AUTOFLT SYSTEM ANOMALIES RESULTS IN MD80 COCKPIT CONFUSION AND GAR DURING CAT III APCH.

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WE WERE TIRED; IT WAS LATE AND THE WX WAS DETERIORATING. WE WERE RETURNING BACK TO ATLANTA AND IT WAS NOW ABOUT 1 HR LATER THAN SCHEDULED; BECAUSE OF ALL THE ICE BLOWING OVER THE SOUTHERN UNITED STATES. IT WAS ONLY OUR SECOND LEG OF THE DAY BUT WE WERE BOTH MAKING LITTLE TRIVIAL MISTAKES -- YAWNING; RUBBING THE EYES; AND WISHING FOR BED. THE LATEST ATIS FROM ATL DIDN'T MAKE ME FEEL ANY BETTER: FOG; DRIZZLE AND LOWERING VISIBILITY. IT WAS THEN THAT I MADE MY FIRST BIG MISTAKE. WHEN YOU'RE EXHAUSTED AND TRYING NOT TO ACT THAT WAY YOU CAN TALK YOURSELF INTO ANYTHING. I SAID 'LET'S FLY A CAT III TO AUTOLAND BECAUSE WE'RE TIRED AND I DON'T WANT TO GO AROUND!' THAT ATTITUDE SET OURSELVES UP FOR OUR NEXT PROB. COMING IN ON THE APCH; EVERYTHING LOOKED GOOD ON MY SIDE. I REVIEWED THE NOTES -- I HAD THE PLANE CONFIGURED AND WE WERE ABOUT TO INTERCEPT THE GLIDE PATH WITH MY FMA IN AUTOLAND MODE. I BELIEVED THAT BTWN THE BOTH OF US WE COULD MUSTER ENOUGH MENTAL ENERGY TO FLY AN AUTOLAND. IT WAS ABOUT THEN THAT MY FO ASKED ME IF I WAS GOING TO ARM THE AUTOLAND. I SAID I DID AND XCHKING HIS FMA NOTED THAT HIS WAS ONLY SHOWING ILS MODE. CONFUSED; I STARTED DOUBLECHKING THE FREQS AND COURSE SETTINGS. THEY WERE CORRECT. HOW ABOUT THE CONFIGN? AGAIN CORRECT. SO I PUSHED THE AUTOLAND BUTTON AGAIN -- NOTHING. SO OFF COMES THE FLT DIRECTORS; AUTOPLT AND THROTTLES; ALL WHILE WE WERE HEADING DOWNHILL TOWARD RWY 8L. THE FO WAS TRYING TO KEEP UP WITH MY FLURRY OF ACTIVITY; BUT I WAS FAST AND DETERMINED BECAUSE I'M A MAN OF MY WORD AND I'M NOT GOING AROUND! FORTUNATELY; WE WEREN'T ALONE. ATL TWR QUERIED US ABOUT OUR POS ON THE GLIDE PATH. MY FO SAID 'LET'S GET OUT OF HERE;' AND IT WAS THEN THAT I FINALLY CAME TO MY SENSES. WHAT THE HECK WAS I DOING? I LOST ALL SITUATIONAL AWARENESS BECAUSE I WAS FIXATED ON THE AUTOPLT. THIS WAS ALL WRONG AND I SAID 'LET'S GO AROUND!' IT WASN'T LONG AFTER THAT THAT TWR ORDERED US TO DO THE SAME. WE PULLED UP; MADE A L TURN; AND CLBED OUT AND WE RETURNED TO FLY A CAT I TO JUST ABOVE MINIMUMS AND LANDED. NOW; IN RETROSPECT; I CAN SEE HOW MENTAL EXHAUSTION CAN MAKE A SIMPLE PROC LIKE GOING AROUND SEEM LIKE A MAJOR INCONVENIENCE. I GOT INFECTED BY A MAJOR CASE OF 'GET HOMEITIS' AND IT ALMOST LED TO DISASTER. SUPPLEMENTAL INFO FROM ACN 724687: THE TWR CTLR ISSUED A LOW ALT ALERT ALONG WITH THE CORRECT ALTIMETER SETTING. I ADVISED THE TWR WE WERE 'ON THE GO' AND THEY CLRED US TO FLY HDG 360 DEGS AND MAINTAIN 4000 FT. AS WE ROLLED OUT ON THE HDG OF 360 DEGS; WE RECEIVED 1 EGPWS 'TERRAIN; PULL UP.' THE CAPT PULLED.