A B737 CAPT BECAME DISORIENTED DURING THE TRANSITION BTWN A HUD APCH TO A VISUAL APCH. THE CAPT LINED UP WITH AN OCCUPIED TXWY RESULTING IN A GAR.

Narrative

WE HAD SET UP FOR AN APCH TO RWY 12R STL. WINDS WERE 150 DEGS AT 13 KTS GUSTS TO 23 KTS. WE BRIEFED A PRACTICE HGS (HEADS UP GUIDANCE SYS); NOTING THAT WE MIGHT NOT BE ABLE TO ENGAGE AIII MODE DUE TO THE XWIND. EVERYTHING WAS NORMAL DOWN TO THE '500 FT AIII MODE CALL.' BY THE 400 FT CALL; WE REALIZED THAT THE GLARE ON THE RWY WAS AN ACFT TAXIING IN OUR DIRECTION; BUT ABOUT TO TURN OFF. WHEN HE DID; THERE WAS ANOTHER ACFT BEHIND HIM. AT THAT POINT; WE BEGAN A GAR. AT THE SAME TIME; WE GOT A CALL FROM TWR TO GO AROUND. WE STAYED IN THE PATTERN FOR A L DOWNWIND LNDG TO RWY 12L. ON TAXI IN; WE WERE GIVEN THE TWR SUPVR'S NUMBER TO CALL AT THE GATE. WHEN I TALKED TO THE TWR SUPVR; I WAS SURPRISED TO HEAR THAT THE GAR WAS GIVEN BECAUSE IT APPEARED TO THEM THAT I WAS LINED UP ON TXWY A. THE WHOLE APCH FLASHED BEFORE MY EYES. HOW COULD THAT BE? WE HAD THE ILS FREQS TUNED; I WAS ON THE HUD. WE HAD IT ALL COVERED. THEN DOUBT STARTED SINKING IN. WHAT WERE THOSE ACFT DOING ON THE 'RWY?' AS I WRITE THIS; I STILL DON'T KNOW FOR SURE WHAT I WAS LINED UP WITH. I FOLLOWED THE HUD CUES DOWN TO ABOUT 300 FT WHERE I DECIDED TO STOW THE HUD AND CONCENTRATE ON THE XWIND LNDG AHEAD. IF I WAS LINED UP ON TXWY A; IT MUST HAVE HAPPENED ON THE SWITCH FROM THE HUD TO ALL VISUAL. CONCENTRATING ON THE ACFT ON THE 'RWY;' THE CRAB ANGLE AND THE RISING SUN REFLECTING OFF THE SNOW WERE ALL FACTORS THAT CONTRIBUTED TO THE POTENTIAL ERROR.