EMB120 CREW OVERSHOOTS ASSIGNED ALT CLBING OUT OF PDX.

Narrative

MY CREW AND I WERE ENRTE FROM PDX TO OUR ALTERNATE OF YKM. WHILE ON THE CLBOUT WE HAD AN ALT INCURSION. THE FO WAS THE PF AND I WAS TALKING TO COMPANY DISPATCH. WE WERE ASSIGNED 15000 FT FROM PDX DEP AND BOTH PLTS ACKNOWLEDGED THE ALT IN THE ALT ALERTER. THE DISPATCHER WAS READING ME THE CURRENT WX AND FORECAST FOR YKM AND WE WERE COORDINATING OUR INTENTIONS. WHILE I WAS DISCUSSING THE SIT WITH DISPATCH WE WERE TOLD TO CONTACT ZSE. THE FO CHKED IN WITH CTR AND RPTED 14000 FT FOR 15000 FT. I HEARD THE ALT ALERT SOUND AND NOTED WE WERE WITHIN 400 FT OF 15000 FT. I CONTINUED TO WRITE DOWN THE WX AND FORECAST FOR YKM. JUST AS I FINISHED; I HEARD THE ALT ALERT SOUND AGAIN. I NOTED THAT WE WERE NOW CLBING TO 16000 FT AND THE FO WAS LEVELING OFF. AFTER LOOKING AGAIN AT THE ALT ALERT BOX AND SEEING 15000 FT IN THE BOX; I ASKED IF WE SHOULD BE AT 15000 FT OR 16000 FT. THE FO REPLIED THAT WE SHOULD BE AT 15000 FT. I WAS ABOUT TO CALL CTR WHEN THEY CALLED US AND ASKED US TO 'SAY ALT.' I REPLIED THAT WE WERE AT 16000 FT. THE CTLR THEN ASKED IF WE WANTED 15000 FT OR 17000 FT FOR A FINAL ALT. I ASKED FOR 17000 FT AND CONTINUED WITHOUT INCIDENT TO YKM. ONCE WE WERE AT 17000 FT; THE FO TOLD ME THAT HE HAD SELECTED THE AUTOPLT; BUT DIDN'T REMEMBER IF THE ALT ARM FUNCTION WAS SELECTED. I DON'T RECALL IF IT WAS SELECTED EITHER. I FEEL THAT THE INCIDENT WAS CAUSED BY IMPROPER USE OF THE AUTOPLT; AND FAILURE TO REACT WHEN THE ALT WAS NOT CAPTURED.