Narrative:

Working ZAU, climbing to assigned altitude 21000', first officer flying aircraft using autoplt. Upon passing 20000' altitude alert went off. I gave required callout, '200 for 210.' first officer acknowledged. I turned my head away from INS to reach in flight bag to net norfolk approach pages. As I looked back to INS panel, saw altitude alert light on, altimeter reading 21400', with aircraft still slowly in climb. Told first officer to get aircraft back to assigned altitude. ZAU asked us our altitude. I answered, '215 correcting to 210.' controller informed us of another aircraft above us at FL220. ZAU controller handed us off to another frequency advising us next controller may have a phone # for me to call when I reached norfolk. Next controller did. Upon landing norfolk, called and explained the deviation our trip had executed to ZAU shift supervisor. He gave me phone # of ZAU quality assurance. Spoke with that individual 10/90. Was informed deviation was under investigation by NTSB for possible safety violation. Both first officer and I understand the gravity and serious nature of this action, and plan to exert greater attention so as to not repeat this deviation. I've suggested to first officer to manually level aircraft at assigned altitude, then engaged autoplt.

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Original NASA ASRS Text

Title: LGT HAS ALT DEVIATION AT LEVEL OFF CREATING LESS THAN STANDARD SEPARATION.

Narrative: WORKING ZAU, CLBING TO ASSIGNED ALT 21000', F/O FLYING ACFT USING AUTOPLT. UPON PASSING 20000' ALT ALERT WENT OFF. I GAVE REQUIRED CALLOUT, '200 FOR 210.' F/O ACKNOWLEDGED. I TURNED MY HEAD AWAY FROM INS TO REACH IN FLT BAG TO NET NORFOLK APCH PAGES. AS I LOOKED BACK TO INS PANEL, SAW ALT ALERT LIGHT ON, ALTIMETER READING 21400', WITH ACFT STILL SLOWLY IN CLB. TOLD F/O TO GET ACFT BACK TO ASSIGNED ALT. ZAU ASKED US OUR ALT. I ANSWERED, '215 CORRECTING TO 210.' CTLR INFORMED US OF ANOTHER ACFT ABOVE US AT FL220. ZAU CTLR HANDED US OFF TO ANOTHER FREQ ADVISING US NEXT CTLR MAY HAVE A PHONE # FOR ME TO CALL WHEN I REACHED NORFOLK. NEXT CTLR DID. UPON LNDG NORFOLK, CALLED AND EXPLAINED THE DEVIATION OUR TRIP HAD EXECUTED TO ZAU SHIFT SUPVR. HE GAVE ME PHONE # OF ZAU QUALITY ASSURANCE. SPOKE WITH THAT INDIVIDUAL 10/90. WAS INFORMED DEVIATION WAS UNDER INVESTIGATION BY NTSB FOR POSSIBLE SAFETY VIOLATION. BOTH F/O AND I UNDERSTAND THE GRAVITY AND SERIOUS NATURE OF THIS ACTION, AND PLAN TO EXERT GREATER ATTN SO AS TO NOT REPEAT THIS DEVIATION. I'VE SUGGESTED TO F/O TO MANUALLY LEVEL ACFT AT ASSIGNED ALT, THEN ENGAGED AUTOPLT.

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.