Narrative:

Aircraft 2 was level at FL210. Aircraft 1 was below aircraft 2 and was climbed to FL200. They were both heading in the same direction and aircraft 2 was overtaking aircraft 1. Aircraft 1 read back FL200 and started his climb. Aircraft 1 climbed through his assigned altitude and separation was lost. Evasive action was taken by both aircraft.

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Original NASA ASRS Text

Title: ZDC CTLR DESCRIBED PLTDEV NEAR FL200; WHEN CLIMBING ACFT FAILED TO STOP AT ASSIGNED ALT AND CONFLICTED WITH TFC LEVEL AT FL210.

Narrative: ACFT 2 WAS LEVEL AT FL210. ACFT 1 WAS BELOW ACFT 2 AND WAS CLBED TO FL200. THEY WERE BOTH HEADING IN THE SAME DIRECTION AND ACFT 2 WAS OVERTAKING ACFT 1. ACFT 1 READ BACK FL200 AND STARTED HIS CLB. ACFT 1 CLBED THROUGH HIS ASSIGNED ALT AND SEPARATION WAS LOST. EVASIVE ACTION WAS TAKEN BY BOTH ACFT.

Data retrieved from NASA's ASRS site as of May 2009 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.