Narrative:

We were descending with multiple leveloffs into the los angeles area. At 12000 ft we were given a descent to what I recall as 10000 ft and read back as the same. ATC called out 2 aircraft restr below us at 10 O'clock position and 12 O'clock position and told us to contact socal. Looking for the traffic, changing the frequency, and noticing my captain was leveling off at 11000 ft, I said we were assigned 10000 ft (which I had also previously set in the altitude alerter). He asked me if that was what I read back, I affirmed this and at approximately 10500 ft we saw an air carrier MD80 passing below and to our right. Checking in with the next controller at 10000 ft, he asked us to confirm our assigned altitude. We said 10000 ft and he told us to expedite down to 9000 ft. Shortly thereafter we saw the second target about 1500 ft above crossing left to right and entering a climbing turn towards us. After ATC gave us further descent and a frequency change, he commented, 'thanks for expediting the descent.' when we returned to our base, we had a call from ATC which informed us we were assigned 11000 ft according to the tapes. We told the controller we knew something was wrong when we saw the MD80 pass close below to our right. In retrospect, I should have confirmed with ATC that we were cleared to 10000 ft when the captain leveled off at 11000 ft. Also, as I have read in training manuals on CRM regarding altitude assignments, it is a good idea for the PNF to keep his finger on the altitude alerter until the PF confirms he heard the same, but most of all, if and when you make a mistake, having those eyes outside is what's going to save you and many others.

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Original NASA ASRS Text

Title: AIR TAXI PLT CONVINCES HIS CAPT TO DSND BELOW ASSIGNED ALT AFTER HE SET WRONG ALT IN ALERTER WINDOW. CAUSED TFC CONFLICT THAT WAS RESOLVED BY CTLR.

Narrative: WE WERE DSNDING WITH MULTIPLE LEVELOFFS INTO THE LOS ANGELES AREA. AT 12000 FT WE WERE GIVEN A DSCNT TO WHAT I RECALL AS 10000 FT AND READ BACK AS THE SAME. ATC CALLED OUT 2 ACFT RESTR BELOW US AT 10 O'CLOCK POS AND 12 O'CLOCK POS AND TOLD US TO CONTACT SOCAL. LOOKING FOR THE TFC, CHANGING THE FREQ, AND NOTICING MY CAPT WAS LEVELING OFF AT 11000 FT, I SAID WE WERE ASSIGNED 10000 FT (WHICH I HAD ALSO PREVIOUSLY SET IN THE ALT ALERTER). HE ASKED ME IF THAT WAS WHAT I READ BACK, I AFFIRMED THIS AND AT APPROX 10500 FT WE SAW AN ACR MD80 PASSING BELOW AND TO OUR R. CHKING IN WITH THE NEXT CTLR AT 10000 FT, HE ASKED US TO CONFIRM OUR ASSIGNED ALT. WE SAID 10000 FT AND HE TOLD US TO EXPEDITE DOWN TO 9000 FT. SHORTLY THEREAFTER WE SAW THE SECOND TARGET ABOUT 1500 FT ABOVE XING L TO R AND ENTERING A CLBING TURN TOWARDS US. AFTER ATC GAVE US FURTHER DSCNT AND A FREQ CHANGE, HE COMMENTED, 'THANKS FOR EXPEDITING THE DSCNT.' WHEN WE RETURNED TO OUR BASE, WE HAD A CALL FROM ATC WHICH INFORMED US WE WERE ASSIGNED 11000 FT ACCORDING TO THE TAPES. WE TOLD THE CTLR WE KNEW SOMETHING WAS WRONG WHEN WE SAW THE MD80 PASS CLOSE BELOW TO OUR R. IN RETROSPECT, I SHOULD HAVE CONFIRMED WITH ATC THAT WE WERE CLRED TO 10000 FT WHEN THE CAPT LEVELED OFF AT 11000 FT. ALSO, AS I HAVE READ IN TRAINING MANUALS ON CRM REGARDING ALT ASSIGNMENTS, IT IS A GOOD IDEA FOR THE PNF TO KEEP HIS FINGER ON THE ALT ALERTER UNTIL THE PF CONFIRMS HE HEARD THE SAME, BUT MOST OF ALL, IF AND WHEN YOU MAKE A MISTAKE, HAVING THOSE EYES OUTSIDE IS WHAT'S GOING TO SAVE YOU AND MANY OTHERS.

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.