A LEAR 45 FLT CREW EXPERIENCED AIR DATA COMPUTER FAILURE THAT RESULTED IN FAILURE OF AUTOPLT ALT HOLD FUNCTION; LEADING TO AN UNCOMMANDED DSCNT.

Narrative

WHILE ESTABLISHED AT FL430 IN AN ACFT ON FIRST FLT OUT OF MAINT FOR A CABIN DOOR ADJUSTMENT; WE RECEIVED A MASTER CAUTION WARNING ALARM AND 2 DOOR WARNING LIGHTS. AFTER FOLLOWING PROCS; WE DECIDED IT WAS AN INDICATING SYS FAULT AND NOT AN ACTUAL DOOR FAULT. WHILE RELAYING THE DOOR INFO TO MAINT VIA SAT PHONE WE RECEIVED ANOTHER MASTER CAUTION WARNING INDICATING MULTIPLE FAILURES. THEY WERE: RUDDER BOOST FAILURE; MACH TRIM FAILURE; PRIMARY PITCH TRIM FAILURE; L ENG COMPUTER FAULT; L STALL WARNING FAILURE AND RED X'S ACROSS ALTIMETER AND AIRSPD INDICATORS. WE REGAINED INSTS WHEN WE SELECTED THE OTHER AIR DATA COMPUTER. THE AUTOPLT DID NOT DISENGAGE AND THE GREEN AUTOPLT ANNUNCIATOR IN THE ADI REMAINED ILLUMINATED. AFTER A QUICK DISCUSSION; WE DECIDED TO RETURN TO BASE AND RELAYED THE REQUEST TO ATC. THEY PROVIDED US WITH VECTORS AND A 180 DEG TURN WHICH WE PROGRAMMED THROUGH THE FMS AND AUTOPLT. IN THE TURN THE ALT HOLD FUNCTION OF THE AUTOPLT FAILED TO MAINTAIN ALT AND THE ACFT BEGAN A GRADUAL DSCNT OUT OF FL430 THAT WENT UNNOTICED BY THE CREW AS WE WORKED CHKLISTS. OUR ATC CTLR NOTIFIED US OF OUR ALT AND WE QUICKLY RETURNED TO FL430. NO LOSS OF SEPARATION WAS RPTED. LESSONS LEARNED: EVEN THOUGH THE AUTOPLT DID NOT INDICATE 'DISENGAGED' WE STILL HAD A FAILURE OF THE ALT HOLD FUNCTION AND WERE DISTR AS OUR WORKLOAD INCREASED. JUST CHKING THE GREEN AUTOPLT ANNUNCIATION DOESN'T MEAN WE ARE LEVEL.