AN INTEGRATED AVIONICS COMPUTER FAILURE IS FOUND TO BE THE CAUSE OF ERRONEOUS INDICATIONS ON BOTH PLT'S FLT DISPLAYS AND FAULTY OP OF THE AUTOPLT SYS ON A CE750. USE OF STANDBY INSTRUMENTATION WAS REQUIRED.

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FMS WAS PROGRAMMED WITH RTE OF FLT WITH NORMAL EFIS INDICATIONS PRIOR TO TKOF. TKOF WAS ACCOMPLISHED IN PITCH AND HDG MODES. AUTOPLT WAS ENGAGED PASSING 2000 FT MSL; AND CLB AND FLT PATH PROGRESSED NORMALLY IN HDG MODE. CLRNC WAS GIVEN TO PROCEED DIRECT TO AN ENRTE FIX; AT WHICH TIME FMS WAS PROGRAMMED FOR DIRECT ROUTING AND NAV MODE WAS SELECTED. NAV SOLUTION FOR DIRECT ROUTING PRESENTED ON MFD APPEARED INVALID; AS IT INDICATED AN INITIAL TURN AWAY FROM PRESENT COURSE. AS THE COPLT AND I DISCUSSED THIS ABNORMALITY; IT BECAME APPARENT THE AUTOPLT WAS NOT RESPONDING TO THE NAV MODE COMMAND; BUT WAS STILL PROCEEDING IN HDG MODE. CORRECT SWITCHOLOGY WAS CONFIRMED; FMS WAS REPROGRAMMED; AND NAV MODE AGAIN SELECTED; BUT SIMILAR RESULTS WERE OBTAINED. IN AN ATTEMPT TO COMPLY WITH OUR CLRNC; THE HDG BUG WAS USED TO INTERCEPT OUR ASSIGNED RTE; BUT IT BECAME APPARENT AN INTERCEPT HDG WAS CAUSING US TO DIVERGE EVEN FURTHER FROM OUR FMS COURSE. ENRTE CHARTS WERE QUICKLY CONSULTED FOR AIRWAY FREQS AND COURSES; CDI'S WERE XCHKED FOR FUNCTIONALITY; AND A COURSE WAS FLOWN WHICH BEGAN TO REINTERCEPT OUR ROUTING. SUSPICIONS TURNED TO OUR FMS-DISPLAYED HDG; WHICH TURNED OUT TO DIFFER 50 DEGS FROM THE MAGNETIC COMPASS. DURING THIS TROUBLESHOOTING PROCESS; THE ACFT FAILED TO AUDIBLY ANNUNCIATE OR CAPTURE THE PRE-SELECTED ALT OF FL270 (AS INDICATED ON OUR PFD'S; WHICH ALSO TURNED OUT TO BE INACCURATE WHEN COMPARED TO OUR STANDBY ALTIMETER); CAUSING A POSSIBLE ALTDEV. THE AUTOPLT WAS DISCONNECTED AND THE ACFT HAND FLOWN TO MAINTAIN FL270; AS INDICATED ON THE STANDBY ALTIMETER; AND COURSE; AS INDICATED ON OUR CDI'S. SUBSEQUENT INVESTIGATION REVEALED A DUAL INS FAILURE WITH ASSOCIATED LOSS OF VALID ADC INDICATIONS; WHICH WAS APPARENTLY CAUSED BY A SINGLE INTEGRATED AVIONICS COMPUTER FAILURE CORRUPTING 2 THEORETICALLY INDEPENDENT SYS. NUMEROUS FAULT WARNINGS SHOULD HAVE ACCOMPANIED SUCH A FAILURE; BUT NONE WERE DISPLAYED AT ANY TIME BEFORE OR DURING THESE MULTIPLE FAILURE MODES. THE POSSIBLE ALTDEV MIGHT HAVE BEEN PREVENTED BY CLOSER ATTN TO (ALL) THE ALTIMETERS; BUT WHEN ALL INST INDICATIONS ARE SUSPECT AND MULTIPLE SYS ARE PARTLY OR WHOLLY INOP; PRIORITIZATION BECOMES VERY CHALLENGING! CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: IT WAS DETERMINED THAT THE MAP DISPLAYS WERE ACCURATE AND MANUAL SLAVING OF THE COMPASS COULD BE DONE. THE REMAINDER OF THE FLT WAS COMPLETED USING VOR NAV. THE MANUFACTURER PARTICIPATED IN TROUBLESHOOTING IN THE AIR. AFTER ARR. IT WAS DETERMINED THAT THE #2 INTEGRATED AVIONICS COMPUTER WAS AT FAULT. DURING FOLLOW-UP TESTING; THE CREW WAS TOLD THAT THE ACCURACY LIMIT FOR THE STANDBY COMPASS WITH PWR ON IS +/-29 DEGS WITH ELECTRIC PWR ON AND +/-10 DEGS WITH PWR OFF. THE COMPASS ON THE ACFT WAS NEAR THESE LIMITS WHICH MADE NAV DIFFICULT WHEN COUPLED WITH HIGH ENRTE XWINDS. NO FAILURES WERE INDICATED ON EICAS. THE 2 SYS SHOULD COMPARE WITH EACH OTHER FOR ACCURACY BUT THERE WAS NO 'MISSCOMPARE' WARNING GIVEN IN SPITE OF THE FAILURE.