C303 PLT FLEW A VOR APCH UTILIZING THE AUTOPILOT ON HIS GPS EQUIPPED ACFT. HE FAILED TO MONITOR THE ALT AND BROKE OUT OF THE CLOUDS 150 FT BELOW THE MDA.

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I HAD 'WAAS' UPGRADES DONE TO MY GPS. MY AUTOPLT CAN FULLY UTILIZE THE CAPABILITIES OF THE 'WAAS' RECEIVERS. RETURNING TO LWM; WX RPTED AS 800 FT SCATTERED; 1100 FT SCATTERED; 2200 FT OVCST; 20 MI -RA. I WAS CLRED FOR THE VOR 23 (MDA = 640 FT MSL). INTERCEPT ALT WAS 2000 FT AND I HAD CLR GND CONTACT AT 2000 FT WITH SOME SCATTERED CLOUDS BELOW. I COUPLED THE AUTOPLT AND 'WAAS' GS; THE AUTOPLT FLEW THE COMPLETE APCH PERFECTLY. I WAS VERY FAMILIAR WITH THE APCH AND KNEW I WOULD BREAK OUT PLENTY EARLY SO I CONCENTRATED ON HOW THE NEW RADIOS WERE OPERATING AND LOOKING OUT THE WINDOW TO SEE HOW THE APCH WAS PROGRESSING. I HAD GOOD GND CONTACT THROUGHOUT THE APCH AND ALL LOOKED GOOD; BUT THERE WERE SCATTERED CLOUDS ALL AROUND. WHEN I FINALLY 'POPPED' THROUGH THAT LAST LITTLE CLOUD AND HAD THE RWY IN SIGHT I LOOKED AT THE ALTIMETER AND I WAS SOME 150 FT BELOW MDA. I BELIEVE I LET THE ACFT DSND ABOUT 150 FT BELOW MDA WITHOUT THE RWY IN SIGHT. I WAS USING THE VOR FOR NAV BUT LETTING THE AUTOPLT TRACK THE GPS COURSE AND GS TO SEE HOW IT WORKED AND WAS DISTR ENOUGH TO FAIL TO MONITOR THE ALTIMETER. I SHOULD HAVE FLOWN THE VOR APCH USING THE VOR WITHOUT THE GPS TO DISTRACT ATTN TO THE ALTIMETER AND PROBABLY COMMENCED THE MISSED APCH.