A LINE MECHANIC REPORTS A #1 ENG INFLIGHT SHUTDOWN DUE TO OIL LOSS AND AIR TURNBACK OF AN ATR-42 ACFT HE HAD SERVICED EARLIER. ENG OIL CAP HAD POPPED OUT OF OIL TANK RECEPTACLE.

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AFTER LOADING AN ATR-42 FOR A CARGO FLT TO ZZZ; THE ACFT LEFT THE FLT LINE AND PROCEEDED TO THE RWY FOR DEP. MY FELLOW WORKER AND I LEFT TO RETURN TO OUR HANGAR AND CLOSE UP SHOP. WE MONITORED THE RADIO UNTIL WE ARRIVED AT THE SHOP AND CLOSED UP. WE DECIDED THAT WE HAD BETTER MAKE SURE THAT THE ACFT WAS OUT OF SIGHT AND WELL ON ITS WAY BEFORE WE LEFT FOR THE DAY. AS WE EXITED THE HANGAR; WE OBSERVED THE ACFT IN THE DOWNWIND LEG FOR A RETURN TO ZZZ. WE TURNED ON THE RADIO JUST AS WE HEARD THE TWR ASK; 'STATE THE NATURE OF YOUR PROB.' WE COULD NOT HEAR THE REPLY. WE BOTH JUMPED IN THE TRUCK AND HEADED FOR THE RAMP. ENRTE TO THE RAMP; WE COULD CLEARLY SEE THAT THE #1 ENG WAS SHUT DOWN. AS THE ACFT ROLLED IN TO BE PARKED; IT WAS CLR THAT IT WAS AN OIL ISSUE; BASED UPON THE AMOUNT OF OIL ON THE COWL AND THE FUSELAGE. UPON OPENING THE #1 ENG COWL; I DISCOVERED THE OIL CAP IN ITS RECEPTACLE; BUT POPPED OUT AND SLIGHTLY ASKEW. I THINK THAT THE FLAP MIGHT HAVE BEEN UP; BUT I WAS A BIT RATTLED AT THE TIME AND DO NOT REMEMBER THIS CLEARLY. THE PLT RELAYED THE EVENTS TO ME THAT UNFOLDED; THAT CAUSED HIM TO DO AN INFLT SHUTDOWN. I GOT ON THE PHONE IMMEDIATELY TO OUR MAINT CTL AND RELAYED TO THEM ALL THAT I KNEW. AFTER A FEW PRELIMINARY QUESTIONS; MAINT CTL STARTED TO SET INTO MOTION THE STEPS I NEEDED TO GO THROUGH IN ORDER TO CHK OUT THE ENG AND REPAIRS IF NEEDED. MY CO-WORKERS AND I THEN PROCEEDED TO CLEAN UP THE ENG; FUSELAGE; AND RAMP. I HAVE DONE OIL SVCING NUMEROUS TIMES ON THIS TYPE OF ACFT AND CLEARLY KNOW HOW TO INSTALL THE OIL CAP. I WAS THE LAST PERSON TO SVC THIS ENG; 2 NIGHTS BEFORE. I FOLLOWED THE COMPANY TALLY SHEET AND COMPLETED ALL OF MY TASKS. I DID NOT; HOWEVER; HAVE THE SHEET IN FRONT OF ME AT THE TIME OF SVCING DUE TO LIMITED SPACE ON THE LADDER DURING SVCING. MY BELIEF IS THAT I MAY NOT HAVE FULLY ENGAGED THE LOCKING FLAP WHILE INSTALLING THE CAP; BASED UPON THE ORIENTATION OF THE CAP WHEN I OPENED THE COWL UPON RETURN TO THE RAMP. A SVC BULLETIN FOR THAT PARTICULAR CAP WAS PERFORMED; WITH NO DISCREPANCIES NOTED. MY SUPVR; DIRECTOR OF SAFETY AND COMPLIANCE; AND DIRECTOR OF MAINT HAVE ALL DISCUSSED THIS WITH ME; AND HAVE ASKED FOR SUGGESTIONS ON HOW TO AVOID THIS PROB IN THE FUTURE. I SUGGESTED THAT ON THE PLT'S PREFLT THEY ASK THE MECH ON DUTY TO CHK AND RE-VERIFY THAT THE OIL CAPS ARE ON PROPERLY AND SECURELY. I BELIEVE THAT FOLLOWING THIS PROC WILL; AND WOULD HAVE PREVENTED THIS SITUATION. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: REPORTER STATED THE OIL CAP IS ON THE OUTBOARD SIDE OF THE #1 ENGINE.