Narrative:

I was on an IFR clearance; but in VMC above the fog layer. While on vectors for the ILS; I was instructed by approach to descend from 3000 ft MSL to 1800 ft MSL. I was attempting to correctly load and activate the approach into the aircraft's new GPS unit (GNS 430); which I am not yet fully proficient with; and inadvertently descended below the assigned altitude. I had just begun to enter the fog layer when ATC advised I was at 1400 ft MSL versus assigned altitude of 1800 ft MSL. I immediately climbed back to assigned altitude and thanked the controller for catching the error. I continued the approach without further incident. Contributing factors: unfamiliar approach; increased cabin workload due to lack of proficiency with new navigation equipment; and reduced diligence of instrument scan while operating 'on top' in VMC; resulted in failure to arrest descent at assigned altitude. Lessons learned: practice! Take advantage of every opportunity to practice with new equipment in VFR conditions to achieve proficiency required to operate it fluently in the IFR environment. Additionally; maintain a diligent instrument scan; even when operating in visual conditions. Finally; at that altitude over level terrain; I could have more effectively used my radar altimeter warning bug as a tool for trapping errors like this.

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Original NASA ASRS Text

Title: PLT EXPERIENCES AN ALTDEV DUE IN PART TO DISTR WITH UNFAMILIAR GPS NAV SYS.

Narrative: I WAS ON AN IFR CLRNC; BUT IN VMC ABOVE THE FOG LAYER. WHILE ON VECTORS FOR THE ILS; I WAS INSTRUCTED BY APCH TO DSND FROM 3000 FT MSL TO 1800 FT MSL. I WAS ATTEMPTING TO CORRECTLY LOAD AND ACTIVATE THE APCH INTO THE ACFT'S NEW GPS UNIT (GNS 430); WHICH I AM NOT YET FULLY PROFICIENT WITH; AND INADVERTENTLY DSNDED BELOW THE ASSIGNED ALT. I HAD JUST BEGUN TO ENTER THE FOG LAYER WHEN ATC ADVISED I WAS AT 1400 FT MSL VERSUS ASSIGNED ALT OF 1800 FT MSL. I IMMEDIATELY CLBED BACK TO ASSIGNED ALT AND THANKED THE CTLR FOR CATCHING THE ERROR. I CONTINUED THE APCH WITHOUT FURTHER INCIDENT. CONTRIBUTING FACTORS: UNFAMILIAR APCH; INCREASED CABIN WORKLOAD DUE TO LACK OF PROFICIENCY WITH NEW NAV EQUIP; AND REDUCED DILIGENCE OF INST SCAN WHILE OPERATING 'ON TOP' IN VMC; RESULTED IN FAILURE TO ARREST DSCNT AT ASSIGNED ALT. LESSONS LEARNED: PRACTICE! TAKE ADVANTAGE OF EVERY OPPORTUNITY TO PRACTICE WITH NEW EQUIP IN VFR CONDITIONS TO ACHIEVE PROFICIENCY REQUIRED TO OPERATE IT FLUENTLY IN THE IFR ENVIRONMENT. ADDITIONALLY; MAINTAIN A DILIGENT INST SCAN; EVEN WHEN OPERATING IN VISUAL CONDITIONS. FINALLY; AT THAT ALT OVER LEVEL TERRAIN; I COULD HAVE MORE EFFECTIVELY USED MY RADAR ALTIMETER WARNING BUG AS A TOOL FOR TRAPPING ERRORS LIKE THIS.

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.