Narrative:

15 mi out at about 3000' MSL, we experienced an electrical failure causing loss of captain's normal flight instruments and numerous other system and gauges. The first officer flight instruments appeared normal, except he lost his command bars and the autoplt failed. I knew we had lost a powerful AC source like a gen or AC buss, but we couldn't find the lost bus or gen. They all appeared normal. Since the WX all over the northeast was very bad and bos was close to limits, I decided to continue approach to pvd as I was very concerned about what was wrong that we couldn't figure out. I asked for an ASR approach to back up our ILS. Pvd approach was very helpful and said they would give us good vectors, but that they weren't really authority/authorized ASR. At that point, I decided to take the pressure off everybody by declaring an emergency. The controller's vectors were excellent in backing up our ILS approach and we landed ok. On taxiing in, I asked the first officer to check circuit breaker's and he found the 'xfer bus,' '#1 normal' was popped, although the left side 'xfer buss off' light was not illuminated. On resetting this circuit breaker, everything returned to normal. If this had happened at altitude, we would have eventually found the circuit breaker, but at night in very bad WX all over and on approach, I declared the emergency to back up our situation. Upon checking other aircraft the next day, I found they all do the same thing and don't turn on the xfer buss off light. Our company is currently checking with manufacturer on this situation with the medium large transport type 3 and 4. The lack of that light coming on in this situation gives the pilot the wrong idea. Callback conversation with reporter revealed the following: this particular aircraft was grounded about 1 week later for repeated xfer circuit breaker problems. The manufacturer said the problem was with the xfer circuit breaker itself. The circuit breaker is made by a subcontractor and they have been advised of the problem. The wiring of the xfer warning light is still being investigated. The reporter claims that they are trained to look for this light whenever an electrical problem develops and the fact that it did not come on with this failure causes cockpit confusion. Reporter also said that a T/right amperage check could have idented the problem, but he missed seeing the slight amperage drop because of the lighting, the time constraint and the scale of the ammeter.

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Original NASA ASRS Text

Title: ACR MLG LOST LEFT AC BUSS WITHOUT THE EXPECTED BUSS TRANSFER LIGHT ILLUMINATING. ACFT WAS ON IAP ILS APCH IN IMC. APCH CONTINUED USING TRACON VECTORS AS A BACKUP.

Narrative: 15 MI OUT AT ABOUT 3000' MSL, WE EXPERIENCED AN ELECTRICAL FAILURE CAUSING LOSS OF CAPT'S NORMAL FLT INSTS AND NUMEROUS OTHER SYS AND GAUGES. THE F/O FLT INSTS APPEARED NORMAL, EXCEPT HE LOST HIS COMMAND BARS AND THE AUTOPLT FAILED. I KNEW WE HAD LOST A POWERFUL AC SOURCE LIKE A GEN OR AC BUSS, BUT WE COULDN'T FIND THE LOST BUS OR GEN. THEY ALL APPEARED NORMAL. SINCE THE WX ALL OVER THE NE WAS VERY BAD AND BOS WAS CLOSE TO LIMITS, I DECIDED TO CONTINUE APCH TO PVD AS I WAS VERY CONCERNED ABOUT WHAT WAS WRONG THAT WE COULDN'T FIGURE OUT. I ASKED FOR AN ASR APCH TO BACK UP OUR ILS. PVD APCH WAS VERY HELPFUL AND SAID THEY WOULD GIVE US GOOD VECTORS, BUT THAT THEY WEREN'T REALLY AUTH ASR. AT THAT POINT, I DECIDED TO TAKE THE PRESSURE OFF EVERYBODY BY DECLARING AN EMER. THE CTLR'S VECTORS WERE EXCELLENT IN BACKING UP OUR ILS APCH AND WE LANDED OK. ON TAXIING IN, I ASKED THE F/O TO CHK CB'S AND HE FOUND THE 'XFER BUS,' '#1 NORMAL' WAS POPPED, ALTHOUGH THE LEFT SIDE 'XFER BUSS OFF' LIGHT WAS NOT ILLUMINATED. ON RESETTING THIS CB, EVERYTHING RETURNED TO NORMAL. IF THIS HAD HAPPENED AT ALT, WE WOULD HAVE EVENTUALLY FOUND THE CB, BUT AT NIGHT IN VERY BAD WX ALL OVER AND ON APCH, I DECLARED THE EMER TO BACK UP OUR SITUATION. UPON CHKING OTHER ACFT THE NEXT DAY, I FOUND THEY ALL DO THE SAME THING AND DON'T TURN ON THE XFER BUSS OFF LIGHT. OUR COMPANY IS CURRENTLY CHKING WITH MANUFACTURER ON THIS SITUATION WITH THE MLG TYPE 3 AND 4. THE LACK OF THAT LIGHT COMING ON IN THIS SITUATION GIVES THE PLT THE WRONG IDEA. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING: THIS PARTICULAR ACFT WAS GNDED ABOUT 1 WK LATER FOR REPEATED XFER CB PROBS. THE MANUFACTURER SAID THE PROB WAS WITH THE XFER CB ITSELF. THE CB IS MADE BY A SUBCONTRACTOR AND THEY HAVE BEEN ADVISED OF THE PROB. THE WIRING OF THE XFER WARNING LIGHT IS STILL BEING INVESTIGATED. THE RPTR CLAIMS THAT THEY ARE TRAINED TO LOOK FOR THIS LIGHT WHENEVER AN ELECTRICAL PROBLEM DEVELOPS AND THE FACT THAT IT DID NOT COME ON WITH THIS FAILURE CAUSES COCKPIT CONFUSION. RPTR ALSO SAID THAT A T/R AMPERAGE CHK COULD HAVE IDENTED THE PROB, BUT HE MISSED SEEING THE SLIGHT AMPERAGE DROP BECAUSE OF THE LIGHTING, THE TIME CONSTRAINT AND THE SCALE OF THE AMMETER.

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.