ALTDEV ALT EXCURSION.

Narrative

WE WERE ON THE FOSSL 3 ARR TO PHX; ABOUT 25 NM E OF FOSSL INTXN. EARLIER WE HAD DSNDED FROM FL350 TO FL310. WE WERE THEN CLRED TO CROSS TONTO INTXN AT 12000 FT. I ENTERED THE XING RESTRICTION IN THE FMCS. THE CAPT THEN WENT OFF FREQ TO GET THE ATIS. WE WERE THEN CLRED DIRECT MAZAT INTXN; AND ADHERE TO THE XING RESTRICTION. I ENTERED THIS IN THE FMCS AND WAS TRYING TO FIGURE WHEN TO DSND TO MEET THE RESTRICTION WHEN THE CAPT THEN SAID 'WHERE ARE WE GOING.' I STARTED TO TELL HIM WHEN HE SAID 'WATCH YOUR ALT.' I GLANCED AT MY ALTIMETER TO CONFIRM LEVEL AT FL310; THE CAPT SAID 'YOU'RE AT FL320' AND PUSHED THE NOSE DOWN AND STARTED A DSCNT. I LOOKED AGAIN AND NOTED WE WERE INDEED AT FL319. SINCE WE WERE JUST UNDER FL320; THE NUMBERS IN THE WINDOW WERE '31' AND THE POINTER WAS APPROX VERT. I HAD MISREAD THE ALTIMETER. WE HAD INDEED BEEN LEVEL AT FL310 PRIOR TO THIS; AS CONFIRMED BY BOTH PLTS. THE AUTOPLT WAS SET IN ALT HOLD; BUT FOR REASONS UNKNOWN TO ME HAD STARTED A SLOW CLB. I WAS SO ENGROSSED IN THE FMCS ENTRIES THAT I HAD NOT NOTICED IT. THE ALT ALERTER HAD NOT GONE OFF SINCE IT WAS ALREADY SET AT 12000 FT FOR THE DSCNT. THE SOLUTION IS NOT TO GET SO ENGROSSED IN THE FMCS THAT YOU FORGET TO SCAN. ALSO; HAVING BOTH PLTS IN THE LOOP WHEN PROGRAMMING THE FMCS WOULD HELP. FINALLY; AUTOMATION THAT IS MORE RELIABLE WOULD HELP.