Narrative:

While descending through approximately FL240, ATC told us traffic was crossing right to left at FL200. We had been cleared to FL210. We were told also to expect karla intersection at 11000 ft, 250 KTS. I read back this information, then I used #2 radio to get the ATIS. A few mins later, ATC asked us if we had been cleared to FL210. I looked at our altitude which was level at FL200. I then looked at the altitude set in the panel and saw FL200 set in. The last time I had pointed to it, it had FL210 set in it. I believe the captain inadvertently set in FL200 when told of the traffic at FL200 while I was on the #2 radio getting the ATIS. The traffic that had been pointed out to us was seen passing us while still descending through FL240, so it was no factor. Supplemental information from acn 400317: we were descending on the BYP2 STAR approximately 40 mi east of karla intersection. Crossing traffic was pointed out to us 1000 below, traveling from our right to left. We saw him while we continued our descent to FL200. I thought we had been cleared to FL200 but instead to only FL210. ATC was pointing out traffic at FL200, but I thought we had been cleared to FL200. No action was necessary since traffic had passed when ATC pointed out the wrong assigned altitude. The altitude change was not noted by the first officer because he was picking up the dfw ATIS at the time. Maybe if the traffic's altitude was not mentioned, I wouldn't have heard FL200 and read it back as a clearance, that might have helped. Instead of ATC telling us the altitude of the traffic, they could just say '1000 ft below.' anyway, fortunately no action was necessary and it was not a near miss.

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Original NASA ASRS Text

Title: FLC OF B737-400 MISHEARS CLRNC FOR DSCNT WHEN PASSING ACFT, ALT IS GIVEN AS WELL.

Narrative: WHILE DSNDING THROUGH APPROX FL240, ATC TOLD US TFC WAS XING R TO L AT FL200. WE HAD BEEN CLRED TO FL210. WE WERE TOLD ALSO TO EXPECT KARLA INTXN AT 11000 FT, 250 KTS. I READ BACK THIS INFO, THEN I USED #2 RADIO TO GET THE ATIS. A FEW MINS LATER, ATC ASKED US IF WE HAD BEEN CLRED TO FL210. I LOOKED AT OUR ALT WHICH WAS LEVEL AT FL200. I THEN LOOKED AT THE ALT SET IN THE PANEL AND SAW FL200 SET IN. THE LAST TIME I HAD POINTED TO IT, IT HAD FL210 SET IN IT. I BELIEVE THE CAPT INADVERTENTLY SET IN FL200 WHEN TOLD OF THE TFC AT FL200 WHILE I WAS ON THE #2 RADIO GETTING THE ATIS. THE TFC THAT HAD BEEN POINTED OUT TO US WAS SEEN PASSING US WHILE STILL DSNDING THROUGH FL240, SO IT WAS NO FACTOR. SUPPLEMENTAL INFO FROM ACN 400317: WE WERE DSNDING ON THE BYP2 STAR APPROX 40 MI E OF KARLA INTXN. XING TFC WAS POINTED OUT TO US 1000 BELOW, TRAVELING FROM OUR R TO L. WE SAW HIM WHILE WE CONTINUED OUR DSCNT TO FL200. I THOUGHT WE HAD BEEN CLRED TO FL200 BUT INSTEAD TO ONLY FL210. ATC WAS POINTING OUT TFC AT FL200, BUT I THOUGHT WE HAD BEEN CLRED TO FL200. NO ACTION WAS NECESSARY SINCE TFC HAD PASSED WHEN ATC POINTED OUT THE WRONG ASSIGNED ALT. THE ALT CHANGE WAS NOT NOTED BY THE FO BECAUSE HE WAS PICKING UP THE DFW ATIS AT THE TIME. MAYBE IF THE TFC'S ALT WAS NOT MENTIONED, I WOULDN'T HAVE HEARD FL200 AND READ IT BACK AS A CLRNC, THAT MIGHT HAVE HELPED. INSTEAD OF ATC TELLING US THE ALT OF THE TFC, THEY COULD JUST SAY '1000 FT BELOW.' ANYWAY, FORTUNATELY NO ACTION WAS NECESSARY AND IT WAS NOT A NEAR MISS.

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.