Narrative:

Prior to departure from cleveland to cincinnati, oh the EICAS showed a caution for 'AC service bus' inoperative. This is primarily a lav motor and we recycled the circuit breaker per the checklist and the problem appeared to be corrected. We noticed that the pre-selected v-spds had been erased and reset these. We also had a momentary spoileron problem which resolved itself (perhaps aided by temporarily selecting all AC hydraulic pumps on) with all problems cleared we departed. After departure switched us to ZOB and we were cleared from 7000- 15000 ft, I informed the first officer that I was going to call operations with our out/off times and that he had the ATC communication. Having done this, operations wanted us to have the flight attendant verify a ticket lift for a specific passenger. I told the first officer that I would be out of communication #1 a little longer for this. After the flight attendant verified the ticket situation, I called operations back to inform them. As I switched back to the ATC communication #1 told the first officer I was back and then saw my altimeter at 15500 ft and we were climbing. The pre-select was still at 15000 ft, but there had been no 1000 audible as the first officer's altimeter was now off by 1000 ft. We apparently had another momentary AC electrical 'blip' which changed his altimeter setting by 1.00 inches and the caution informed the first officer that we were almost 1000 ft high and to descend immediately. About this time center cleared us up to the flight levels and inquired about the altitude. We replied that we had experienced an AC electrical bus failure and we were trying to sort out the problem. Callback conversation with reporter revealed the following information: reporter states he had a similar incident in the same kind of aircraft a short time after this reported incident. At that time even the decimal digits changed up from 30.20 to 31.47. This was more noticeable due to the decimal change. The AC bus did not indicate in that instance and the electrical surge must have been extremely momentary. Reporter states he had no idea what causes this problem. He of course reported it to his chief pilot who indicated he has had other reports of similar incidents. He will be consulting with the manager.

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Original NASA ASRS Text

Title: MDT HAS CAUTION LIGHT FOR AC SVC BUS. RESET CIRCUIT BREAKER BUT ALTIMETER CHANGES BY 1 INCH.

Narrative: PRIOR TO DEP FROM CLEVELAND TO CINCINNATI, OH THE EICAS SHOWED A CAUTION FOR 'AC SVC BUS' INOP. THIS IS PRIMARILY A LAV MOTOR AND WE RECYCLED THE CIRCUIT BREAKER PER THE CHKLIST AND THE PROB APPEARED TO BE CORRECTED. WE NOTICED THAT THE PRE-SELECTED V-SPDS HAD BEEN ERASED AND RESET THESE. WE ALSO HAD A MOMENTARY SPOILERON PROB WHICH RESOLVED ITSELF (PERHAPS AIDED BY TEMPORARILY SELECTING ALL AC HYD PUMPS ON) WITH ALL PROBS CLRED WE DEPARTED. AFTER DEP SWITCHED US TO ZOB AND WE WERE CLRED FROM 7000- 15000 FT, I INFORMED THE FO THAT I WAS GOING TO CALL OPS WITH OUR OUT/OFF TIMES AND THAT HE HAD THE ATC COM. HAVING DONE THIS, OPS WANTED US TO HAVE THE FLT ATTENDANT VERIFY A TICKET LIFT FOR A SPECIFIC PAX. I TOLD THE FO THAT I WOULD BE OUT OF COM #1 A LITTLE LONGER FOR THIS. AFTER THE FLT ATTENDANT VERIFIED THE TICKET SIT, I CALLED OPS BACK TO INFORM THEM. AS I SWITCHED BACK TO THE ATC COM #1 TOLD THE FO I WAS BACK AND THEN SAW MY ALTIMETER AT 15500 FT AND WE WERE CLBING. THE PRE-SELECT WAS STILL AT 15000 FT, BUT THERE HAD BEEN NO 1000 AUDIBLE AS THE FO'S ALTIMETER WAS NOW OFF BY 1000 FT. WE APPARENTLY HAD ANOTHER MOMENTARY AC ELECTRICAL 'BLIP' WHICH CHANGED HIS ALTIMETER SETTING BY 1.00 INCHES AND THE CAUTION INFORMED THE FO THAT WE WERE ALMOST 1000 FT HIGH AND TO DSND IMMEDIATELY. ABOUT THIS TIME CTR CLRED US UP TO THE FLT LEVELS AND INQUIRED ABOUT THE ALT. WE REPLIED THAT WE HAD EXPERIENCED AN AC ELECTRICAL BUS FAILURE AND WE WERE TRYING TO SORT OUT THE PROB. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: RPTR STATES HE HAD A SIMILAR INCIDENT IN THE SAME KIND OF ACFT A SHORT TIME AFTER THIS RPTED INCIDENT. AT THAT TIME EVEN THE DECIMAL DIGITS CHANGED UP FROM 30.20 TO 31.47. THIS WAS MORE NOTICEABLE DUE TO THE DECIMAL CHANGE. THE AC BUS DID NOT INDICATE IN THAT INSTANCE AND THE ELECTRICAL SURGE MUST HAVE BEEN EXTREMELY MOMENTARY. RPTR STATES HE HAD NO IDEA WHAT CAUSES THIS PROB. HE OF COURSE RPTED IT TO HIS CHIEF PLT WHO INDICATED HE HAS HAD OTHER RPTS OF SIMILAR INCIDENTS. HE WILL BE CONSULTING WITH THE MGR.

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.