Narrative:

Air carrier X was en route to orl at FL410. He was cleared to FL330 by the controller. The pilot asked if that would be at his discretion. He did not want to start down. The controller then amended the clearance to FL390, rather than trying to get him down below traffic, which was at FL370. The controller was then relieved from the position by the next controller, myself. The first transmission I made was to clear air carrier X to FL370, after he had passed his traffic. He still had to be descended to FL270, which is the normal procedure, but had additional traffic at FL350. The pilot read back FL330. I did not catch the incorrect readback. Standard sep was lost when air carrier X descended below FL370 on top of the traffic at FL350. A possible contributing factor to the pilot reading back FL330 is that he had previously been cleared to FL330 before being stopped at FL390. When he got clearance to descend from me he was probably expecting FL330 again, and for whatever reason he heard what he was expecting and not what was actually stated. At the same time, I too was guilty of poor listening. Possibly contributions to my failure to catch the bad readback: 1) I had just sat down to a moderately busy sector after eating lunch, 2) at the time of the readback I was looking for the sign-on sheet so I could sign on the position--a definite distraction!, 3) I was suffering from a cold, and 4) I was working a day shift after having worked a swing shift the night before. Supplemental information from acn 143090: en route to mco at FL410 ZJX requested our descent to FL330. Passing approximately FL375 we were reclred to maintain FL370. About 3 mins later ZJX cleared us 'direct otk, descend and maintain FL330.' the copilot read back this clearance and I turned toward otk and started the descent. Passing through FL360, ZJX called and requested our altitude, stated 'maintain FL370.' at this time I saw air carrier Y crossing right to left at FL350. There was no time for evasive action. Our altitude at this time was 35500'. I started a climb back to FL370 and later called the quality assurance specialist and was informed of an operational error by ZJX. Our readback 'descend to FL350' was confirmed by tape.

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Original NASA ASRS Text

Title: ACR X HAD LESS THAN STANDARD SEPARATION FROM ACR Y. SYSTEM ERROR.

Narrative: ACR X WAS ENRTE TO ORL AT FL410. HE WAS CLRED TO FL330 BY THE CTLR. THE PLT ASKED IF THAT WOULD BE AT HIS DISCRETION. HE DID NOT WANT TO START DOWN. THE CTLR THEN AMENDED THE CLRNC TO FL390, RATHER THAN TRYING TO GET HIM DOWN BELOW TFC, WHICH WAS AT FL370. THE CTLR WAS THEN RELIEVED FROM THE POS BY THE NEXT CTLR, MYSELF. THE FIRST XMISSION I MADE WAS TO CLR ACR X TO FL370, AFTER HE HAD PASSED HIS TFC. HE STILL HAD TO BE DSNDED TO FL270, WHICH IS THE NORMAL PROC, BUT HAD ADDITIONAL TFC AT FL350. THE PLT READ BACK FL330. I DID NOT CATCH THE INCORRECT READBACK. STANDARD SEP WAS LOST WHEN ACR X DSNDED BELOW FL370 ON TOP OF THE TFC AT FL350. A POSSIBLE CONTRIBUTING FACTOR TO THE PLT READING BACK FL330 IS THAT HE HAD PREVIOUSLY BEEN CLRED TO FL330 BEFORE BEING STOPPED AT FL390. WHEN HE GOT CLRNC TO DSND FROM ME HE WAS PROBABLY EXPECTING FL330 AGAIN, AND FOR WHATEVER REASON HE HEARD WHAT HE WAS EXPECTING AND NOT WHAT WAS ACTUALLY STATED. AT THE SAME TIME, I TOO WAS GUILTY OF POOR LISTENING. POSSIBLY CONTRIBUTIONS TO MY FAILURE TO CATCH THE BAD READBACK: 1) I HAD JUST SAT DOWN TO A MODERATELY BUSY SECTOR AFTER EATING LUNCH, 2) AT THE TIME OF THE READBACK I WAS LOOKING FOR THE SIGN-ON SHEET SO I COULD SIGN ON THE POS--A DEFINITE DISTR!, 3) I WAS SUFFERING FROM A COLD, AND 4) I WAS WORKING A DAY SHIFT AFTER HAVING WORKED A SWING SHIFT THE NIGHT BEFORE. SUPPLEMENTAL INFO FROM ACN 143090: ENRTE TO MCO AT FL410 ZJX REQUESTED OUR DSNT TO FL330. PASSING APPROX FL375 WE WERE RECLRED TO MAINTAIN FL370. ABOUT 3 MINS LATER ZJX CLRED US 'DIRECT OTK, DSND AND MAINTAIN FL330.' THE COPLT READ BACK THIS CLRNC AND I TURNED TOWARD OTK AND STARTED THE DSNT. PASSING THROUGH FL360, ZJX CALLED AND REQUESTED OUR ALT, STATED 'MAINTAIN FL370.' AT THIS TIME I SAW ACR Y XING RIGHT TO LEFT AT FL350. THERE WAS NO TIME FOR EVASIVE ACTION. OUR ALT AT THIS TIME WAS 35500'. I STARTED A CLB BACK TO FL370 AND LATER CALLED THE QUALITY ASSURANCE SPECIALIST AND WAS INFORMED OF AN OPERROR BY ZJX. OUR READBACK 'DSND TO FL350' WAS CONFIRMED BY TAPE.

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.