Narrative:

C500 assigned FL300. Aircraft read back clearance. Due to poor radio; heard FL300; pilot actually read back FL200. Aircraft told to stop descent when conflict alert activated. Supplemental information from acn 766796: I was on a vector during descent from FL330. I thought the controller had told me to descend and maintain FL200. My readback was that I would descend and maintain FL200. Upon reaching FL293; the controller asked me to confirm FL300. I leveled off at FL292 and responded that I had understood I was to descend and maintain FL200. The controller asked me to maintain present altitude and turn right 15 or 20 degrees (I don't recall which). After a couple mins; the controller asked me to maintain FL290; which was later followed by lower altitude in preparation for arrival into vgt airport. There was no indication of a traffic or airspace conflict other than the short vector for which I don't remember a reason being given. Although there was minimal noise on the intercom due to the cockpit mikes being selected to the 'open microphone;' I think the cause was simply my misunderstanding the new assigned altitude. A contributing factor was that the controller did not correct me when I read back that I was descending to FL200. To prevent this from occurring in the future; I plan to keep the intercom in the 'closed microphone' position unless flight crew conversation is necessary. This will improve clarity of all radio communications.

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Original NASA ASRS Text

Title: ZLA CTLR PROVIDING OJT EXPERIENCED OPERROR AT FL300 WHEN INCORRECT DESCENT ALT WAS MISSED; RESULTING IN CONFLICT.

Narrative: C500 ASSIGNED FL300. ACFT READ BACK CLRNC. DUE TO POOR RADIO; HEARD FL300; PLT ACTUALLY READ BACK FL200. ACFT TOLD TO STOP DSCNT WHEN CONFLICT ALERT ACTIVATED. SUPPLEMENTAL INFO FROM ACN 766796: I WAS ON A VECTOR DURING DSCNT FROM FL330. I THOUGHT THE CTLR HAD TOLD ME TO DSND AND MAINTAIN FL200. MY READBACK WAS THAT I WOULD DSND AND MAINTAIN FL200. UPON REACHING FL293; THE CTLR ASKED ME TO CONFIRM FL300. I LEVELED OFF AT FL292 AND RESPONDED THAT I HAD UNDERSTOOD I WAS TO DSND AND MAINTAIN FL200. THE CTLR ASKED ME TO MAINTAIN PRESENT ALT AND TURN R 15 OR 20 DEGS (I DON'T RECALL WHICH). AFTER A COUPLE MINS; THE CTLR ASKED ME TO MAINTAIN FL290; WHICH WAS LATER FOLLOWED BY LOWER ALT IN PREPARATION FOR ARR INTO VGT ARPT. THERE WAS NO INDICATION OF A TFC OR AIRSPACE CONFLICT OTHER THAN THE SHORT VECTOR FOR WHICH I DON'T REMEMBER A REASON BEING GIVEN. ALTHOUGH THERE WAS MINIMAL NOISE ON THE INTERCOM DUE TO THE COCKPIT MIKES BEING SELECTED TO THE 'OPEN MIKE;' I THINK THE CAUSE WAS SIMPLY MY MISUNDERSTANDING THE NEW ASSIGNED ALT. A CONTRIBUTING FACTOR WAS THAT THE CTLR DID NOT CORRECT ME WHEN I READ BACK THAT I WAS DSNDING TO FL200. TO PREVENT THIS FROM OCCURRING IN THE FUTURE; I PLAN TO KEEP THE INTERCOM IN THE 'CLOSED MIKE' POS UNLESS FLT CREW CONVERSATION IS NECESSARY. THIS WILL IMPROVE CLARITY OF ALL RADIO COMS.

Data retrieved from NASA's ASRS site as of January 2009 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.