A B737-300 WAS RETURNED TO SVC WITH #5 TRAILING EDGE MID FLAP SEAL PLATE (ELEPHANT EAR) MISSING A COTTER PIN.

Narrative

ON AUG/MON/05 ON A 'C' CHK BLOCK VISIT; I WAS ASSIGNED THE R WING INSPECTION TASKS. VARIOUS PANELS WERE REMOVED. APPROX 1 HR BEFORE PUSHBACK FOR ENG RUNS; I INSPECTED ALL R WING PANEL TASK CARDS. WHILE CHKING #5 FLAP TRACK FAIRING (3601R) FOR INSTALLATION; CHKING THE DEBRIS CURTAIN; THE GND WIRE; AND CORRECT PANEL ENGAGEMENT. AS I WAS DOING THIS; HYD FUNCTIONALS WERE IN PROGRESS; AND FLAPS BEGAN TO MOVE TO FULL 'UP' POS; TAKING MY ATTN TO LOOK FOR THE LAST INSPECTION CRITERIA FOR PANEL 3601; THE INBOARD MID-FLAP SEAL PLATE (ELEPHANT EAR) HOOK ENGAGEMENT AS FLAPS MOVE UP TO FULL UP. THIS IS WHERE THE INCIDENT HAPPENED MOVING FROM THE FRONT OF PANEL 3601 TO THE REAR OF PANEL 3601; MISSING THE COTTER PIN INSTALLATION. MY ACTIONS FROM MOVING FROM THE FRONT TO BACK OF PANEL 3601R AT THE TIME FLAPS WERE MOVING IS WHAT CAUSED THE EVENT. INSTEAD OF LETTING FUNCTIONALS COMPLETE AND THEN OPERATING FLAPS FOR THE SOLE PURPOSE OF CHKING THE #5 FLAP TRACK HOOK ENGAGEMENT WOULD HAVE BEEN THE CORRECT THING TO DO AND PREVENT A RECURRENCE. THE MISSING COTTER PIN WAS DISCOVERED BY CONTRACT MAINT ON AUG/XX/05 AND MISSING COTTER PIN WAS INSTALLED ON PANEL 3601R.