Narrative:

We were being vectored and on the downwind leg 4000 ft AGL. The slc tower gave us a vector to base with a 7 mi final. I thought we had been cleared for a visual approach and started to descend. My first officer told me we did not have clearance to descend (there was no conflicting traffic). I descended 400 ft below our clearance altitude when slc cleared us for the visual.

Google
 

Original NASA ASRS Text

Title: ACR LGT ON DOWNWIND VECTOR AT 8000 FT FOR VISUAL TO ARPT. CAPT, PF, THOUGHT CLRED FOR THE VISUAL AND BEGAN DSCNT. FO ADVISED NOT CLRED YET AND AT THAT TIME TWR CLRED ACFT FOR THE VISUAL.

Narrative: WE WERE BEING VECTORED AND ON THE DOWNWIND LEG 4000 FT AGL. THE SLC TWR GAVE US A VECTOR TO BASE WITH A 7 MI FINAL. I THOUGHT WE HAD BEEN CLRED FOR A VISUAL APCH AND STARTED TO DSND. MY FO TOLD ME WE DID NOT HAVE CLRNC TO DSND (THERE WAS NO CONFLICTING TFC). I DSNDED 400 FT BELOW OUR CLRNC ALT WHEN SLC CLRED US FOR THE VISUAL.

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.