B737-400 FLT CREW ON DEP FROM DEN LOSE BOTH IRS'S.

Narrative

AFTER LNDG AT DEN; I ASKED THE FO TO DETERMINE IF WE EXCEEDED OUR QUICK TURN LIMIT. WE DETERMINED THAT WE WERE OVER OUR WT; AND AFTER CONFERRING WITH ZZZ DISPATCH WE REVISED OUR DEP TIME TO REFLECT THE TIME LIMIT IMPOSED ON THE QUICK TURN LIMITS. AFTER THE MAINT INSPECTION WAS PERFORMED; WE PROCEEDED TO DO A NORMAL BEFORE START AND PUSHBACK PROC AND THEN TAXIED OUT. WE WERE ADVISED TO EXPECT A DIFFERENT RWY THAN ORIGINALLY ANTICIPATED. I ASKED GND WHERE THEY WOULD LIKE US TO HOLD WHILE WE UPDATED OUR DATA. WE TAXIED TO THAT LOCATION; ENTERED THE DATA AND PROCEEDED TO TAKE OFF. AS A SIDE NOTE; WHEN I AM #1 FOR TKOF; IT IS A MEMORY JOGGER FOR ME TO ENGAGE THE AUTOTHROTTLES; EXTEND THE LNDG LIGHTS AND PUSH RECALL FOR ANY ABNORMALITIES. UPON COMPLETING THESE ITEMS NO FAULT WAS FOUND. WE PROCEEDED TO TAKE OFF WITH BLEEDS OFF; IMPROVED CLB PROFILE; THE FO WAS FLYING AND AFTER FLYING AN APPROPRIATE DEP FOR THE CONDITIONS; I RE-ESTABLISHED THE BLEEDS AND TOLD THE FO THE BLEEDS ARE ESTABLISHED; UPON WHICH THE FO SAID THAT WE WERE WAITING FOR THE APU TO COOL DOWN. AFTER AT LEAST 1 MIN HAD ELAPSED; I SHUT THE APU DOWN AND SAW WE WERE COMING UP TO 10000 FT AND I PROCEEDED WITH MY 10000 FT FLOW. IT IS AT THIS TIME TO THE BEST OF MY RECOLLECTION; I FIRST NOTICED THAT WE HAD A SPD TRIM FAIL LIGHT AND SHORTLY THEREAFTER; AN AUTOPLT DISCONNECT AND LOSS OF NAV/FLT GUIDANCE. UPON LOOKING ON THE OVERHEAD PANEL; I NOTICED BOTH IRS'S WERE IN THE OFF POS. INSTINCTIVELY; I TURNED THEM BOTH ON HOPING TO REALIGN OR TO GET ANY NAV/FLT GUIDANCE BACK. WE WERE UNABLE TO RE-ALIGN OR ESTABLISH NORMAL NAV/FLT GUIDANCE INFO. AT THIS TIME I DECIDED IT WAS BEST TO RETURN TO DEN. WE CONTACTED DEN OPS AND TOLD THEM TO CONTACT DISPATCH AND TELL THEM WE ARE RETURNING TO DEN AND WE WILL CONTACT THEM ON THE GND. THE WX AT DEN WAS CLR WITH WINDSHEAR AND SHIFTING WINDS. WE ASKED DEN APCH FOR VECTORS BACK TO RWY 26K THE RWY WHICH WE HAD PREVIOUSLY LANDED ON. KNOWING THAT THIS WOULD BE AN OVERWT LNDG; WE BURNT OFF AS MUCH FUEL AS WE COULD BY LOWERING THE LNDG GEAR AND SELECTING FLAPS 1 DEG TO KEEP US WITHIN THE FLT REALM OF ATC'S REQUEST. IT TOOK BOTH OF US A WHILE TO REACQUAINT OURSELVES WITH FLYING THE WET COMPASS. AFTER CONSIDERING OUR OPTIONS; OF FLYING AROUND TRYING TO BURN OFF EXCESSIVE FUEL IN ORDER TO AVOID AN OVERWT LNDG; I THOUGHT IN THE INTEREST OF SAFETY; IT WOULD BE MORE APPROPRIATE TO LAND THE ACFT IN AN OVERWT CONDITION THAN TO FLY AROUND WITH LIMITED RAW DATA; NO 'TACAS.' AT SOME POINT ON ARR THE FO ASKED IF I COULD TAKE CTLS FOR A MOMENT. I TOOK OVER THE CTL OF THE ACFT; AND TOLD HIM I WOULD FLY THE ACFT TO LNDG. WE VISUALLY ACQUIRED THE LNDG RWY AND ASKED FOR A VISUAL APCH. WE WERE GRANTED A VISUAL TO OUR RWY. WE LANDED IN AN OVERWT CONDITION BUT A SMOOTH TOUCHDOWN WAS ACCOMPLISHED. DISPATCH WAS REPEATEDLY CALLING US FOR INFO ABOUT OUR SIT WHILE WE WERE TAXIING IN. WHEN WE HAD A SPARE MOMENT AT THE GATE; WE BRIEFLY ANSWERED SOME OF HER QUESTIONS. AFTER SHUTDOWN; WE MADE THE NECESSARY MAINT LOGBOOK ENTRIES; AND FOLLOWED UP WITH CALLS TO APPROPRIATE PERSONNEL INCLUDING ZZZ MAINT CTL; ZZZ DISPATCH; AND THE ON-CALL. WHILE I HAVE NO CONCLUSIVE EVIDENCE OF WHAT ACTUALLY HAPPENED; I CAN ONLY SPECULATE THAT IN THE HIGH ALT ENVIRONMENT OF DENVER; WHILE ACCOMPLISHING A BLEEDS OFF TKOF AND BEING CONCERNED ABOUT A PRESSURIZATION ISSUE; I MAY HAVE INADVERTENTLY TURNED OFF THE IRS'S WHILE PERFORMING A FLOW. I HAVE BEEN CONCERNED ABOUT CHRONIC FATIGUE FOR SOME TIME; AND I THINK IT PROBABLY WAS A CONTRIBUTING FACTOR IN THIS INCIDENT. WHILE I WISH I COULD STATE EXACTLY WHAT HAPPENED; AND HOW IT COULD BE AVOIDED; I TRULY DO NOT KNOW WHAT TRANSPIRED TO CAUSE THIS INCIDENT. I THINK A SIMULATOR TRAINING SESSION OF BASIC PITCH PWR AIRSPD; WET COMPASS; BASIC RAW DATA AND PARTIAL PANEL MANEUVERS WOULD BE BENEFICIAL TO CREW MEMBERS WHEN THE NORMAL IRS INFO IS NOT AVAILABLE.