Narrative:

On IFR flight plan; under ATC control; approach assigned me a descent from 8000 ft to 4000 ft. Aircraft 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 air carrier aircraft with FMS. While descending; I began monitoring the VNAV page of the kln 94 since descent 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 altitude alerting system on the M5 and the kln 94 VNAV will build a profile all the way to the set point -- in my case; destination lou. No protection is provided for intermediate leveloff as in the air carrier aircraft. I focused mainly on the descent profile; being accustomed to this on FMS aircraft; and missed my assigned altitude. Approach queried my altitude when I was at 3500 ft (4000 ft assigned) and I realized I had blown it! A problem I see with integration of technology into GA aircraft is the piecemeal approach to functionality. Very few aircraft have a good overall system. Most IFR GPS use a VNAV function that provides very few protections to this scenario. They are advisory in nature; but the pilot may be led to focus on them. Secondly; it is hard to imagine why an affordable altitude alerting system is not available for GA. Better yet; to have one built into a navigation system. Current IFR GPS system seem to go 1/2 way to being FMS. Lastly; stepping from a technologically advanced aircraft into a partially advanced aircraft can lead a pilot astray as he/she loses system protections that have been previously available.

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Original NASA ASRS Text

Title: 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.

Narrative: 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.

Data retrieved from NASA's ASRS site as of January 2009 and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.