FO RPTS ON PROBS WITH THE MD90 LOOP FIRE DETECTION SYS.

Narrative

THE 'A' LOOP OF THE R ENG FIRE DETECTION SYS WAS MCO'ED (MAINT CARRY OVER) INOP. WHEN WE PERFORMED THE FIRE TEST DURING PREFLT (WITH THE 'B' LOOP SELECTED PER THE MEL); THE TEST FAILED. WE CALLED CONTRACT MAINT. HE GOT A GOOD TEST WHEN HE SELECTED THE 'A' LOOP (WHICH WAS PREVIOUSLY MCO'ED). SO THE CONTRACT MAINT MAN WROTE ''A' LOOP CHKS GOOD;' AND MCO'ED THE 'B' LOOP ON THE R ENG. AFTER WE STARTED THE R ENG; WE GOT ANOTHER 'A' LOOP FAILURE. WE SHUT DOWN; CALLED CONTRACT MAINT AND THIS TIME HE SELECTED THE 'B' LOOP AND THE TEST WAS NORMAL; SO HE MCO'ED THE 'A' LOOP AGAIN. (SO WE WERE BACK WHERE WE STARTED FROM -- 'A' LOOP INOP AND 'B' LOOP SELECTED.) EXTENSIVE TROUBLESHOOTING COULD NOT REVEAL WHY THE 'A' LOOP WAS BAD OR WHY THE 'B' LOOP TESTED BAD THEN TESTED GOOD. THIS AIRPLANE (MD90) IS EQUIPPED WITH A FIRE DETECTION SYS THAT CONTINUALLY CHKS THE LOOPS FOR PROBS; AND WILL AUTOMATICALLY SELECT THE GOOD LOOP IF THE OTHER DEVELOPS A PROB. THIS AIRPLANE HAD A PREVIOUS HISTORY OF THE 'B' LOOP BEING INOP. ALTHOUGH WE TESTED THE 'B' LOOP SEVERAL TIMES; AND IT WORKED AGAIN; I WAS UNCOMFORTABLE TAKING A PLANE THAT HAD THE 'A' LOOP ON MCO AND THE 'B' LOOP OPERATING BUT WITH A RECENT HISTORY OF PROBS. IT WAS ALSO DISCONCERTING THAT WE DID NOT KNOW WHY THE 'B' LOOP FAILED AT THE GATE; AND THEN 'FIXED ITSELF.' THIS AIRPLANE (MD90) DOES HAVE THE REPUTATION OF HAVING SPURIOUS ELECTRICAL FAULTS THAT GO AWAY ON THEIR OWN. I AM USED TO THIS; BUT NOT WITH THE FIRE DETECTION SYS. MY ULTIMATE CONCERN WAS THAT IF WE HAD A 'B' LOOP FAILURE WE DIDN'T KNOW ABOUT; THEN WE WOULD LOSE FIRE DETECTION ON THE R ENG.