ACFT DSNDS BELOW PUBLISHED ALT DURING APCH TO SLC. B737 (148 PT 43).

Narrative

WHEN WE RECEIVED OUR APCH CLRNC; BOTH PLTS UNDERSTOOD THE CTLR TO SAY '6000 FT UNTIL SANND.' FO READ BACK '6 TO SANND.' I CONFIRMED THE CORRECT (I THOUGHT) ALT OF 6000 FT WAS SET IN THE MODE CTL PANEL AND BEGAN A DSCNT OUT OF 8000 FT FOR 6000 FT. I NOTED THE APCH CHART INDICATED 8000 FT UNTIL SAND; BUT DECIDED NOT TO QUESTION IT BECAUSE 1) I KNEW THAT THERE WAS NO TERRAIN CLRNC PROB AT 6000 FT AND 2) I WAS CERTAIN I HAD HEARD THE CTLR SAY 6000 FT AND 3) THE FREQ WAS VERY BUSY AND I WASN'T SURE I COULD 'BREAK IN' FOR A QUESTION (BAD DECISION)! AS WE WERE LEVELING AT 6000 FT; THE CTLR ASKED US WHAT OUR ALT WAS. WHEN WE TOLD HIM; HE INDICATED THAT WE SHOULD HAVE BEEN AT 8000 FT. HE THEN TOLD US TO MAINTAIN 6000 FT AND CLRED US FOR THE ILS RWY 16R. FACTORS: 1) I BELIEVE THE CTLR MIS-SPOKE AND SAID 6000 FT WHEN HE MEANT 8000 FT. 2) FO READ BACK '6 TO SANND' (WHICH WAS OUR APPROX DISTANCE FROM SANND) INSTEAD OF '6000 FT UNTIL SANND' WHICH MIGHT HAVE CAUGHT THE CTLRS ATTN. 3) ON MY APCHS; THE ALT GIVEN BY ATC TO MAINTAIN UNTIL ESTABLISHED IS BELOW THE ALT FOR THE APPROPRIATE APCH SEGMENT OR GS INTERCEPT ALT. THIS TENDS TO MAKE ME COMPLACENT TO THIS SIT. I SHOULD HAVE ASKED MY FO TO CONFIRM THE ALT ASSIGNMENT SINCE IT CONFLICTED WITH WHAT I EXPECTED.