LR55 DEVIATES FROM ASSIGNED ALT AFTER EXPERIENCING ADI FAILURE IN ATL AIRSPACE.

Narrative

DURING A ROUTINE NIGHT FLT; OUR PRIMARY ADI AND GYRO FAILED. INITIAL INDICATIONS WERE ABNORMAL BANK OF 10 DEGS OR MORE FOLLOWED BY A GRADUAL INCREASE IN BANK AND PITCH INDICATIONS. AUTOPLT FUNCTIONS WERE ALSO ABNORMAL AS INDICATORS DEPICTED NORMAL ROLL AND PITCH ENGAGEMENT; BUT WAS ACTUALLY DISABLED DUE TO VERT GYRO FAILURE. AFTER SEVERAL ATTEMPTS OF RE-ENGAGEMENT; AUTOPLT WAS RENDERED USELESS. WHILE DISTR WITH PRIMARY ATTITUDE GYRO INDICATIONS AND AUTOPLT FAILURE; THE PF WAS DISORIENTED AND ATTEMPTED TO RECOVER FROM AN UNUSUAL ATTITUDE AS DEPICTED ON THE PRIMARY ADI. EVEN THOUGH VISUAL AIDS TO OUTSIDE HORIZON AND 2 SUPPORTING BACK-UP ATTITUDE INDICATORS REVEALED A NORMAL STRAIGHT AND LEVEL CRUISE CONFIGN; AN ATTEMPT WAS MADE TO RECOVER TO NORMAL. APPROX 500 FT WAS LOST AND 40 DEGS L DEV OFF COURSE. ZTL WAS IMMEDIATELY NOTIFIED AS TO THE NATURE OF THE PROB; IE; GYRO FAILURE; AND DEV RECOVERY; IE; RETURNING TO NORMAL ON COURSE AND ALT ASSIGNMENTS. THE PNF ANNOUNCED HIS ACTION TO TAKE THE CTLS FROM THE PF AND IMMEDIATELY INITIATED A RECOVERY TO NORMAL STRAIGHT AND LEVEL CRUISE CONFIGN. ONCE ACFT WAS RE-ESTABLISHED ON COURSE; CTLS WERE RETURNED TO THE ORIGINAL PF; AFTER THE PRIMARY ADI WAS COVERED AND COMPLETELY OUT OF PRIMARY INST SCAN. THIS WHOLE EVENT COULD HAVE BEEN PREVENTED IF THE PRIMARY ADI HAD BEEN COVERED UP EARLIER AND/OR CTLS RELINQUISHED TO THE FO WITH NORMAL FLT INSTS UNTIL ACFT TROUBLESHOOTING HAD BEEN ACCOMPLISHED.