FLT CREW OF LGT MISTAKENLY SET 12000 FT VICE 16000 FT IN ALT ALERT WINDOW WHILE ON CHINS ARR TO SEA FOR A S LNDG. DSNDED THROUGH ALT RESTR AT RADDY INTXN.

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ON THE CHINS 5 ARR INTO SEATTLE AT FL200; WE WERE 'CLRED TO DSND VIA THE CHINS 5 ARR.' I THEN SET 12000 FT INTO THE ALT WINDOW FOR THE VNAV DSCNT. NOTING THAT THE ACFT WAS NOT RESPONDING ADEQUATELY AT THE TOP OF DSCNT POINT (REACTING SLOWLY) I SELECTED FLT LEVEL CHANGE TO BEGIN THE DSCNT THEREBY DROPPING OUT OF VNAV. I DID THIS BELIEVING WE WOULD NOT MAKE THE XING RESTR OF 16000 FT; 280 KTS AT THE RADDY INTXN. WE BEGAN THE DSCNT CHKLIST AT THIS TIME. SEATTLE WAS LNDG S AT THIS TIME. DURING DSCNT; I NOTED THE 12000 FT RESTR NOTED AT RADDY INTXN FOR ACFT LNDG TO THE N. THIS IS WHERE THE PROB OCCURRED. HAD I STAYED IN VNAV; THE ACFT WOULD HAVE LEVELED OFF AT 16000 FT SINCE; PRIOR TO THE ARR; WE CONFIRMED ALL XING RESTR WERE CORRECTLY SET IN THE FMS. BEING IN FLT LEVEL CHANGE; HOWEVER; ALLOWED TO DSND TO 12000 FT WHICH WAS THE ALT SET IN THE ALT WINDOW. THE CONFUSION AROSE WHEN I MISTAKENLY NOTED 12000 FT AS A XING RESTR FOR ACFT LNDG N RATHER THAN 16000 FT FOR ACFT LNDG S. AFTER ADVISING US OF THE DEV; ZSE CLRED US TO 12000 FT. THERE WERE NO TFC CONFLICTS OR TERRAIN CONFLICTS DURING THIS DEV. TO HELP AVOID THIS PROB; IT WOULD BE HELPFUL IF XING RESTRS FOR THE SAME INTXN FOR N OR S OPS WERE NOTED IN 2 DIFFERENT BOXES ON THE ARR CHART; NOT IN THE SAME BOX. ALSO; IF 'LNDG N' AND 'LNDG S' WERE NOTED IN BOLD FACE; THIS WOULD BE HELPFUL. OTHER FACTORS CONTRIBUTING WOULD BE DISTR (IE; CHKLISTS) TURB; AND PLT FATIGUE (LONG DUTY DAY; LATE AT NIGHT BODY TIME). BETTER CHART NOTATIONS AND BETTER SITUATIONAL AWARENESS WOULD HELP PREVENT THIS FROM RECURRING.