ACR FLC DEV FROM ALT AFTER LEVEL OFF.

Narrative

CAPT WAS FLYING ACFT. A FAILURE OCCURRED TO MAINTAIN ASSIGNED ALT OF 10000 FT MSL. I HAD MADE APPROPRIATE CALLOUTS ON APCH TO ALT. IT APPEARS THAT THE VERT SPD COMMAND WHEEL ON AUTOPLT DID NOT FALL INTO DETENT OR WAS NUDGED OUT TO ALLOW A SLOW RATE CLB. WHILE ATTENDING TO OTHER ESSENTIAL DUTIES; I VERBALLY NOTED ALT AT 10200 FT AND CLBING. CORRECTIVE MEASURES WERE STARTED; HOWEVER; 10400 FT WAS TOP OF CLB. A TCASII TA SHORTLY FOLLOWED BY ATC INQUIRY OCCURRED. SUPPLEMENTAL INFO FROM ACN 316640: I NORMALLY USE HDG SELECT MODE BUT HDG SELECT ON THIS ACFT CAUSED ACFT TO FLY 'WING DOWN.' AS ACFT WAS LEVELING AT 10000 FT ATC REQUESTED A TURN TO 170 DEGS AND ADVISED OF TFC AT 1-2 O'CLOCK. THE VERT SPD WHEEL WAS ROTATED TO THE DETENT AND THE CREW LOOKED FOR THE TFC. ACFT CONTINUED TO CLB UNTIL THE ON BOARD ALT ALERT ACTIVATED. PF SHOULD VERIFY THAT THE DETENT IS ENGAGED AND THAT THE ACFT ACTUALLY LEVELS OFF BEFORE SHIFTING ATTN OUTSIDE OF COCKPIT. USE OF TURN KNOB AND THOUGHTS ABOUT 10000 FT ANNOUNCEMENT COULD ALSO HAVE BEEN A FACTOR.