A DC9-40 FLC TURNS TOO EARLY FOR VECTORING TO DTW. FO HAD USED THEIR GPS FOR THE POS WITHOUT REF TO THE VOR DME.

Narrative

WE WERE ESTABLISHED ON THE CETUS 2 ARR INTO DTW. TRACKING THE 147 DEG RADIAL INBOUND TO DXO VOR; APCH CTL INSTRUCTED US TO TURN TO A HDG OF 030 DEGS. UPON REACHING GLOZE INTXN (DXO 8 DME); THE FO WAS THE PF. AS I WAS HEADS DOWN REVIEWING THE ILS RWY 21L APCH PLATE; THE FO STARTED A R TURN TO A HDG OF 030 DEGS AS ASSIGNED BY ATC. AS HE TURNED; HE STATED 'PASSING GLOZE.' SINCE HE WAS ON THE ASSIGNED HDG; I DIALED IN THE LOC FREQ FOR RWY 21L. ABOUT THE SAME TIME APCH CALLED AND SAID; 'HEY; YOU MISSED THE TURN FOR GLOZE; TURN BACK TO A HDG OF 300 DEGS.' WE ACKNOWLEDGED HIM AND TURNED BACK IMMEDIATELY; AND ALSO APOLOGIZED FOR THE EARLY TURN. CONTRIBUTING FACTORS INCLUDED THE GPS AND THE CREW. THE FO MISINTERPED THE GPS. HE SAW THE GPS READING 8 MI FROM GLOZE; AND THOUGHT IT WAS READING 8 MI FROM DXO VOR. THINKING WE WERE AT GLOZE; HE TURNED TO THE 030 DEG ASSIGNED HDG. ACTUALLY HE TURNED 16 MI FROM DXO INSTEAD OF 8 MI. I DIDN'T NOTICE BECAUSE I WAS REVIEWING THE APCH PLATE AND HAD MY HEAD DOWN WHEN HE TURNED. BEFORE I PICKED UP ON WHAT HAPPENED; APCH CALLED AND GAVE US THE HDG BACK TO GLOZE. IN MY OPINION; THIS OCCURRED BECAUSE THE GPS ACTUALLY LOWERED OUR LEVEL OF SITUATIONAL AWARENESS. RELYING TOO MUCH ON THE GPS DURING THE ARR; WE FAILED TO CONFIRM OUR POS WITH THE VOR. HAD WE DONE THIS; I WOULDN'T BE WRITING THIS RPT.