Narrative:

Frequency was congested and a slightly garbled and weak center transmitter. I thought we had been cleared to 10000'. Captain set 10000' in altitude alert window (he was handling radios) and I descended to 10000'. Informed after level-off that we had been cleared to 11000'. Controller then cleared us to 10000'. I should have asked for verification of assigned altitude because 11000' is normal going into las from the east. However, localizer was OTS, so I thought they were doing something different because the localizer was out. Supplemental information from acn 138356: I am embarrassed and befuddled how 2 competent adults can bust an altitude. I'm sure we were cleared to 11000' and I have no explanation for this altitude deviation. I am usually attentive and conscientious, but I evidently must not be enough. This was a shocking experience for both of us.

Google
 

Original NASA ASRS Text

Title: FLT CREW OF ACR MLG ARRIVING LAS MISTAKES CLRNC TO 11000' AS CLRNC TO 10000' AND DESCENDS TOO LOW.

Narrative: FREQ WAS CONGESTED AND A SLIGHTLY GARBLED AND WEAK CENTER XMITTER. I THOUGHT WE HAD BEEN CLRED TO 10000'. CAPT SET 10000' IN ALT ALERT WINDOW (HE WAS HANDLING RADIOS) AND I DSNDED TO 10000'. INFORMED AFTER LEVEL-OFF THAT WE HAD BEEN CLRED TO 11000'. CTLR THEN CLRED US TO 10000'. I SHOULD HAVE ASKED FOR VERIFICATION OF ASSIGNED ALT BECAUSE 11000' IS NORMAL GOING INTO LAS FROM THE E. HOWEVER, LOC WAS OTS, SO I THOUGHT THEY WERE DOING SOMETHING DIFFERENT BECAUSE THE LOC WAS OUT. SUPPLEMENTAL INFO FROM ACN 138356: I AM EMBARRASSED AND BEFUDDLED HOW 2 COMPETENT ADULTS CAN BUST AN ALT. I'M SURE WE WERE CLRED TO 11000' AND I HAVE NO EXPLANATION FOR THIS ALT DEVIATION. I AM USUALLY ATTENTIVE AND CONSCIENTIOUS, BUT I EVIDENTLY MUST NOT BE ENOUGH. THIS WAS A SHOCKING EXPERIENCE FOR BOTH OF US.

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.