A FLC OF A DSNDING DHC8 OVERSHOOTS THEIR ASSIGNED ALT ON THE STAR PROC NE OF SEA; WA.

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OUR FLT WAS INBOUND TO SEATTLE; WA; FROM WENATCHEE; WA; ON THE EPHRATA 4 ARR. WE WERE GIVEN A DSCNT CLRNC TO 10000 FT; RESTR BY A XING ALT AT FLAAK INTXN OF 11000 FT. APPROX 10 MI OUTSIDE OF FLAAK I LEFT THE FO; WHO WAS THE PF; RESPONSIBLE FOR BOTH FLYING AND ATC COMS AS I CONTACTED OUR COMPANY MAINT CTL WITH NEWS OF AN INOP DEICE BOOT INDICATOR. COMPANY FREQ WAS CONGESTED SO IT TOOK ME A FEW MINS TO ESTABLISH CONTACT; THEN TO ADEQUATELY EXPLAIN WHAT THE EXACT PROB APPEARED TO BE. WHILE CONVERSING WITH THE MAINT CTLR AND EXPLAINING THE WAY IN WHICH I HAD WRITTEN UP THE FAULTY INDICATOR IN THE MAINT LOGBOOK; I FELT THE ACFT ATTITUDE CHANGE FROM A DSCNT PITCH ATTITUDE TO AN IMMEDIATE CLB PITCH ATTITUDE. I GLANCED AT OUR ALT AND SAW A CLB THROUGH 9400 FT. I THEN HEARD ATC INQUIRE ABOUT OUR ALT. I IMMEDIATELY WRAPPED UP MY DISCUSSION WITH OUR MAINT CTLR AND ASKED THE FO WHAT HAD HAPPENED. APPARENTLY HE HAD FAILED TO SELECT 'ALT SELECT' ON THE FLT DIRECTOR; WHICH THEN DIDN'T GIVE HIM A LEVELOFF COMMAND AT 10000 FT. HE ALSO FAILED TO NOTICE HE WAS DSNDING THROUGH 10000 FT UNTIL WE REACHED 9300 FT WHEREUPON HE IMMEDIATELY INITIATED A CLB BACK TO OUR ASSIGNED ALT. WITH THE TCASII SET TO 20 NM; THERE APPEARED TO BE NO IMMEDIATE CONFLICT WITH OTHER XPONDER EQUIPPED ACFT. FACTORS CONTRIBUTING TO THIS INCIDENT: 1) ALTHOUGH HIGHLY EXPERIENCED WITH OVER 10000 HRS; THE FO WAS FAIRLY NEW TO THE COMPANY; AND PROBABLY NOT QUITE COMFORTABLE YET WITH THE ACFT AND COMPANY PROCS REGARDING THE USE OF THE FLT DIRECTOR. 2) A PREVIOUS FLC HAD WRITTEN UP THE SAME MAINT PROB BUT THE MECHS HAD BEEN UNABLE TO DUPLICATE IT ON THE GND; SO THE WRITE-UP WAS CLRED WITHOUT FURTHER ACTION. WHEN THE SAME PROB OCCURRED ON OUR FLT; WE WERE ALREADY RUNNING OVER AN HR LATE. GIVEN THAT; I WAS MOTIVATED TO TALK DIRECTLY TO OUR MAINT CTLR REGARDING THE DETAILS OF THE FAILURE; IN ORDER TO EXPEDITE A RESOLUTION ONCE WE REACHED SEATTLE. IN ADDITION; COMPANY PROC ALSO DICTATED A CALL TO THE MAINT CTLR SO THAT DELAYS ON THE GND WERE MINIMIZED IN TERMS OF COORD WITHIN THE MAINT STRUCTURE. 3) I FAILED TO ADEQUATELY SUPERVISE THE FO WHILE I WAS TALKING WITH MAINT CTL. I WAS LULLED INTO COMPLACENCY BY THE FO'S LEVEL OF EXPERIENCE AND HIS PREVIOUS 2 DAYS OF OUTSTANDING PERFORMANCE. IN RETROSPECT; I SHOULD NOT HAVE 'LEFT THE ACFT' AS I DID TO CALL OUR MAINT FOLKS AT THAT POINT IN THE FLT. GIVEN THE FO'S RELATIVE INEXPERIENCE IN THE AIRPLANE AND WITH THE COMPANY; I SHOULD HAVE KEPT AN EXTREMELY WIDE EYE AND EAR OPEN INSIDE THE ACFT AS I WAS MAKING THE CALL TO COMPANY MAINT PERSONNEL; OR I SHOULD HAVE DELAYED THE CALL ALTOGETHER UNTIL WE LANDED. WE WERE EXTREMELY FORTUNATE IN THIS INSTANCE THAT THERE WERE NO CONFLICTS; BUT IT SURE GOT MY HEART RACING NONETHELESS.