TWO MECHANICS AND A MAINTENANCE CONTROLLER REPORT ON THE LACK OF INFORMATION AVAILABLE ABOUT A REQUIRED CAT-3 DOWNGRADE TO A CAT-1 STATUS OF THE B737-300 WHEN THE STALL MANAGEMENT COMPUTER (SMC) IS REPLACED IN THE #1 POSITION UNTIL A HGS CAT-3 RECERTIFICATION IS ACCOMPLISHED.

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A FELLOW MECHANIC AND I WERE WORKING ACFT. IT ARRIVED WITH A MEL DEFERRAL FOR A SINGLE CHANNEL AUTOSLAT FAILURE. THIS WAS A REPEAT DISCREPANCY AND NO PROBLEMS WERE DISCOVERED NOR PARTS REPLACED DURING PREVIOUS MAINTENANCE. TOGETHER WE ACCOMPLISHED THE MM SYSTEM TESTS FOR BOTH THE #1 AND #2 AUTOSLAT SYSTEMS WITH NO FAILURES. WE CHECKED THE CONDITION AND OPERATION OF BOTH AOA VANES. NEITHER STALL MANAGEMENT COMPUTER SHOWED ANY RECORDED FAULTS. WE DECIDED TO REPLACE THE #1 SMC FOR TROUBLESHOOTING AND AGAIN PERFORMED THE SYSTEM TESTS. I SIGNED OFF THE DISCREPANCY IN THE LOGBOOK WHILE MECH #2 CALLED TO CLEAR THE MEL. WE FAILED TO ACCOMPLISH THE REQUIRED HGS CAT III RECERTIFICATION AFTER REPLACING THE #1 SMC. SINCE WE WERE WORKING AN AUTOSLAT PROBLEM; WE DIDN'T THINK ABOUT HGS. WHEN VERIFYING THE CORRECT PART NUMBER IN COMPUTER SYSTEM AND COMPLETING THE PARTS TAG WE SAW NO NOTES ABOUT CAT III SENSITIVITY. WHEN CLEARING THE MEL WITH MAINT CTL; CAT III WAS NOT DISCUSSED. I WAS OFF WORK THE FOLLOWING NIGHT. WHEN MECH #2 RETURNED TO WORK THAT EVENING; SOMETHING JOGGED HIS MEMORY TO GO BACK AND CHECK THE CAT III COMPONENT LIST. HE DISCOVERED OUR ERROR; CALLED MAINT CTL IMMEDIATELY AND HAD THE ACFT DOWNGRADED TO CAT I STATUS.SUPPLEMENTAL INFO FROM ACN 814602: DURING TROUBLESHOOTING WE COULD NOT GET AN AUTOSLAT FAILURE. UNABLE TO GET A FAILURE WE DECIDED TO CHANGE THE #1 STALL MANAGEMENT COMPUTER FOR TROUBLESHOOTING. WE CLOCKED OUT AND WENT HOME. I CAME BACK THE NEXT NIGHT. I WAS A LEAD THIS NIGHT AND WHILE HELPING WITH THE PAPERWORK FOR THE NIGHT; I DECIDED TO CHECK COMPUTER TO SEE IF THE AUTOSLAT FAILURE HAD RETURNED. WHILE DOING THIS I STARTED THINKING ABOUT CHANGING THE STALL COMPUTER AND QUESTIONING MYSELF ABOUT WHETHER OR NOT IT WAS CAT III. SO I CHECKED A LOGBOOK COVER TO FIND OUT IT WAS. THIS I SHOULD KNOW BEING CAT III CERTIFIED. AT THIS POINT I INFORMED THE SUPERVISOR ON DUTY THAT WE MISSED A CAT III UPGRADE AND WE CALLED MAINT CTL TO CONFIRM THIS AND START CORRECTIVE ACTION. MAINT CTL SAID HE WOULD DOWNGRADE THE ACFT THAT NIGHT AND HAVE IT RECERTIFIED ON THE FOLLOWING DAY WHEN IT GOES THROUGH A MAINT BASE. CONTRIBUTING FACTORS: 1) WE WERE TROUBLESHOOTING A NON CAT III PROBLEM/MEL. 2) TAG ON THE STALL COMPUTER DOES NOT HAVE THE CAT III SENSITIVE REMINDER. 3) COMPUTER SYSTEM HAS NO NOTATION THAT CAT III IS REQUIRED IF COMPUTER IS PUT INTO #1 POSITION. 4) WORKING MULTIPLE PROBLEMS ON MULTIPLE ACFT SO IT JUST SLIPPED OUR MINDS DUE TO THE CAT III UPGRADE. THESE ARE SOME FACTORS THAT MOST LIKELY HELPED LEAD UP TO OUR OVERSIGHT; BUT DEFINITELY NOT EXCUSES OR REASONS FOR OVERSIGHT.SUPPLEMENTAL INFO FROM ACN 814600: THE ACFT HAD A SINGLE CHANNEL AUTOSLAT FAIL ON MEL. WHEN ISSUING THE CLEARING NUMBER; I NEGLECTED TO REMIND MAINT THE ACFT WOULD REQUIRE CAT III RECERTIFICATION. ACFT FLEW ALL DAY WITH A CAT III STATUS BEFORE THE ERROR WAS DISCOVERED BY ZZZ MAINT. ACFT WAS DOWNGRADED TO CAT I UNTIL MAINT COULD PERFORM THE CAT III RECERTIFICATION. THE STALL MANAGEMENT COMPUTER IS NOT NORMALLY A CAT III COMPONENT ONLY WHEN ITS INSTALLED IN THE NUMBER ONE POSITION. I FEEL THAT IF A NOTE WERE ADDED TO MAINT COMPUTER PROGRAM TO REMIND MAINT THAT THIS PART IS CAT III IF PLACED IN THE NUMBER ONE POSITION; IT COULD PREVENT THIS TYPE OF OVERSIGHT.