Narrative:

I was working mol int one man, at time of incident. I gave air carrier X the proper clearance to clear air carrier Y, I missed the altitude read back, I move to D side to sequence tickets, when I looked back air carrier X altitude 187. Air carrier Y FL190, 1 mi. To prevent recurrence I suggest no one man sectors, if I hadn't tried to do two jobs, I would have been able to prevent this incident. Supplemental information from acn 155100. We had been descending in IMC conditions from approximately FL260 until we broke to at about FL200. As the descent continued through FL190 I spotted a widebody transport approximately 2 mi ahead and right of our position. Center then inquired about our altitude, and we replied 'passing 18.7.' center then ordered us to maintain FL180 and advised us that the clearance we acknowledged was to 'cross 15 northwest of fak at FL210.' the first officer immediately responded that he had read back '15 northwest of fak at 11000'.' the controller then advised us that we had been involved in 'a system error' and to contact washington center via landline for additional information. We were then cleared to 17000' and switched to a new frequency. This episode is a classic example of poor communications procedures and complete reliance upon the infallibility of the system. For some inexplicable reason, this controller failed to detect our misunderstanding of his clearance, and we were remiss in accepting his lack of response as confirmation of our understanding. Failure to acknowledge 'read backs' is rather common occurrence and seemingly has become the accepted norm. This event could have been avoided by each party being aware of the content of the other's transmission; not issuing a modified crossing restriction when it was apparent that the original clearance altitude would be reached before the fix limit; and acknowledging each transmission in accordance with accepted rt procedures.

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Original NASA ASRS Text

Title: ACR X HAD LESS THAN STANDARD SEPARATION FROM ACR Y. SYSTEM ERROR. ACR X FLT CREW UNAUTH DESCENT THROUGH OCCUPIED ALT. PLTDEV.

Narrative: I WAS WORKING MOL INT ONE MAN, AT TIME OF INCIDENT. I GAVE ACR X THE PROPER CLRNC TO CLR ACR Y, I MISSED THE ALT READ BACK, I MOVE TO D SIDE TO SEQUENCE TICKETS, WHEN I LOOKED BACK ACR X ALT 187. ACR Y FL190, 1 MI. TO PREVENT RECURRENCE I SUGGEST NO ONE MAN SECTORS, IF I HADN'T TRIED TO DO TWO JOBS, I WOULD HAVE BEEN ABLE TO PREVENT THIS INCIDENT. SUPPLEMENTAL INFO FROM ACN 155100. WE HAD BEEN DSNDING IN IMC CONDITIONS FROM APPROX FL260 UNTIL WE BROKE TO AT ABOUT FL200. AS THE DSNT CONTINUED THROUGH FL190 I SPOTTED A WDB APPROX 2 MI AHEAD AND R OF OUR POS. CTR THEN INQUIRED ABOUT OUR ALT, AND WE REPLIED 'PASSING 18.7.' CTR THEN ORDERED US TO MAINTAIN FL180 AND ADVISED US THAT THE CLRNC WE ACKNOWLEDGED WAS TO 'CROSS 15 NW OF FAK AT FL210.' THE F/O IMMEDIATELY RESPONDED THAT HE HAD READ BACK '15 NW OF FAK AT 11000'.' THE CTLR THEN ADVISED US THAT WE HAD BEEN INVOLVED IN 'A SYS ERROR' AND TO CONTACT WASHINGTON CTR VIA LANDLINE FOR ADDITIONAL INFO. WE WERE THEN CLRED TO 17000' AND SWITCHED TO A NEW FREQ. THIS EPISODE IS A CLASSIC EXAMPLE OF POOR COMS PROCS AND COMPLETE RELIANCE UPON THE INFALLIBILITY OF THE SYS. FOR SOME INEXPLICABLE REASON, THIS CTLR FAILED TO DETECT OUR MISUNDERSTANDING OF HIS CLRNC, AND WE WERE REMISS IN ACCEPTING HIS LACK OF RESPONSE AS CONFIRMATION OF OUR UNDERSTANDING. FAILURE TO ACKNOWLEDGE 'READ BACKS' IS RATHER COMMON OCCURRENCE AND SEEMINGLY HAS BECOME THE ACCEPTED NORM. THIS EVENT COULD HAVE BEEN AVOIDED BY EACH PARTY BEING AWARE OF THE CONTENT OF THE OTHER'S XMISSION; NOT ISSUING A MODIFIED XING RESTRICTION WHEN IT WAS APPARENT THAT THE ORIGINAL CLRNC ALT WOULD BE REACHED BEFORE THE FIX LIMIT; AND ACKNOWLEDGING EACH XMISSION IN ACCORDANCE WITH ACCEPTED RT PROCS.

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.