THE FO OF A DH8 FORGOT TO ARM THE ALT CAPTURE ON THE AUTOPLT AND MISSED THE ASSIGNED ALT. WHEN THE ACFT WAS 300 FT LOW; THE APCH CTLR INTERVENED. THIS RPTR CITES COMPANY SCHEDULING PRACTICES; LACK OF ALT CALLOUTS; AND NONESSENTIAL CONVERSATIONS DURING STERILE COCKPIT.

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NORFOLK APCH ADVISED US TO DSND FROM 5000 FT TO 3000 FT. USING THE AUTOPLT; I STARTED MY DSCNT WITH USE OF VERT SPD AND HDG SELECTOR. I FORGOT TO ENGAGE THE ALT SELECT BUTTON. THE CAPT DID NOT CALL MY 1000 FT TO GO. I DSNDED THROUGH 3000 FT TO 2700 FT. ATC ADVISED US OF TFC AT OUR 1 O'CLOCK 5 MI; AND ASKED OUR ALT. CAPT SAID WE WERE AT 2700 FT CLBING BACK TO 3000 FT. THIS SIT COULD HAVE BEEN AVOIDED BY MY ATTN DURING THE DSCNT; STANDARD CALLOUTS; AND LACK OF CONVERSATION BELOW 10000 FT. ANOTHER FACTOR WAS THIS WAS MY SIXTH DAY STRAIGHT OF DUTY. I WAS SUPPOSED TO HAVE 3 DAYS OFF AFTER A 5 DAYS ON SCHEDULE. SCHEDULING CALLED ME DURING MY TRIP AND JUNIOR MANNED ME FOR MY SIXTH DAY ON. THIS TRIP ENDED UP BEING 7 HRS 16 MINS OF FLYING AND OVER 12 HRS OF DUTY. I WOULDN'T MIND A SHORT TRIP WITH A COUPLE OF LEGS; BUT MY LEVEL OF FLYING AND NONFLYING WITH SUCH A LONG TRIP AFTER A LONG WK IS NOT SAFE. THE COMPANY HAS TO FILL THESE TRIPS DUE TO LACK OF PLTS; BUT THEY DON'T LOOK AT THE BIG PICTURE AS TO FATIGUE AND SAFETY.