A B757-200 ON LNDG HAD THE R OVER-WING ESCAPE SLIDE DEPLOY. THE LEFT OVER-WING SLIDE WAS SUBSEQUENTLY FOUND TO BE UNLOCKED. THE SLIDES HAD BEEN INSTALLED AT DEP MAINT STATION BY LINE MAINT TECHNICIANS.

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OVER-WING ESCAPE SLIDE DEPLOYED IN FLT. INSTALLED OVER-WING SLIDE; L SIDE. INSTALLATION WAS NORMAL PER MAINT MANUAL WITH POSITIVE LOCKING OF TUBULAR HANDLE. EXTERIOR DOOR WAS FAIRED TO ACFT SKIN WITH NO ABNORMAL SIGNS OF A PROB. ACTUATED SQUARE OVER-WING ARMING HANDLE AND LEFT ACFT. IN FOLLOWING MAINT PROCS ON INSTALLATION. IT'S JUST POSSIBLE A 16 HR DAY; COLD WEATHER (HANGAR DOOR OPEN; TEMP 32 DEGS) AND ABSENCE OF A SUPVR WERE FACTORS. WE MECHANICS WERE UNAWARE THAT THERE WAS A WRITE-UP ON ACCESS DOOR (WEAK LATCHES).CALLBACK CONVERSATION WITH RPTR ACN 726111 REVEALED THE FOLLOWING INFO: RPTR STATED WHEN CLOSING THE SLIDE DOOR; ONE TECHNICIAN IS PUSHING THE DOOR TO THE LOCK POSITION AND THE OTHER PULLING THE YELLOW LOCK HANDLE TO THE CLOSE AND LOCK POSITION. THE DOOR WAS CHECKED FOR BEING FAIRED WITH THE FUSELAGE BODY FAIRING AND WAS FLUSH. A SECOND CHECK IS MADE BY PRYING ON THE EDGE WITH A SCREWDRIVER TO ENSURE THE DOOR IS IN FACT LOCKED. THERE IS NO VISIBLE INDICATION ON THE DOOR INDICATING THE CLOSED AND LOCKED CONDITION. SHOULD HAVE WITNESS HOLES DRILLED AT THE LATCHES TO VISUALLY CHECK LOCKING. A CORRECTION IS IN ORDER AS IT WAS THE R OVER-WING SLIDE THAT DEPLOYED ON LANDING AND THE L OVER-WING SLIDE DOOR WAS FOUND UNLOCKED.CALLBACK CONVERSATION WITH RPTR ACN 726112 REVEALED THE FOLLOWING INFO: RPTR STATED THAT CHANGING THE OVER-WING SLIDE PACK IS NOT A NORMAL JOB FOR LINE MECHANICS; AS IT IS NORMALLY ACCOMPLISHED IN A HEAVY MAINT CHECK ON A TIME CONTROL BASIS. THIS SLIDE WAS REPLACED PER THE MAINT MANUAL AND DID NOT DISPLAY ANY DIFFICULTY. THE SLIDE DOOR WAS CLOSED LOCKED AND THE HANDLE PULLED DOWN IN THE CLOSED AND LOCKED POSITION. THERE WAS NEVER ANY SPECIAL TRAINING ON THE OVER-WING SLIDE REMOVAL AND INSTALLATION. RPTR STATED THE BIG PROBLEM IS MAKING ABSOLUTELY CERTAIN THE SLIDE DOOR IS CLOSED AND LOCKED; AS THE ONLY THING THAT CAN BE VISIBLY CHECKED IS IF THE DOOR FAIRS WITH THE FUSELAGE BODY FAIRING. IN THIS CASE IT FAIRED PERFECTLY. CALLBACK CONVERSATION WITH RPTR ACN 726116 REVEALED THE FOLLOWING INFO: RPTR STATED THEY DO NOT HAVE ADEQUATE KNOWLEDGE ON THE OVER-WING SLIDE INSTALLATION; ARMING; AND DISARMING. NO TRAINING ON OVER-WING SLIDES HAS BEEN RECEIVED BY THE RPTR. THE SLIDE DOOR WAS CLOSED AND WAS FAIRED WITH THE FUSELAGE BODY FAIRING AND THAT'S THE ONLY REAL INDICATION THE DOOR IS CLOSED AND LOCKED. THE LOCKING HANDLE WAS IN THE DOWN AND LOCKED POSITION. RPTR STATED THIS SLIDE DOOR NEEDS A POSITIVE LOCK INDICATION.