AN IFR PA-32 PILOT WAS UNEXPECTEDLY ASSIGNED A STAR TO HIS DESTINATION. HE SET THE ANTICIPATED LOWER ALTITUDE AND WITH THE AUTOPILOT ENGAGED THE AIRCRAFT DESCENDED. AFTER DESCENDING 1;000' ATC CALLED HIM TO RETURN TO HIS ASSIGNED ALTITUDE.

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I WAS FLYING BACK FROM ZZZ TO ZZZ1 ON AN IFR FLT PLAN. I WAS IN RADIO CONTACT WITH CTR. I HAD BEEN PREVIOUSLY CLRED DIRECT TO ZZZ1 MORE THAN 200 MI OUT FROM ZZZ1 FROM A PREVIOUS CTLR. I HAD BEEN FLYING AT 13000 FT AND WAS MONITORING MYSELF WITH A PULSE OXIMETER DURING THE FLT WITH NO HYPOXIA. I RECEIVED A CLRNC 'CLRED PRESENT POS DIRECT ABC VOR; THEN ABC TWO ARR.' I WAS SURPRISED ABOUT THE STAR PROC BECAUSE ON A NUMBER OF FLTS TO ZZZ1 I HAD NOT RECEIVED SUCH A CLRNC. I IMMEDIATELY OPENED THE APCH PLATES BOOKLET AND FOUND THE ABC TWO ARR. I WAS FLYING A NEW PIPER 6X WITH THE AVIDYNE GLASS COCKPIT. I WAS UNDER SOME PRESSURE BECAUSE I WAS INPUTTING THE ABC INTXN INTO THE GARMIN 430 TO FLY DIRECT TO THAT INTXN. AND; AT THE SAME TIME; I WAS TRYING TO REVIEW THE ABC TWO ARR. IN MY HASTE I WAS NOT RECALLING HOW TO ENTER THE ARR STAR USING THE GARMIN 430; MAINLY BECAUSE I HAD NOT FLOWN ANY STAR PROCS IN A FEW YRS DESPITE FLYING ON IFR FLT PLANS ROUTINELY. AFTER TRYING A FEW OPTIONS WITHOUT LUCK; I DID REMEMBER THE PROC AND PROGRAMMED THE STAR. I ALSO PUT IN THE ALT 4000 FT IN THE AUTOPLT ANTICIPATING LOWER OVER THE RTE. I MISUNDERSTOOD THE ARR PROC AND ENGAGED THE AUTOPLT TO DSND WITHOUT A CLRNC. THE CTLR CALLED THE ALTDEV AFTER I HAD DSNDED ABOUT 1000 FT BELOW THE ASSIGNED ALT. I IMMEDIATELY CLBED BACK TO ALT. IN LOOKING BACK; A COUPLE OF KEY INSIGHTS: FIRST; I HAVE TRIED TO STAY PROFICIENT WITH IFR BUT OBVIOUSLY I MISSED THE MARK. I ACTUALLY UNDERWENT AN INST PROFICIENCY CHK WITH A CFI ONLY 30 HRS PRIOR TO THIS FLT. BUT WORKING OUT OF A NON-CTLED FIELD; WE DID NOT HAVE THE EASE OF GETTING A STAR; ALTHOUGH WE COULD HAVE STUDIED THEM ON THE GND DURING THE GND REVIEW TIME. THE STAR ARR; LIKE ANY PROC; NEEDS TO BE REVIEWED. I WAS LUCKY THE CTLR WAS ALERT AND CALLED THE DEV. SECOND; AND THIS IS RUDIMENTARY -- CHK WITH THE CTLR BEFORE ANY ALT CHANGE. I KNOW I HAD A QUESTION AND DID NOT FOLLOW THE HUNCH PRIOR TO ACTIVATING THE AUTOPLT. THE AIM GIVES PARTICULAR HIGHLIGHT TO FACT THAT THE PLT WILL MAINTAIN LAST ASSIGNED ALT UNTIL RECEIVING AUTH TO DSND WITH THE WORDS 'DSND VIA' SO AS TO COMPLY WITH PUBLISHED RESTRS. THIRDLY; ALTHOUGH I WAS FLYING AT 13000 FT USING A PULSE OXIMETER TO MONITOR HYPOXIA; I SHOULD HAVE ASKED FOR LOWER ENRTE EVEN THOUGH THIS WOULD HAVE PUT ME IN THE CLOUDS AND POSSIBLE MILD TURB; A SIGMET WAS OUT FOR TURB ACROSS 13 STATES THAT MORNING STRETCHING FROM NEW MEXICO TO AS FAR E AS TENNESSEE; ALABAMA AND THIS WAS ON MY MIND. I THOUGHT BEING ON TOP PREFERABLE TO BEING DOWN LOWER. THE ALT; WHILE NOT SEEMING TO CAUSE ANY SYMPTOMS; CANNOT BE RULED OUT FOR SOME PERFORMANCE DEGRADATION; ESPECIALLY BEING SINGLE PLT IFR. LASTLY; MISTAKES ARE SIGN POSTS FOR LEARNING EVENTS; I AM PLANNING ON REMEDIAL LEARNING ACTIONS WITH A CFI REVIEW AND SEVERAL AVIATION SAFETY COURSES ONLINE. WHILE I WAS COMFORTABLE WORKING THE GARMIN 430 I DIDN'T REALIZE MY DEFICIENCY UNTIL I WAS UNDER THE GUN TO PERFORM. A GOOD PLT IS ALWAYS LEARNING; EVEN IF THE PRIDE IS INJURED.