THREE MECHANICS DESCRIBE THE EVENTS SURROUNDING THE RELEASE OF A B737-400 AIRCRAFT AFTER A LANDING GEAR SWING CHECK; WITH THE DRAIN MAST CIRCUIT BREAKER PULLED AND THE COLLAR STILL INSTALLED.

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ACFT WAS BROUGHT TO THE HANGAR FOR A PLT WRITE-UP INDICATING HE EXPERIENCED A DROP IN HYD PRESSURE DURING THE GEAR RETRACTION (WHICH IS A NORMAL INDICATION) AND PRESSURE RETURNING TO NORMAL. ACFT WAS PREPPED FOR JACKING. ANOTHER TECHNICIAN AND MYSELF INSTALLED THE JACKS (GEAR WAS ALREADY PINNED) AND ANOTHER TECHNICIAN TOOK CARE OF PULLING THE CIRCUIT BREAKERS. WE JACKED THE PLANE. WE HOOKED UP THE MULE AND STARTED TO SWING THE GEAR; BUT THE MULE WAS NOT WORKING CORRECTLY. SO WE STOPPED AND TRIED TO GET THE MULE TO WORK CORRECTLY. THEN IT WAS BREAK TIME. DURING THAT TIME; TECHNICIAN WAS CALLED AWAY TO GO ON A FIELD TRIP. ANOTHER TECHNICIAN STEPPED IN TO HELP US. WE GOT THE MULE TO OPERATE PROPERLY AND DID THE GEAR SWING AND FOUND NO DEFECTS. WE STARTED THE DOWN JACK AND CLEAN-UP TO GET THE ACFT UP TO THE GATE TO FLY. I TOOK CARE OF THE JACK PADS AND CHKED TO MAKE SURE THE HYD LEVELS WERE GOOD. TECHNICIAN INDICATED TO ME THAT HE TOOK CARE OF THE BREAKERS AND GEAR PINS. I WENT UPSTAIRS AND DOUBLECHKED THE BREAKERS AND HEADED TO DO THE PAPERWORK. GRABBED THE LOGBOOK; CHKED THE BREAKERS AGAIN WHEN I GOT IN THE PLANE; AND TAXIED TO THE GATE FOR DEP. SUPPLEMENTAL INFO FROM ACN 814834: I WAS ASSIGNED TO ACFT FOR LNDG GEAR WORK. MY INITIAL RESPONSIBILITIES WERE TO ASSIST WITH PREPARING THE ACFT FOR JACKING. I HELPED PREPOSITION THE JACKS; PULLED CIRCUIT BREAKERS AND INSTALLED CIRCUIT BREAKER COLLARS FOR THE JACK. AFTER JACKING THE ACFT; WE HOOKED UP THE HYD MULE FOR A GEAR SWING. THE MULE WAS NOT OPERATING PROPERLY; SO WE BRIEFLY REVIEWED THE OPERATORS MANUAL BEFORE GOING TO BREAK. JUST PRIOR TO BREAK; I WAS APCHED ABOUT GOING ON A FIELD TRIP. DUE TO TIME CONSTRAINTS; I QUICKLY TOLD THE OTHER MECHS ASSIGNED TO THE AIRPLANE THAT I WOULD BE LEAVING FOR A FIELD TRIP. I THEN GATHERED ALL THE NECESSARY TOOLS FOR THE TRIP AND EXPEDITED MY WAY TO SECURITY AND ON TO THE GATE TO CATCH THE LAST FLT OF THE DAY. BEFORE I LEFT FOR THE FIELD TRIP; DOCUMENTATION FOR CORRECTIVE ACTION HAD YET TO BE STARTED BECAUSE WE WERE STILL IN THE PROCESS OF TROUBLESHOOTING AND HAD NOT YET DECIDED ON A SPECIFIC COURSE OF ACTION. SUPPLEMENTAL INFO FROM ACN 814832: I WAS CALLED OVER TO AN ACFT BY A TECHNICIAN WHO WAS HAVING PROBS GETTING THE HYD MULE TO OPERATE PROPERLY. I TRAINED THE TECHNICIAN ON OP OF THE MULE AND; THEREFORE; HE WAS ABLE TO PROPERLY SWING THE GEAR AND CLEAR THE WRITE-UP. ABOUT THAT TIME ANOTHER TECHNICIAN WAS CALLED OUT ON A FIELD TRIP. I DECIDED TO STAY AND HELP DOWN-JACK THE ACFT. THE SUPVR THEN CAME OVER AND INFORMED US THAT THE ACFT WAS SCHEDULED TO FLY OUT AT XA00. I TOLD THE TECHNICIAN TO GO AHEAD AND FINISH UP THE PAPERWORK AND THAT I WOULD TIDY UP THE ACFT SO THAT WE COULD GET UP TO THE GATE IN TIME FOR ITS DEP. I PROCEEDED TO REMOVE ALL THE EQUIP AWAY FOR THE ACFT; CLOSED UP ANY PANELS; STOWED THE GEAR PINS AND REMOVED THE CIRCUIT BREAKER COLLARS FROM THE CIRCUIT BREAKERS. I MUST HAVE INADVERTENTLY MISSED REMOVING THE DRAIN MAST CIRCUIT BREAKER COLLAR; AS I WAS INFORMED LATER.