Narrative:

Flight from las to ont was in a descent for an altitude restriction. We were flying on J60 from hec to pde in VMC conditions. ZLA gave us a clearance to cross dawna intersection at 12000'. We set 12000' in the altitude alert. Later center revised the clearance to cross dawna at 13000' and maintain 12000'. I set in 13000' into the altitude alert and read the clearance back. Unfortunately the aircraft radios were giving us problems and the captain interpreted the new crossing restriction as cross dawna at or below 13000' and maintain 12000'. At 14000' I called out 1000' to go. We were still in a very high rate of descent. At this point I observed an aircraft Y which I believed to be a VFR target. It was a high wing and was moving from 11:00 to 12:00 position. We were in VMC and the traffic had not been called out. Later we were informed that in fact it was an IFR traffic. I called out the traffic and my attention was now focused outside to watch the aircraft. When I saw we were descending through 13000', I called out maintain 13000' and the captain pulled up trying to stop the descent and return to 13000'. Unfortunately we descended down to 12600'. Supplemental information from acn 144461. On landing ontario ground control asked me to call ATC on the phone.

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Original NASA ASRS Text

Title: ACR MLG OVERSHOOTS ASSIGNED ALT IN DESCENT ON ZIGGY ONE STAR TO ONT, LOSS OF SEPARATION WITH IFR ACFT Y RESULTS.

Narrative: FLT FROM LAS TO ONT WAS IN A DSNT FOR AN ALT RESTRICTION. WE WERE FLYING ON J60 FROM HEC TO PDE IN VMC CONDITIONS. ZLA GAVE US A CLRNC TO CROSS DAWNA INTXN AT 12000'. WE SET 12000' IN THE ALT ALERT. LATER CTR REVISED THE CLRNC TO CROSS DAWNA AT 13000' AND MAINTAIN 12000'. I SET IN 13000' INTO THE ALT ALERT AND READ THE CLRNC BACK. UNFORTUNATELY THE ACFT RADIOS WERE GIVING US PROBLEMS AND THE CAPT INTERPRETED THE NEW XING RESTRICTION AS CROSS DAWNA AT OR BELOW 13000' AND MAINTAIN 12000'. AT 14000' I CALLED OUT 1000' TO GO. WE WERE STILL IN A VERY HIGH RATE OF DSNT. AT THIS POINT I OBSERVED AN ACFT Y WHICH I BELIEVED TO BE A VFR TARGET. IT WAS A HIGH WING AND WAS MOVING FROM 11:00 TO 12:00 POS. WE WERE IN VMC AND THE TFC HAD NOT BEEN CALLED OUT. LATER WE WERE INFORMED THAT IN FACT IT WAS AN IFR TFC. I CALLED OUT THE TFC AND MY ATTN WAS NOW FOCUSED OUTSIDE TO WATCH THE ACFT. WHEN I SAW WE WERE DSNDING THROUGH 13000', I CALLED OUT MAINTAIN 13000' AND THE CAPT PULLED UP TRYING TO STOP THE DSNT AND RETURN TO 13000'. UNFORTUNATELY WE DSNDED DOWN TO 12600'. SUPPLEMENTAL INFO FROM ACN 144461. ON LNDG ONTARIO GND CTL ASKED ME TO CALL ATC ON THE PHONE.

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.