AN AIRBUS 320 ON TKOF AND INITIAL CLB DIVERTED DUE TO LOSS OF R AND L #1 ENG FAN COWLING CAUSED BY UNLOCKED COWLING LATCHES.

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SHORTLY AFTER TKOF FROM ZZZ RWY 19L; PASSING ABOUT 4000 FT MSL; FLT ATTENDANT CALLED TO TELL US THE 'DOOR ON THE ENG HAS COME OPEN.' AFTER BRIEF CONVERSATION BTWN FLT ATTENDANT AND FO; I SENT FO BACK TO INVESTIGATE. HE RETURNED TO THE COCKPIT QUICKLY; RPTING THAT BOTH SECTIONS OF THE #1 ENG COWLING HAD BEEN TORN OFF THE ENG; AND THAT THERE WAS NO DAMAGE VISIBLE ON LEADING EDGE SLATS OR TOP OF WING. WE TOLD ZZZ DEP WE'D LOST THE ENG COWL AND REQUESTED RETURN TO ZZZ. DID NOT DECLARE AN EMER AND TOLD APCH CRASH VEHICLES WOULDN'T BE NECESSARY SINCE THERE WERE NO ABNORMAL COCKPIT INDICATIONS AND CTLABILITY WAS NOT AT ISSUE; RETURNED TO ZZZ FOR UNEVENTFUL LNDG ON RWY 12. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: THE RPTR STATES THESE LATCHES ARE EXTREMELY DIFFICULT TO DETERMINE IF THEY ARE LOCKED AS NO PART OF THE LATCH HANGS OUT TO INDICATE AN UNLOCKED POS AND THE LATCH ASSEMBLY CONFORMS TO THE COWLING SHAPE. THE RPTR SAID THE COWLING WHEN LOCKED OR UNLOCKED HAS NO GAP AT THE 6 O'CLOCK POS WHERE THE LATCHES ARE LOCATED. THE RPTR SAID THIS TYPE OF INCIDENT (COWLING LOSS) HAS HAPPENED TO 2 OTHER CARRIERS. CALLBACK CONVERSATION WITH RPTR FROM ACN 386041 REVEALED THE FOLLOWING INFO: THE RPTR WAS ASSIGNED TO WORK #1 ENG THRUST REVERSER AND ANOTHER MECH WAS ASSIGNED TO SVC THE INTERNAL DRIVE GENERATOR AND AS A RESULT; THE FAN COWLING WAS OPENED. THE RPTR SAID THE INTERNAL DRIVE GENERATOR WAS SVCED BUT THE COWLING WAS LEFT OPEN; WHICH THE RPTR THEN CLOSED BUT DID NOT LATCH IN ORDER TO GND TEST THE THRUST REVERSER REPAIR. THE RPTR ADMITS BEING THE LAST PERSON TO CLOSE THE COWLING AND NOT LOCKING THE LATCHES. THE RPTR SAID THE AREA WAS CLEANED AND THE TOOLS PICKED UP AND THE COWLING APPEARED LOCKED AS NOTHING WAS HANGING DOWN AND NO GAP VISIBLE.