AN ACR PILOT FLYING A MAULE M5 WITH ADVANCED NAV DESCRIBES HIS ALT DEVIATION AND PIECEMEAL GA NAV SYSTEMS THAT OFFER FEW PROTECTIONS ENJOYED ON FMS ACFT.

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ON IFR FLT PLAN; UNDER ATC CTL; APCH ASSIGNED ME A DSCNT FROM 8000 FT TO 4000 FT. ACFT OPERATED WAS A MAULE M5 EQUIPPED WITH KLN 94 GPS; WHICH I OPERATE FOR PERSONAL PLEASURE AND BUSINESS FLTS. MY FULL TIME JOB IS FLYING ACR ACFT WITH FMS. WHILE DSNDING; I BEGAN MONITORING THE VNAV PAGE OF THE KLN 94 SINCE DSCNT WAS INITIATED A BIT LATER THAN NORMAL. I RARELY USE THIS PAGE SINCE ITS FUNCTIONALITY SEEMS AWKWARD COMPARED TO THE FMS I USE AT WORK. A VMC DAY SEEMED A GOOD TIME TO USE IT. THERE IS NO ALT ALERTING SYS ON THE M5 AND THE KLN 94 VNAV WILL BUILD A PROFILE ALL THE WAY TO THE SET POINT -- IN MY CASE; DEST LOU. NO PROTECTION IS PROVIDED FOR INTERMEDIATE LEVELOFF AS IN THE ACR ACFT. I FOCUSED MAINLY ON THE DSCNT PROFILE; BEING ACCUSTOMED TO THIS ON FMS ACFT; AND MISSED MY ASSIGNED ALT. APCH QUERIED MY ALT WHEN I WAS AT 3500 FT (4000 FT ASSIGNED) AND I REALIZED I HAD BLOWN IT! A PROB I SEE WITH INTEGRATION OF TECHNOLOGY INTO GA ACFT IS THE PIECEMEAL APPROACH TO FUNCTIONALITY. VERY FEW ACFT HAVE A GOOD OVERALL SYS. MOST IFR GPS USE A VNAV FUNCTION THAT PROVIDES VERY FEW PROTECTIONS TO THIS SCENARIO. THEY ARE ADVISORY IN NATURE; BUT THE PLT MAY BE LED TO FOCUS ON THEM. SECONDLY; IT IS HARD TO IMAGINE WHY AN AFFORDABLE ALT ALERTING SYS IS NOT AVAILABLE FOR GA. BETTER YET; TO HAVE ONE BUILT INTO A NAV SYS. CURRENT IFR GPS SYS SEEM TO GO 1/2 WAY TO BEING FMS. LASTLY; STEPPING FROM A TECHNOLOGICALLY ADVANCED ACFT INTO A PARTIALLY ADVANCED ACFT CAN LEAD A PLT ASTRAY AS HE/SHE LOSES SYS PROTECTIONS THAT HAVE BEEN PREVIOUSLY AVAILABLE.