ACFT EQUIP PROB. DUAL IRS FAILURE WITH LOSS OF FLT NAV INSTS.

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INABILITY TO COMPLY WITH ATC INSTRUCTION AND TO FOLLOW PUBLISHED SID. ACFT STRAYED OFF COURSE INTO ARR AIRSPACE. NO ACTUAL CONFLICT OR EVASIVE ACTION REQUIRED AS ACFT ASSIGNED ALT WAS AMENDED TO 10000 FT (INBOUND ACFT DSNDING TO 11000 FT) ALT WAS AMENDED WHILE ACFT WAS PASSING THROUGH 6000 FT OR 8000 FT AND LEVELED AT 10000 FT; THOUGH ACFT DID ENTER ARR SECTOR AIRSPACE. ON ROTATION AFTER VR PRIOR TO V2 BOTH IRS SYS FAILED. BOTH CAPT AND FO ADI'S FROZEN IN ROLL AND PITCH LEVEL. FLT DIRECTOR BARS RETRACTED. BOTH CAPT AND FO HSI'S ROTATED TO 360 DEGS AND FROZE ON THAT HEADING. VOR COURSE SELECTORS INOP. BOTH CAPT AND FO RMI'S ROTATED TO 360 DEGS AND FROZE. BOTH NEEDLES PARKED AT 270 DEG POS. TCASII FAIL LIGHT ILLUMINATED. VOR/ILS -- NAV SELECTOR SWITCHES INOP; UNABLE TO RETURN TO RAW DATA NAV. NO PROCS FOUND IN ABNORMAL (EMER) OR SUPPLEMENTAL PROCS CHKLIST FOR BOTH IRS FAILURE. MAINT CTL UNABLE TO PROVIDE ANY ASSISTANCE. ACFT REMAINED VFR AND A TURN WAS INITIATED IN THE APPROX DIRECTION OF THE SID HEADING; THOUGH INITIAL ACTION WAS SOLELY TO MAINTAIN ACFT CTL (WINGS LEVEL; CONSTANT PITCH ATTITUDE). PHX DEP CLRED US DIRECT BXK. A TURN TOWARD BXK WAS NOT INITIATED PROMPTLY AS WE HAD JUST DETERMINED THAT AFTER BXK WAS TUNED INTO THE #2 NAV BOTH RMI NEEDLES WERE INOP. THE CAPT KNOWING THE APPROX LOCATION OF BXK THEN DIRECTED ME TO FOLLOW THE HWY. ATC NOTING OUR ERRANT PATH AMENDED OUR ALT TO 10000 FT. AFTER CLRING INBOUND TFC WE WERE CLBED TO 15000 FT AND SWITCHED TO ZAB. WE NOTIFIED THEM ON INITIAL CONTACT OF OUR PROB AND REQUESTED A HEADING AND TO REMAIN AT 15000 FT WHILE WE WORKED ON THE PROB. HAVING ALREADY CALLED FOR A CHKLIST AND NOT FINDING ONE; THE SUPPLEMENTAL PROCS AND ACFT MANUAL WERE REFED BUT NO INFO ON BOTH IRS FAILURE WAS FOUND. MAINT CTL WAS CONTACTED AND AFTER THEY ALSO COULD GIVE NO ADVICE; A DECISION TO RETURN TO PHX WAS MADE. COMPANY AND FLC WERE NOTIFIED AND A CLRNC TO PHX WAS RECEIVED. THE ACFT LANDED WITHOUT FURTHER INCIDENT. THE ONLY ABNORMAL OCCURRENCE PRIOR TO THE INCIDENT WAS THAT THE FMC WAS REPROGRAMMED BY MAINT IMMEDIATELY PRIOR TO OUR DEP; THOUGH THE PREVIOUS REVISION WAS STILL CURRENT AND WAS BEING USED BY US. THIS WAS THE 4TH LEG AND 6TH HR OF FLT WHILE ON OUR 11TH HR ON DUTY AFTER A LEGAL REST PERIOD THOUGH FATIGUE DID NOT SEEM TO BE A PROB. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: PREFLT SET-UP; PROGRAMMING; TAXI OUT AND TKOF WERE ALL NORMAL. A DUAL IRS FAILURE OCCURRED AT 50 FT AFTER TKOF WITH A RESULTANT FAILURE OF ALL PRIMARY FLT INSTS AND NAV EQUIP. AT FIRST THE FLC EXPERIENCED CONFUSION; BUT CONTINUED TO FLY THE ACFT AS CLOSE AS POSSIBLE TO THE SID USING STANDBY ATTITUDE AND MAGNETIC COMPASS; BUT WERE NOT VERY ACCURATE ON HDG CTL. AT APPROX 4000 FT THE FLC NOTIFIED ATC OF THEIR PROB AND ABOUT THAT TIME WERE ENTERING INTO THE ARR TFC CORRIDOR. ATC CLRED THEM DIRECT TO BXK; BUT WITHOUT NAV; THEIR ONLY MEANS OF DIRECTION WAS TO FOLLOW A HWY. THE FLC; TRYING TO CORRECT THE SERIOUS PROB; COULD NOT FIND ANY PROC; EMER OR SUPPLEMENTAL; TO GET BACK THE IRS'S AND ASSOCIATED INSTS. WHEN NO PROC WAS FOUND IN THE OPS MANUAL THEY CONTACTED THEIR COMPANY MAINT; WHO COULD NOT ADVISE OF ACTION TO TAKE. IN AN ATTEMPT TO REGAIN THE FMC; THEY TURNED IT OFF AND BACK ON WHILE RESETTING THE HDGS. LATER; MAINT ADVISED THEM THAT IT WOULD BE NECESSARY TO RESET THE HDGS TWICE IN ORDER TO GET BACK THE HDG INFO. FLC HAS HAD NO FEEDBACK ABOUT THE REASON FOR THE DUAL IRS FAILURE.