AN ALT EXCURSION PERFORMED BY THE PIC OF A B737 WHILE IN DSCNT IN PROX OF TSTM ACTIVITY 30 MI SE OF PHX; AZ.

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LAST LEG OF A 4 DAY TRIP. ARRIVING PHX AREA FROM SE. TRYING TO MAKE UP TIME. DSNDING AT 320 KIAS; WITH AUTOPLT/AUTOTHROTTLE ENGAGED. THERE WAS A LARGE TSTM ON STAR ROUTING; SO ZAB CLRED US HDG 300 DEGS TO FLY AROUND STORM ON THE NE SIDE. WINDS ALOFT WERE FROM THE SW. THEY THEN CLRED US TO CROSS 30 NM E OF PXR VORTAC AT 250 KTS AND 11000 FT. THE FO NEEDED TO TALK TO COMPANY ON #2 COM FOR WE COULD NOT GET A HOLD OF THEM EARLIER. I ASKED HIM TO REACH THE DSCNT CHKLIST PRIOR TO LEAVING ME ALONE WITH THE #1 COM RESPONSIBILITIES. I KNEW THINGS COULD SNOWBALL TO A VERY HIGH WORKLOAD FOR 1 PLT; SO I WAS PLANNING ACCORDINGLY. THE CTLR THEN TOLD ME TO CONTACT PHX APCH. AT THIS POINT; I BELIEVE FATIGUE BECAME A FACTOR BECAUSE I CANNOT CLRLY REMEMBER THE EXACT SEQUENCE OF EVENTS AND EVERYTHING SEEMED TO HAPPEN IN DOUBLE TIME. I NOTICED THAT WE WERE WARM ON ENERGY MGMNT BUT FELT I COULD TURN THE ACFT DIRECTLY TOWARD THE VORTAC AND STILL MAKE THE 250 KTS; 11000 FT RESTR; SO I DID THAT. ALSO; AS I ROLLED OUT; I FELT I NO LONGER NEEDED THE SPD BRAKES DEPLOYED; SO I STOWED THEM. THE ACFT WAS BEGINNING TO SLOW TOWARDS 250 KTS AND WAS AT APPROX 12500 FT STARTING TO PITCH UP. I THEN LOOKED DOWN AND CHANGED THE FREQ TO PHX APCH. WHEN I LOOKED UP; THE ACFT WAS RAISING ITS NOSE FASTER BUT APPEARED TO BE LEVELING AT 12000 FT INSTEAD OF 11000 FT. I PRESSED LIGHT FORWARD PRESSURE ON THE YOKE TO ASSIST THE AUTOPLT TO GO MORE FOR THE ALT. AT THIS POINT THE AUTOTHROTTLES STARTED TO ADVANCE THE THRUST LEVERS TO HOLD 250 KTS. WE WERE GETTING CLOSE TO THE 30 DME FIX; SO I SHUT OFF THE AUTOTHROTTLES AND AUTOPLT AND HAND FLEW THE ACFT DOWN 1000 FT MORE OF DSCNT. I CHKED IN WITH PHX APCH CTL; 'LEVEL AT 11000 FT WITH ATIS INFO.' PHX APCH IMMEDIATELY CLRED ME 'DSND 7000 FT; FLY HDG 240 DEGS; INTERCEPT RWY 25L CTRLINE.' I READ BACK THE CLRNC. AS I SET 7000 FT ON THE MCP ALT ALERTER; THE FO SAID HE WAS BACK ON THE FREQ. WHEN I LOOKED DOWN AT THE ALTIMETER WE WERE LEVEL AT 10000 FT. I WAS VERY PERPLEXED. I COULD NOT UNDERSTAND HOW WE HAD GOTTEN THERE. I ASKED THE FO TO VERIFY OUR LAST CLRNC WITH ZAB AS 11000 FT AND NOT 10000 FT. HIS FIRST RESPONSE WAS; 'YOU HAVE BEEN CLRED TO 7000 FT.' I DETERMINED THE BEST THING TO DO WAS FLY THE AIRPLANE NOW AND UNRAVEL WHAT HAPPENED LATER WHEN WE WERE ON THE GND. WHEN WE TALKED IT OVER AFTER THE FLT; WE REALIZED THAT 1) THE FO COULD NOT HELP ME DETERMINE WHAT HAPPENED OTHER THAN SAY THE LAST TIME HE LOOKED UP HE SAW ME LEVELING AT 11000 FT 'WITH THE AUTOPLT OFF;' AND 2) I HAD MOST LIKELY MISINTERPED THE AUTOPLT'S RESPONSE AND HAD DSNDED THE ACFT MANUALLY FROM 11000 FT TO 10000 FT; BUT NEITHER OF US COULD REMEMBER HEARING AN ALT WARNING HORN. IN RETROSPECT; I PROBABLY PUT TOO MUCH WORKLOAD ON A TIRED MIND. NEITHER CTLR EXPRESSED CONCERN; THE ALT WARNING HORN WAS NOT RECALLED SOUNDING BY EITHER PLT. IN THE FUTURE I WILL SLOW DOWN THE ACFT EARLIER AND ASK THE FO TO STAY WITH ME ON THE FREQ IF I THINK THINGS COULD SNOWBALL IN THE NEXT FEW MINS. IN SHORT; MAKE A MANAGEABLE WORKLOAD FOR THE RESOURCES ON HAND.