Narrative:

At FL370 the left reverser unlocked EICAS message appeared. Immediately after this happened the autoplt and autothrottles disconnected and the airplane went into a left roll; but no buffet was noted. After autoplt let loose and the left roll occurred two pages of master cautions msgs appeared and then disappeared and then reappeared. The msgs came and went; at times so fast that I could not even focus on them; nor could I tell what kind of problem we had. I noticed on the overhead panel the battery discharge light came on. The aircraft was flying like a bad MD80; very heavy on the controls and slow to respond to inputs. I checked to see if we were losing hydraulics; all systems were normal. I declared an emergency with center and requested a heading directly to the airport. First officer was running the checklist for the loss of 'standby power' but from what we had it was difficult to tell if this was the true issue at hand. As ATC cleared us directly to airport; I was hand flying the aircraft which was not responding normally to inputs. I continued to check the hydraulic quantities because I had a deep concern about the flight controls. The entire time this was going on; I had system coming and going. At one point I went to make a PA and my communication panel was completely dead. I could not talk to ATC; the crew or make a PA -- nothing was working. EICAS message continued to come and go throughout descent until the first officer started the APU. Once the APU was up and running; things started to settle down a little; but the airplane continued to fly like a box of rocks. Controls during the entire descent and landing were very stiff and slow to respond. We landed just under the maximum landing weight and taxied to the gate. Maintenance met the aircraft and we explained what had happened. Maintenance went to bite check the boxes and talk to tech service. As we sat waiting for their return the power flashed twenty or more times. First officer and I both thought it was maintenance working on the aircraft. They later returned to tell us the bite checks were good. As they were talking to us the power started flashing again. It was at this point we learned it was not maintenance causing the power to flash and everyone was quite surprised. Maintenance then went back to work trying to find out what the issue was. After a few hours the maintenance team informed us the bpcu had failed. The bpcu is the main brain that controls how the power is placed on the bus systems. As this bus power control unit was failing it could not figure out how or where it wanted to place the power. It was powering and unpowering different buses at different times as it was failing. This gave us a vast amount of EICAS messages and at the same time it also kicked off the feel computers and the rudder ratio inputs according to maintenance. We were quite fortunate that we had good WX and easy arrival into airport or this could have become very ugly. The maintenance team did a great job of problem solving this issue. First officer with less than 200 hours on the aircraft did an outstanding job of helping me work through this very abnormal emergency. At the end of the day during a discussion with dispatch; I learned that another air carrier had a similar type issue which almost resulted in the loss of the aircraft. In 12 years of training on the aircraft I have never had an instructor talk about this malfunction.callback conversation with reporter revealed the following information: the reporter stated that early on in this event he was most concerned about keeping the aircraft upright for an off airport landing if necessary. According to the air carrier maintenance starting the APU was probably what allowed the electrical system to assume some order because of relay movements. At the height of the event the IRU's were on battery only; VHF #1 was unpowered; the ADI's tumbled and the standby attitude gyro caged. An interesting sideline to this event was that when power was restored the crew could not find the FMS diversion page because it buried so deeply in the pegasus menus. This event in total was something that one would think could not happen in a modern aircraft but it now appears that asingle point failure can cause a potentially serious problem.

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Original NASA ASRS Text

Title: A B757 CREW DECLARED AN EMER AND DIVERTED FOLLOWING THE LOSS OF ELECTRICAL POWER CAUSED BY THE FAILURE OF THE BPCU POWER DISTRIBUTION UNIT.

