AN MD80 IN CRUISE AT FL310 HAD R AUTOPLT DISCONNECT WITH FO'S MOMENTARY FLAGS IN THE AIRSPD INDICATOR.

Narrative

IN CRUISE AT FL310; AUTOPLT #2 DISCONNECTED. FO (PF) HAD MOMENTARY FLAGS IN AIRSPD INDICATOR. HIS ALTIMETER AND VSI WERE SHOWING A DSCNT WHILE MINE WAS SHOWING A CLB. ATC WAS NOTIFIED IMMEDIATELY. DURING RECOGNITION AND ANALYSIS OF PROB; MAX ALT GAIN WAS 300-400 FT. ATC ADVISED US THERE WAS NO TFC CONFLICT. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: THE RPTR STATED THE FIRST INDICATION OF A PROB WAS THE R AUTOPLT TRIPPED OFF AND THEN MOMENTARY FLAGS APPEARED IN THE FO'S AIRSPD INDICATOR. THE RPTR SAID IT WAS NOTED THE FO'S INSTS INDICATED A SLIGHT DSCNT WHILE THE CAPT'S INSTS WERE SHOWING A CLB. THE RPTR STATED ATC WAS ADVISED OF THE ALT EXCURSION OF 400 FT. SUPPLEMENTAL INFO FROM ACN 604262: I (THE FO) WAS FLYING. THE DFGC SWITCH WAS ON #2 AND WE WERE IN NAV TRACK AND ALT HOLD MODES. I TOOK OVER MANUALLY. ATC WAS IMMEDIATELY NOTIFIED OF THE PROB AND QUERIED ABOUT WHAT THEY SHOWED OUR ALT TO BE. THIS INFO; ALONG WITH THE STANDBY INSTS CONFIRMED THAT THE FO'S FLT INSTS WERE ERRATIC AND UNRELIABLE; AND NOW THE FO'S 'OFF' FLAGS WERE BEGINNING TO APPEAR INTERMITTENTLY. IT WAS LATER DIAGNOSED THAT THE ACFT WAS EXPERIENCING A FAILURE OF THE #2 CENTRAL AIR DATA COMPUTER. THE FLT WAS COMPLETED WITH BOTH THE CAPT'S AND FO'S INSTS OPERATING OFF OF THE #1 CENTRAL AIR DATA COMPUTER. ONCE STABILIZED BACK AT FL310; ATC WAS QUERIED AND RESPONDED THAT TFC SEPARATION WAS NEVER JEOPARDIZED.