Narrative:

During descent, ATC cleared us from 15000' to 11000'. The descent altitude was misunderstood to be 10000'. Both pilot and first officer understood cleared to 10000'. Approaching 10500 ATC asked us to maintain 11000'. We immediately initiated an expedited climb up to 11000'. Callback conversation with reporter revealed the following: reporter says that correct phraseology was used by controller. Controller said descend to 'one one thousand'. Flight crew read back 'one one thousand', but somehow had mindset of 'one zero thousand' and were descending to that altitude when controller got them stopped before any less than standard separation occurred. In response to analyst question the reporter says that there was another 'zero' involved in the clearance from controller and that was an airspeed reduction to 'two one zero'. Reporter says he has often noted a problem of pilot and controller misunderstanding when these two altitudes are involved.

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Original NASA ASRS Text

Title: FLT CREW DESCENDED TO WRONG ALT. SET UP POSSIBLE CONFLICT, BUT CORRECTED BY CTLR BEFORE LESS THAN STANDARD SEPARATION. PLT DEVIATION.

Narrative: DURING DSCNT, ATC CLRED US FROM 15000' TO 11000'. THE DSCNT ALT WAS MISUNDERSTOOD TO BE 10000'. BOTH PLT AND F/O UNDERSTOOD CLRED TO 10000'. APCHING 10500 ATC ASKED US TO MAINTAIN 11000'. WE IMMEDIATELY INITIATED AN EXPEDITED CLIMB UP TO 11000'. CALLBACK CONVERSATION WITH REPORTER REVEALED THE FOLLOWING: REPORTER SAYS THAT CORRECT PHRASEOLOGY WAS USED BY CTLR. CTLR SAID DSND TO 'ONE ONE THOUSAND'. FLT CREW READ BACK 'ONE ONE THOUSAND', BUT SOMEHOW HAD MINDSET OF 'ONE ZERO THOUSAND' AND WERE DSNDING TO THAT ALT WHEN CTLR GOT THEM STOPPED BEFORE ANY LESS THAN STANDARD SEPARATION OCCURRED. IN RESPONSE TO ANALYST QUESTION THE REPORTER SAYS THAT THERE WAS ANOTHER 'ZERO' INVOLVED IN THE CLRNC FROM CTLR AND THAT WAS AN AIRSPEED REDUCTION TO 'TWO ONE ZERO'. REPORTER SAYS HE HAS OFTEN NOTED A PROBLEM OF PLT AND CTLR MISUNDERSTANDING WHEN THESE TWO ALTS ARE INVOLVED.

Data retrieved from NASA's ASRS site as of August 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.