Narrative:

When we received our approach clearance, both pilots understood the controller to say '6000 ft until sannd.' first officer read back '6 to sannd.' I confirmed the correct (I thought) altitude of 6000 ft was set in the mode control panel and began a descent out of 8000 ft for 6000 ft. I noted the approach chart indicated 8000 ft until sand, but decided not to question it because 1) I knew that there was no terrain clearance problem at 6000 ft and 2) I was certain I had heard the controller say 6000 ft and 3) the frequency 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 controller asked us what our altitude was. When we told him, he indicated that we should have been at 8000 ft. He then told us to maintain 6000 ft and cleared us for the ILS runway 16R. Factors: 1) I believe the controller mis-spoke and said 6000 ft when he meant 8000 ft. 2) first officer read back '6 to sannd' (which was our approximately distance from sannd) instead of '6000 ft until sannd' which might have caught the controllers attention. 3) on my approachs, the altitude given by ATC to maintain until established is below the altitude for the appropriate approach segment or GS intercept altitude. This tends to make me complacent to this situation. I should have asked my first officer to confirm the altitude assignment since it conflicted with what I expected.

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Original NASA ASRS Text

Title: 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.

Data retrieved from NASA's ASRS site as of July 2007 and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.