Narrative:

I was the captain on flight and experienced an electrical glitch during cruise flight, which led to an unintentional altitude deviation. This problem manifested itself with a partial failure of the #2 CADC, which resulted in a temporary failure of the thrust rating indicator. The automatic throttles retarded to the new erroneous EPR limit of 1.34 EPR. Simultaneously the altitude alert warning failed, along with several of the first officer's flight INS. The autoplt and automatic throttles remained engaged and began a slow descent. My immediate attention was focused on the engine gauges to determine which engine had failed and why. By the time I had determined that both engines were operating normally, the first officer realized that we had experienced a partial #2 CADC failure, and immediately switched the CADC selector to 'both on 1.' at that time we had descended from our cruising altitude of 35000' to 34100', due to the engines having spooled down. By this time ZLC air traffic controller on frequency 121.15 noticed on his scope that we no longer were cruising at FL350 and asked why. By now I was hand-flying the aircraft and climbing back up to FL350. We told ATC that we were experienced an electrical problem and were in the process of climbing back up to FL350. 3 mins later our partial CADC failure healed itself, and all failure flags disappeared. During our unintentional descent there was no conflict with any other air traffic, and our TCAS further showed that there was no other aircraft within 40 mi of us.

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Original NASA ASRS Text

Title: ELECTRICAL MALFUNCTION ALLOWED AUTOPLT TO START A DESCENT OF ACFT NOT NOTICED BY FLT CREW.

Narrative: I WAS THE CAPT ON FLT AND EXPERIENCED AN ELECTRICAL GLITCH DURING CRUISE FLT, WHICH LED TO AN UNINTENTIONAL ALT DEVIATION. THIS PROB MANIFESTED ITSELF WITH A PARTIAL FAILURE OF THE #2 CADC, WHICH RESULTED IN A TEMPORARY FAILURE OF THE THRUST RATING INDICATOR. THE AUTO THROTTLES RETARDED TO THE NEW ERRONEOUS EPR LIMIT OF 1.34 EPR. SIMULTANEOUSLY THE ALT ALERT WARNING FAILED, ALONG WITH SEVERAL OF THE F/O'S FLT INS. THE AUTOPLT AND AUTO THROTTLES REMAINED ENGAGED AND BEGAN A SLOW DSNT. MY IMMEDIATE ATTN WAS FOCUSED ON THE ENG GAUGES TO DETERMINE WHICH ENG HAD FAILED AND WHY. BY THE TIME I HAD DETERMINED THAT BOTH ENGS WERE OPERATING NORMALLY, THE F/O REALIZED THAT WE HAD EXPERIENCED A PARTIAL #2 CADC FAILURE, AND IMMEDIATELY SWITCHED THE CADC SELECTOR TO 'BOTH ON 1.' AT THAT TIME WE HAD DSNDED FROM OUR CRUISING ALT OF 35000' TO 34100', DUE TO THE ENGS HAVING SPOOLED DOWN. BY THIS TIME ZLC AIR TFC CTLR ON FREQ 121.15 NOTICED ON HIS SCOPE THAT WE NO LONGER WERE CRUISING AT FL350 AND ASKED WHY. BY NOW I WAS HAND-FLYING THE ACFT AND CLBING BACK UP TO FL350. WE TOLD ATC THAT WE WERE EXPERIENCED AN ELECTRICAL PROB AND WERE IN THE PROCESS OF CLBING BACK UP TO FL350. 3 MINS LATER OUR PARTIAL CADC FAILURE HEALED ITSELF, AND ALL FAILURE FLAGS DISAPPEARED. DURING OUR UNINTENTIONAL DSNT THERE WAS NO CONFLICT WITH ANY OTHER AIR TFC, AND OUR TCAS FURTHER SHOWED THAT THERE WAS NO OTHER ACFT WITHIN 40 MI OF US.

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.