ALTDEV ALT EXCURSION ALT OVERSHOT IN A READJUSTED CLB MODE.

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WHILE IN CRUISE AT 16000 FT; CTR GAVE US DISCRETION TO 11000 FT. HE THEN GAVE US THE SUX ALTIMETER SETTING WHICH WAS 0.15 INCHES LOWER THAN THE CURRENT SETTING. RESETTING THE ALTIMETER PUT US 150 FT LOW. I ENGAGED THE CLB MODE ON THE FLT DIRECTOR TO BRING US BACK TO 16000 FT. THAT IS THE ALT I WAS GOING TO STAY AT FOR ANOTHER 35 MI AND THEN BEGIN MY DSCNT. I BELIEVED THAT THE ALT HAD CAPTURED AT 16000 FT AND NOW DIALED IN 11000 FT IN THE ALT ALERTER. AT THIS TIME I BEGAN MY COMPANY MANIFEST FOR THE RETURN TRIP; MY FO WAS GETTING SUX ATIS. AS I LOOKED UP FROM THE PAPERWORK; THE FLT DIRECTOR WAS STILL IN THE CLB MODE WITH NO ALT TO CAPTURE DUE TO 11000 FT SET IN. I DISENGAGED THE AUTOPLT AT 17500 FT AND BEGAN A RAPID DSCNT TOWARDS MY ASSIGNED ALT OF 11000 FT. CTR THEN HANDED US OFF TO SUX APCH AT 11000 FT. THE CTR MADE NO MENTION OF THE ALT DISCREPANCY AT THAT TIME. THIS INCIDENT SHOULD HAVE BEEN AVOIDED BY ME NOT DOING PAPERWORK WHILE THE OTHER CREW MEMBER HAD HIS HEAD DOWN ALSO DOING PAPERWORK. I FELT RUSHED AND OUT OF MY NORMAL ROUTINE DUE TO A 30 MIN LATE DEP OUT OF OUR ORIGINATING STATION. I WAS TRYING TO SAVE TIME; I NORMALLY DO THE MANIFEST ON THE GND. ANOTHER CONTRIBUTING FACTOR IS THE COMPANY POLICY OF RESERVE CAPT OR CAPT QUALIFIED FO. THE COMPANY HAS THESE CREW MEMBERS FLY BOTH L AND R SEAT IN A PARTICULAR ACFT. THIS WAS MY FIRST TRIP AS A CAPT IN 65 DAYS. I HAD FLOWN AS A FO THAT PREVIOUS AMOUNT OF TIME. THIS POLICY DOES LEAD TO CONFUSING ROLES IN THE COCKPIT. IT IS HARD TO CHANGE JOB ROLES ON A DAILY BASIS. THIS IS NOT TO SAY THAT I HAVE NOT ADAPTED TO THE ROLES IN THE PAST; BUT MY JUDGEMENT WAS NOT WHAT IT SHOULD HAVE BEEN ON THIS FLT. SUPPLEMENTAL INFO FROM ACN 309393: CAPT INITIATED CLB BACK TO 16000 FT ON NEW ALTIMETER SETTING (APPROX 150 FT TO CLB). AFTER OBSERVING THIS; I BEGAN TO COPY DEST ATIS AND DETERMINE LNDG DATA. AFTER A SHORT TIME I FELT THE AIRPLANE PITCHING VERTLY; LOOKED UP AND FOUND THE ACFT AT 17500 FT AND CLBING. AFTER EXCITEDLY NOTIFYING THE CAPT; WE BEGAN A RAPID DSCNT BACK TO 16000 FT AND THEN CONTINUED TO 11000 FT. THE PROB TURNED OUT TO BE THIS: CAPT HAD SET THE ALT SELECT TO 11000 FT FIRST; THEN USED CLB MODE TO INITIATE OUR CLB BACK TO 16000 FT. SINCE THE AUTOPLT NEVER CAME TO THE SELECTED ALT; IT JUST KEPT CLBING. NEITHER I NOR THE CAPT NOTICED THIS; AS WE WERE BOTH 'HEADS DOWN' IN THE COCKPIT. WITH SUCH A SHORT CLB; WE SHOULD HAVE BOTH BEEN MORE ATTENTIVE TO WHAT THE AUTOPLT WAS DOING.