AN SW3 FLT CREW DEVIATE FROM ASSIGNED ALT.

Narrative

ENRTE FROM FXE TO MLB ATC INSTRUCTED US TO DSND FROM FL230 TO FL200. THE AUTOPLT WAS INOP AND MY COPLT WAS PF. AFTER DSNDING AND RE-ESTABLISHED IN STRAIGHT AND LEVEL FLT; I DECIDED TO USE SOME TIME TO BECOME MORE FAMILIAR WITH THE GPS. THIS WAS ONLY MY THIRD FLT IN THIS ACFT AND IN THIS TYPE ACFT AND I WAS INEXPERIENCED WITH THE MODEL GPS ON BOARD. AFTER APPROX A MIN OR SO OF PLAYING WITH THE GPS; ATC CALLED AND SAID 'SAY ALT.' I OBSERVED THE VSI SHOWING A DSCNT AND THE ALTIMETER APCHING 19500 FT. THE PF MADE THE SAME OBSERVATION AND IMMEDIATELY BEGAN TO CORRECT. I THINK THIS DEV RESULTED FROM A COMBINATION OF INATTENTION; INEXPERIENCE; AND TO A LESSER EXTENT; FAULTY AUTOPLT AND ALT ALERT SYS. MY COPLT IS VERY INEXPERIENCED WITH ONLY 320 TOTAL FLT HRS. I TOOK THIS FACT INTO CONSIDERATION THROUGHOUT THE FLT BUT SINCE HE HAD ALREADY BEEN ON THE CTLS FOR ABOUT 1 HR AND THE ACFT WAS ESTABLISHED IN STRAIGHT AND LEVEL FLT AT 20000 FT; I SAW NO PARTICULAR REASON FOR CONCERN WHILE I FOCUSED MY ATTN ON THE GPS FOR A FEW MINS. I WAS UNAWARE OF MY COPLT'S PREOCCUPATION WITH MY ACTIVITIES AND THE BREAKDOWN IN HIS OWN SCAN UNTIL WE WERE QUERIED BY ATC. THAT IS BECAUSE THE GRADUAL DSCNT GAVE NO PHYSICAL SENSATION THAT THE ACFT WAS DSNDING; THERE WAS NO WARNING FROM THE ALT ALERTER; AND THE DEV ITSELF TRANSPIRED OVER THE COURSE OF A SHORT TIME. IN RETROSPECT; I WISH THE AUTOPLT HAD BEEN WORKING. I WISH MY COPLT HAD PAID MORE ATTN TO HIS PRIMARY RESPONSIBILITY OF FLYING THE PLANE INSTEAD OF WHAT I WAS DOING; AND I WISH I HAD PAID MORE ATTN TO MY PRIMARY RESPONSIBILITY OF FLYING THE COPLT. OPPORTUNITIES TO LEARN THE GPS ARE LIMITED ON THE GND DUE TO HEAT AND GND PWR REQUIREMENTS BUT I COULD HAVE DONE A BETTER JOB BY GIVING CLR CRM GUIDANCE PRIOR TO DIVERTING MY ATTN FROM OPERATING THE ACFT.