B737-200 CREW DSNDED BELOW PUBLISHED STAR ALT ARRIVING AT CZYZ.

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WE WERE ENRTE FROM ORD TO YYZ AT FL290. WE RECEIVED CLRNC TO 11000 FT VIA THE PROFILE DSCNT RWY 23/24L. THIS CLRNC REQUIRED US TO CROSS YWT VOR BTWN 17000 FT AND 13000 FT. THE FO WAS VERY NEW TO THE AIRLINE WITH ABOUT 4 MONTHS EXPERIENCE. HE HAD BEEN A PLT IN THE NAVY FOR THE PREVIOUS 14 YRS. HE HAD FLOWN INTO TORONTO BEFORE. HE WAS THE PF. HE GAVE A VERY THOROUGH BRIEF ON HIS DSCNT PLAN INCLUDING HIS INTENTION TO CROSS YWT BTWN 17000 FT AND 13000 FT. PRIOR TO DSCNT; AN IRREGULAR FUEL SIT DEVELOPED. FUEL BEGAN QUICKLY XFERRING FROM THE #2 MAIN TANK BACK INTO THE CTR TANK. THIS IS AN IRREGULAR AND POTENTIALLY HAZARDOUS SIT FOR THE B737 AND DEMANDED MY IMMEDIATE ATTN. I LEFT THE COPLT IN CHARGE OF THE ACFT RTE AND FLYING; AS WELL AS THE ATC RADIO RESPONSIBILITIES. I LEFT THE FREQ TO CALL OUR MAINT PERSONNEL ON ANOTHER RADIO. THE COPLT LET ME KNOW HE WAS STARTING HIS DSCNT. WE AGAIN HAD ANOTHER BRIEF DISCUSSION ABOUT THE YWT XING AND HE SEEMED TO COMPLETELY UNDERSTAND THE MANEUVER. DURING MY RUNNING OF THE IRREGULAR CHKLIST; I NOTICED THE ACFT DSNDING BELOW 13000 FT PRIOR TO WATERLOO VOR. I INSTRUCTED THE FO TO RETURN TO 1300 FT FOR THE XING RESTR AT YWT. WHEN I ASKED HIM WHY HE DSNDED BELOW 13000 FT; HE SAID HE THOUGHT THAT ALL XING RESTRS WERE NOT APPLICABLE BECAUSE WE WERE ORIGINALLY CLRED TO 11000 FT. I FOUND THAT STRANGE BECAUSE WE WERE CLRED TO 11000 FT VIA THE PROFILE DSCNT WHEN HE DID HIS BRIEF. HE IS AN EXCELLENT PLT AND I ATTRIBUTE THIS TO A LIMITED EXPERIENCE BASE WITH CIVILIAN ENVIRONMENTS AND ESPECIALLY A HIGH LOAD ARPT LIKE YYZ. THE FACTORS THAT CAUSED THIS THAT WERE ALSO CONTRIBUTING WERE MY DIVERTED ATTN TO A FUEL PROB AND ALSO ON ARPT WITH RAPIDLY DETERIORATING WX. WE DISCUSSED THIS WITH THE TORONTO CTLR AND HE INDICATED IT CAUSED NO PROBS.