Narrative:

While performing a before-start checklist on medium large transport, I discovered that the primary trim switches, which move the horizontal stabilizer, would not repos the stabilizer to the takeoff setting. All 3 sets of switches (captain yoke, first officer yoke, 'suitcase handles') would not move the stabilizer, however the secondary trim switches on the pedestal did work. I checked the 3 circuit breaker's for the 3 phase primary trim motor and they were in. I called a mechanic, and he could offer no solution, but he did pull all 3 circuit breaker's and one of them took extreme pressure to reset it. After doing this, the primary trim worked ok, from all 3 trim positions. This seems to be an internal failure of a circuit breaker, which gives no external indication to the flight crew. The mechanic didn't think this was worthy of a log book write up, and the aircraft did perform normally on the next flight. I called our maintenance tech center a few days later, and asked them to run a trace on that system and suggested that the circuit breaker's should be further checked and replaced, as a failure of this system on landing could have a dangerous effect on aircraft control. This is the first time I've had a system failure caused by an apparent internal circuit breaker failure in 28 yrs of airline flying, and would like to know how common this type of failure is.

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Original NASA ASRS Text

Title: REPORTER INDICATES AN INTERNAL FAILURE OF A CIRCUIT BREAKER THAT DID NOT REVEAL A MALFUNCTION.

Narrative: WHILE PERFORMING A BEFORE-START CHKLIST ON MLG, I DISCOVERED THAT THE PRIMARY TRIM SWITCHES, WHICH MOVE THE HORIZ STABILIZER, WOULD NOT REPOS THE STABILIZER TO THE TKOF SETTING. ALL 3 SETS OF SWITCHES (CAPT YOKE, F/O YOKE, 'SUITCASE HANDLES') WOULD NOT MOVE THE STABILIZER, HOWEVER THE SECONDARY TRIM SWITCHES ON THE PEDESTAL DID WORK. I CHKED THE 3 CB'S FOR THE 3 PHASE PRIMARY TRIM MOTOR AND THEY WERE IN. I CALLED A MECH, AND HE COULD OFFER NO SOLUTION, BUT HE DID PULL ALL 3 CB'S AND ONE OF THEM TOOK EXTREME PRESSURE TO RESET IT. AFTER DOING THIS, THE PRIMARY TRIM WORKED OK, FROM ALL 3 TRIM POSITIONS. THIS SEEMS TO BE AN INTERNAL FAILURE OF A CB, WHICH GIVES NO EXTERNAL INDICATION TO THE FLT CREW. THE MECH DIDN'T THINK THIS WAS WORTHY OF A LOG BOOK WRITE UP, AND THE ACFT DID PERFORM NORMALLY ON THE NEXT FLT. I CALLED OUR MAINT TECH CENTER A FEW DAYS LATER, AND ASKED THEM TO RUN A TRACE ON THAT SYS AND SUGGESTED THAT THE CB'S SHOULD BE FURTHER CHKED AND REPLACED, AS A FAILURE OF THIS SYS ON LNDG COULD HAVE A DANGEROUS EFFECT ON ACFT CTL. THIS IS THE FIRST TIME I'VE HAD A SYS FAILURE CAUSED BY AN APPARENT INTERNAL CB FAILURE IN 28 YRS OF AIRLINE FLYING, AND WOULD LIKE TO KNOW HOW COMMON THIS TYPE OF FAILURE IS.

Data retrieved from NASA's ASRS site as of July 2007 and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.