Narrative: AT FL370 THE L REVERSER UNLOCKED EICAS MSG APPEARED. IMMEDIATELY AFTER THIS HAPPENED THE AUTOPLT AND AUTOTHROTTLES DISCONNECTED AND THE AIRPLANE WENT INTO A L ROLL; BUT NO BUFFET WAS NOTED. AFTER AUTOPLT LET LOOSE AND THE L ROLL OCCURRED TWO PAGES OF MASTER CAUTIONS MSGS APPEARED AND THEN DISAPPEARED AND THEN REAPPEARED. THE MSGS CAME AND WENT; AT TIMES SO FAST THAT I COULD NOT EVEN FOCUS ON THEM; NOR COULD I TELL WHAT KIND OF PROBLEM WE HAD. I NOTICED ON THE OVERHEAD PANEL THE BATTERY DISCHARGE LIGHT CAME ON. THE ACFT WAS FLYING LIKE A BAD MD80; VERY HEAVY ON THE CTLS AND SLOW TO RESPOND TO INPUTS. I CHKED TO SEE IF WE WERE LOSING HYDRAULICS; ALL SYSTEMS WERE NORMAL. I DECLARED AN EMER WITH CTR AND REQUESTED A HEADING DIRECTLY TO THE ARPT. FO WAS RUNNING THE CHKLIST FOR THE LOSS OF 'STANDBY POWER' BUT FROM WHAT WE HAD IT WAS DIFFICULT TO TELL IF THIS WAS THE TRUE ISSUE AT HAND. AS ATC CLRED US DIRECTLY TO ARPT; I WAS HAND FLYING THE ACFT WHICH WAS NOT RESPONDING NORMALLY TO INPUTS. I CONTINUED TO CHK THE HYDRAULIC QUANTITIES BECAUSE I HAD A DEEP CONCERN ABOUT THE FLT CTLS. THE ENTIRE TIME THIS WAS GOING ON; I HAD SYSTEM COMING AND GOING. AT ONE POINT I WENT TO MAKE A PA AND MY COMMUNICATION PANEL WAS COMPLETELY DEAD. I COULD NOT TALK TO ATC; THE CREW OR MAKE A PA -- NOTHING WAS WORKING. EICAS MESSAGE CONTINUED TO COME AND GO THROUGHOUT DESCENT UNTIL THE FO STARTED THE APU. ONCE THE APU WAS UP AND RUNNING; THINGS STARTED TO SETTLE DOWN A LITTLE; BUT THE AIRPLANE CONTINUED TO FLY LIKE A BOX OF ROCKS. CONTROLS DURING THE ENTIRE DESCENT AND LANDING WERE VERY STIFF AND SLOW TO RESPOND. WE LANDED JUST UNDER THE MAX LANDING WT AND TAXIED TO THE GATE. MAINT MET THE ACFT AND WE EXPLAINED WHAT HAD HAPPENED. MAINT WENT TO BITE CHECK THE BOXES AND TALK TO TECH SVC. AS WE SAT WAITING FOR THEIR RETURN THE POWER FLASHED TWENTY OR MORE TIMES. FO AND I BOTH THOUGHT IT WAS MAINT WORKING ON THE ACFT. THEY LATER RETURNED TO TELL US THE BITE CHECKS WERE GOOD. AS THEY WERE TALKING TO US THE POWER STARTED FLASHING AGAIN. IT WAS AT THIS POINT WE LEARNED IT WAS NOT MAINT CAUSING THE POWER TO FLASH AND EVERYONE WAS QUITE SURPRISED. MAINT THEN WENT BACK TO WORK TRYING TO FIND OUT WHAT THE ISSUE WAS. AFTER A FEW HOURS THE MAINT TEAM INFORMED US THE BPCU HAD FAILED. THE BPCU IS THE MAIN BRAIN THAT CONTROLS HOW THE POWER IS PLACED ON THE BUS SYSTEMS. AS THIS BUS POWER CONTROL UNIT WAS FAILING IT COULD NOT FIGURE OUT HOW OR WHERE IT WANTED TO PLACE THE POWER. IT WAS POWERING AND UNPOWERING DIFFERENT BUSES AT DIFFERENT TIMES AS IT WAS FAILING. THIS GAVE US A VAST AMOUNT OF EICAS MESSAGES AND AT THE SAME TIME IT ALSO KICKED OFF THE FEEL COMPUTERS AND THE RUDDER RATIO INPUTS ACCORDING TO MAINT. WE WERE QUITE FORTUNATE THAT WE HAD GOOD WX AND EASY ARR INTO ARPT OR THIS COULD HAVE BECOME VERY UGLY. THE MAINT TEAM DID A GREAT JOB OF PROBLEM SOLVING THIS ISSUE. FO WITH LESS THAN 200 HOURS ON THE ACFT DID AN OUTSTANDING JOB OF HELPING ME WORK THROUGH THIS VERY ABNORMAL EMER. AT THE END OF THE DAY DURING A DISCUSSION WITH DISPATCH; I LEARNED THAT ANOTHER ACR HAD A SIMILAR TYPE ISSUE WHICH ALMOST RESULTED IN THE LOSS OF THE ACFT. IN 12 YEARS OF TRAINING ON THE ACFT I HAVE NEVER HAD AN INSTRUCTOR TALK ABOUT THIS MALFUNCTION.CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: THE RPTR STATED THAT EARLY ON IN THIS EVENT HE WAS MOST CONCERNED ABOUT KEEPING THE ACFT UPRIGHT FOR AN OFF ARPT LNDG IF NECESSARY. ACCORDING TO THE ACR MAINT STARTING THE APU WAS PROBABLY WHAT ALLOWED THE ELECTRICAL SYSTEM TO ASSUME SOME ORDER BECAUSE OF RELAY MOVEMENTS. AT THE HEIGHT OF THE EVENT THE IRU'S WERE ON BATTERY ONLY; VHF #1 WAS UNPOWERED; THE ADI'S TUMBLED AND THE STANDBY ATTITUDE GYRO CAGED. AN INTERESTING SIDELINE TO THIS EVENT WAS THAT WHEN POWER WAS RESTORED THE CREW COULD NOT FIND THE FMS DIVERSION PAGE BECAUSE IT BURIED SO DEEPLY IN THE PEGASUS MENUS. THIS EVENT IN TOTAL WAS SOMETHING THAT ONE WOULD THINK COULD NOT HAPPEN IN A MODERN ACFT BUT IT NOW APPEARS THAT ASINGLE POINT FAILURE CAN CAUSE A POTENTIALLY SERIOUS PROBLEM.

Data retrieved from NASA's ASRS site as of May 2009 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.