ACFT EQUIP PROB. IRU FAILURE. MULTIPLE FAILURES RESULTED AND ACFT RETURNED TO ARPT OF ORIGIN.

Narrative

ON FEB/XX/96; I COMMANDED A FOKKER 100 UNDER THE CALL SIGN X FLT FROM RDU TO PVD. UPON XING THE FIRST FIX; TYI VOR; A LEVEL ONE ALERT WAS DISPLAYED ON THE MULTI- FUNCTION DISPLAY UNIT (MFDU) CRT. THE ALERT STATED 'COMPARE HDGS.' THIS REVEALED THAT THE FO'S AND THE STANDBY INSTS WERE IN AGREEMENT; BUT THE CAPT'S SIDE WAS OFF BY 20 DEGS. SHORTLY THEREAFTER; MULTIPLE LEVEL TWO ALERTS WERE DISPLAYED; INCLUDING: EFIS; AFCAS; YAW DAMPER #1; AUTOTHROTTLE #1; TCASII; AND IRU #1 FAIL. THE AUTOPLT DISCONNECTED; AND THE FO WHO WAS THE PF TOOK OVER AFTER THE AUTOPLT COULD NOT BE REENGAGED. ALL ATTITUDE INFO OF THE CAPT'S PRIMARY FLT DISPLAY (PFD) CRT FAILED; AND ALL NAV INFO ON THE CAPT'S NAV DISPLAY CRT ALSO FAILED. COMPLIANCE WITH ALL LEVEL TWO ALERTS YIELDED CONTINUED FAULTS; AND THE INERTIAL REF UNIT (IRU) #1 COULD NOT BE REALIGNED. DUE TO THE PARTIAL LOSS OF NAV CAPABILITY AND THE FLT INFO; I MADE THE DECISION TO RETURN TO RDU. I CONTACTED MY FLT DISPATCH; WHO ALSO CONCURRED WITH MY DECISION. I ADVISED ATC OF THE SIT; AND FOR THE NEED TO RETURN TO RDU. PRIORITY HANDLING WAS NOT NEEDED OR REQUESTED. A NORMAL LNDG FOLLOWED AT RDU APPROX 25 MINS LATER UNDER VFR CONDITIONS. COMPANY MAINT DETERMINED THAT THE IRU HAD INDEED FAILED. A NEW IRU WAS INSTALLED; AND THE FLT WAS REDISPATCHED APPROX 45 MINS LATER. CREW COORD AND ATC ASSISTANCE WAS VERY GOOD. UNFORTUNATELY; THERE WAS LITTLE GUIDANCE IN THE ABNORMAL SECTION OF THE SYS FLT MANUAL FOR THE 'COMPARE HDGS' ALERT; OR IRU FAILURE. I WAS ABLE TO TRANSFER THE FO'S FLT AND NAV INFO TO MY SIDE HOWEVER. THE COMPANY CHIEF PLT ADVISED ME THAT HE WILL LOOK INTO THIS SIT; AND AN UPDATE TO THE ABNORMALS SECTION OF THE FLT MANUAL WILL BE AVAILABLE SOON. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: WHEN THE 'COMPARE HDGS' MEMO CAME UP ON THE ELECTRONIC SCREEN; THE FLC DIDN'T KNOW WHAT ACTION TO TAKE; AND THERE WAS NO PROC IN THEIR MANUAL FOR THAT PROB. THEY DIDN'T TAKE ANY ACTION AND SHORTLY THEREAFTER THE #1 IRU FAILED COMPLETELY; TAKING WITH IT THE RELATED FLT INSTS; ETC. THE RPTR CAPT TRIED TO RESET THE IRU; BUT UNSUCCESSFULLY. HE THEN TRANSFERRED THE FO'S INSTS TO HIS PFD AND NAV DISPLAY. THE FO CONTINUED TO FLY AND CAPT HANDLED COM WITH ATC AND HIS COMPANY TO RETURN TO THEIR ORIGINAL ARPT. THE ACFT WAS CAPABLE OF CONTINUING BECAUSE THE #2 IRU WAS GOOD; BUT CONCERN FOR POTENTIAL FUTURE PROBS (ENG FAIL; ETC) CAUSED THE CAPT TO TAKE THE MOST PRUDENT COURSE -- TO RETURN TO THE JUST DEPARTED ARPT. THE RPTR'S CHIEF PLT IS INCLUDING AN ABNORMAL PROC IN THEIR MANUAL WHICH; THOUGH NOT COMPLETED; WILL INCLUDE SWITCHING THE FLT INSTS TO THE OTHER PLT'S IRU IN ANTICIPATION OF AN IRU FAILURE. USING THAT PROC WILL INSURE THE INSTS WILL BE ACCURATE WHEN THE IRU FAILS. RPTR SAYS THAT THE TRAINING ON ACFT SYS SHOULD BE MORE THOROUGH AND COMPREHENSIVE.