Narrative:

While at cruise altitude of FL310, 40 mi east of billings, the master caution light illuminated. The indication of both IRU's dc backup failure was confirmed. The abnormal checklist was read, but the problem could not be resolved. A call to our ZZZ maintenance people was initiated. After further research, we were instructed to pull the #2 IRS AC circuit breaker. When that circuit breaker was pulled, all power to the autoplts was lost, as well as other system, including the loss of #2 IRS. The aircraft started to descend immediately. Altitude loss was corrected at approximately FL307 and a climb back to FL310 started within seconds. Due to the original problem, a diversion to billings, mt, was necessary. A report has been filed with our company in order to advise our maintenance department as well as other pilots about the incident and what to expect if that circuit breaker is pulled. Callback conversation with reporter revealed the following: the reporter stated that both inertial reference units were operating normally with the exception of the master caution IRS warning and both dc fail lights on the inertial reference mode select panel. The reporter stated an attempt was made to clear the dc fail lights, but no checklist covered this problem, so a call was made to the system maintenance controller advising the situation. The reporter said the maintenance controller advised pulling and resetting the #2 inertial reference AC circuit breaker which was tried with undesirable results. The reporter said the autoplt disconnected, the #2 IRS was lost and all system related to the #2 IRS. The reporter said that the maintenance controller should have been aware of the consequence of pulling this circuit breaker. The reporter stated the company has been alerted to this problem.

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Original NASA ASRS Text

Title: A B737-300 IN CRUISE AT FL310 ATTEMPTED TO CLR AN INERTIAL REF SYS FAULT BY RESETTING A CIRCUIT BREAKER CAUSING THE LOSS OF THE IRS AND AUTOPLT RESULTING IN AN ALT EXCURSION.

Narrative: WHILE AT CRUISE ALT OF FL310, 40 MI E OF BILLINGS, THE MASTER CAUTION LIGHT ILLUMINATED. THE INDICATION OF BOTH IRU'S DC BACKUP FAILURE WAS CONFIRMED. THE ABNORMAL CHKLIST WAS READ, BUT THE PROB COULD NOT BE RESOLVED. A CALL TO OUR ZZZ MAINT PEOPLE WAS INITIATED. AFTER FURTHER RESEARCH, WE WERE INSTRUCTED TO PULL THE #2 IRS AC CIRCUIT BREAKER. WHEN THAT CIRCUIT BREAKER WAS PULLED, ALL PWR TO THE AUTOPLTS WAS LOST, AS WELL AS OTHER SYS, INCLUDING THE LOSS OF #2 IRS. THE ACFT STARTED TO DSND IMMEDIATELY. ALT LOSS WAS CORRECTED AT APPROX FL307 AND A CLB BACK TO FL310 STARTED WITHIN SECONDS. DUE TO THE ORIGINAL PROB, A DIVERSION TO BILLINGS, MT, WAS NECESSARY. A RPT HAS BEEN FILED WITH OUR COMPANY IN ORDER TO ADVISE OUR MAINT DEPT AS WELL AS OTHER PLTS ABOUT THE INCIDENT AND WHAT TO EXPECT IF THAT CIRCUIT BREAKER IS PULLED. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING: THE RPTR STATED THAT BOTH INERTIAL REF UNITS WERE OPERATING NORMALLY WITH THE EXCEPTION OF THE MASTER CAUTION IRS WARNING AND BOTH DC FAIL LIGHTS ON THE INERTIAL REF MODE SELECT PANEL. THE RPTR STATED AN ATTEMPT WAS MADE TO CLR THE DC FAIL LIGHTS, BUT NO CHKLIST COVERED THIS PROB, SO A CALL WAS MADE TO THE SYS MAINT CTLR ADVISING THE SIT. THE RPTR SAID THE MAINT CTLR ADVISED PULLING AND RESETTING THE #2 INERTIAL REF AC CIRCUIT BREAKER WHICH WAS TRIED WITH UNDESIRABLE RESULTS. THE RPTR SAID THE AUTOPLT DISCONNECTED, THE #2 IRS WAS LOST AND ALL SYS RELATED TO THE #2 IRS. THE RPTR SAID THAT THE MAINT CTLR SHOULD HAVE BEEN AWARE OF THE CONSEQUENCE OF PULLING THIS CIRCUIT BREAKER. THE RPTR STATED THE COMPANY HAS BEEN ALERTED TO THIS PROB.

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.