE. FOLLOW UP INVESTIGATION REVEALED SIMILAR FINDINGS AT THE SAME LOCATION ON 2 OTHER ACFT WITH NO RELATED DEFECTS. AT THIS TIME THE SOURCE OF THE FOD IS BELIEVED TO BE FROM EXTERNAL AIR-CONDITIONING CARTS/HOSES. FOLLOW UP INTO AN APPROVED FIX, SUCH AS AN ENTRY POINT SCREEN IS ONGOING.