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.