Narrative:

We were cleared to climb to 10000 ft and to turn right to a 180 degree heading. The autoplt was engaged. We were climbing in the turn as I began to reset my altimeter per company procedure. ATC called traffic at 12 O'clock at 11000 ft. I stopped resetting the altimeter to spot the traffic. At 9000 ft (MSL) I engaged the vertical speed function of the autoplt at plus 1000 FPM. As the aircraft neared 10000 ft, the FMA annunciated altitude capture and altitude hold. I then returned my attention to resetting my altimeter. Just then, the captain called 'there's your altitude!' even though 'altitude hold' was annunciated on the FMA, the aircraft continued to climb through 10000 ft. I immediately pressed the altitude hold button hoping to stop the climb. The aircraft continued to climb, so I disconnected the autoplt. The aircraft climbed approximately 300 ft above our assigned altitude. At the first sign of autoplt malfunction, I should have disconnected the autoplt and manually brought the aircraft back to the desired flight path. Engaging the 'altitude hold' function a second time in attempting to 'fix' the problem wasted valuable time and caused a greater altitude deviation. Supplemental information from acn 262750: the FGS/autoplt approachs level-off altitudes (in climb and descent) at a much higher rate than we do when we fly by hand. When it doesn't capture and we recognize this there is an excellent chance for an altitude overshoot. Perhaps I could have taken over the aircraft and started the pushover, but I don't think I could have done it much more quickly than the first officer without risk to the flight attendants up in the back.

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Original NASA ASRS Text

Title: AN ACR MLG OVERSHOT ITS ALT ON CLBOUT.

Narrative: WE WERE CLRED TO CLB TO 10000 FT AND TO TURN R TO A 180 DEG HDG. THE AUTOPLT WAS ENGAGED. WE WERE CLBING IN THE TURN AS I BEGAN TO RESET MY ALTIMETER PER COMPANY PROC. ATC CALLED TFC AT 12 O'CLOCK AT 11000 FT. I STOPPED RESETTING THE ALTIMETER TO SPOT THE TFC. AT 9000 FT (MSL) I ENGAGED THE VERT SPD FUNCTION OF THE AUTOPLT AT PLUS 1000 FPM. AS THE ACFT NEARED 10000 FT, THE FMA ANNUNCIATED ALT CAPTURE AND ALT HOLD. I THEN RETURNED MY ATTN TO RESETTING MY ALTIMETER. JUST THEN, THE CAPT CALLED 'THERE'S YOUR ALT!' EVEN THOUGH 'ALT HOLD' WAS ANNUNCIATED ON THE FMA, THE ACFT CONTINUED TO CLB THROUGH 10000 FT. I IMMEDIATELY PRESSED THE ALT HOLD BUTTON HOPING TO STOP THE CLB. THE ACFT CONTINUED TO CLB, SO I DISCONNECTED THE AUTOPLT. THE ACFT CLBED APPROX 300 FT ABOVE OUR ASSIGNED ALT. AT THE FIRST SIGN OF AUTOPLT MALFUNCTION, I SHOULD HAVE DISCONNECTED THE AUTOPLT AND MANUALLY BROUGHT THE ACFT BACK TO THE DESIRED FLT PATH. ENGAGING THE 'ALT HOLD' FUNCTION A SECOND TIME IN ATTEMPTING TO 'FIX' THE PROB WASTED VALUABLE TIME AND CAUSED A GREATER ALTDEV. SUPPLEMENTAL INFO FROM ACN 262750: THE FGS/AUTOPLT APCHS LEVEL-OFF ALTS (IN CLB AND DSCNT) AT A MUCH HIGHER RATE THAN WE DO WHEN WE FLY BY HAND. WHEN IT DOESN'T CAPTURE AND WE RECOGNIZE THIS THERE IS AN EXCELLENT CHANCE FOR AN ALT OVERSHOOT. PERHAPS I COULD HAVE TAKEN OVER THE ACFT AND STARTED THE PUSHOVER, BUT I DON'T THINK I COULD HAVE DONE IT MUCH MORE QUICKLY THAN THE FO WITHOUT RISK TO THE FLT ATTENDANTS UP IN THE BACK.

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.