FLIGHT CREW REPORTS INADVERTENTLY ENTERING CLASS D AIRSPACE WHILE ATTEMPTING TO MAINTAIN VMC AND USING GARMIN 530 TO AVOID CONTROLLED AIRSPACE. CONTROLLED AIRSPACE DID NOT APPEAR UNTIL MAP SCALE WAS REDUCED TO 20 NM.

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THE PILOT FLYING PERFORMED ALL FLIGHT DUTIES WHILE THE PILOT NOT FLYING PERFORMED RADIO COMMUNICATIONS AND NAVIGATION DUTIES. AFTER DEPARTING THE LATERAL EXTENTS OF THE ATL CLASS B AIRSPACE TO THE SOUTH; WE FLEW DIRECT MCN AND CLIMBED TO 7500 FT MSL. THE PLAN WAS TO CRUISE AT 11500 FT ALONG VICTOR AIRWAYS TO ENSURE AVOIDANCE OF ALL POTENTIAL AIRSPACE CONFLICT. ENROUTE IT BECAME CLEAR THAT MAINTAINING CLOUD SEPARATION WOULD NOT ALLOW THE PLANNED CRUISING ALTITUDE. AT THIS TIME THE PILOT NOT FLYING BEGAN TO REVIEW THE PLANNED FLIGHT PATH TO FIND POTENTIAL AIRSPACE CONFLICTS. THE GARMIN 530 WAS USED TO REVIEW THE FLIGHT PATH. NO CONFLICTS WERE FOUND. IN HINDSIGHT; NO CONFLICTS WERE IDENTIFIED BECAUSE THE ZOOM LEVEL OF THE GARMIN 530 WAS NOT AT A LEVEL THAT SHOWS THE LATERAL EXTENTS OF CONTROLLED AIRSPACE. AT THIS POINT; A FEW MILES NORTH OF MCN AND WRB; THE PILOT FLYING BEGAN A SPIRALING DESCENT TO MAINTAIN CLOUD CLEARANCE. DURING THE DESCENT; THE PROXIMITY TO MCN AND WRB WAS NOTED BY THE PILOT FLYING AND QUESTIONED THE PILOT NOT FLYING ABOUT POTENTIAL AIRSPACE CONFLICTS DUE TO THE LIKELIHOOD THAT THE FINAL ALTITUDE WOULD BE BELOW THE NORMAL CLASS D ALTITUDES. NO AIRSPACE BOUNDARY WAS SHOWN ON THE GARMIN 530; AS IF THEY WERE UNCONTROLLED. THAT CLEARLY DID NOT MAKE SENSE. AT THAT POINT; THE ZOOM LEVEL WAS NOTED BY THE PILOT TO BE AT 35 NAUTICAL MILES AND QUICKLY CHANGED TO 20 NAUTICAL MILES. THE TRSA AND CLASS D BOUNDARIES OF BOTH MCN AND WRB WERE IMMEDIATELY APPARENT AT THIS ZOOM LEVEL. AN IMMEDIATE WESTBOUND HEADING AND DESCENT TO 1;500 FT MSL WAS BEGUN TO QUICKLY EXIT THE TRSA AND LATERAL EXTENTS OF THE AIRSPACE; BUT WE WERE CLEARLY DEEP WITHIN THE TRSA AIRSPACE. AFTER CLEARING THE SUBJECT AIRSPACE; WE PROCEEDED ON A SOUTHBOUND HEADING ON COURSE AND USED OUR PAPER CHARTS TO FULLY ENSURE THAT NO FURTHER AIRSPACE CONFLICTS WOULD OCCUR. USE OF FLIGHT FOLLOWING WOULD HAVE PREVENTED THE WEATHER INDUCED DESCENT THAT RESULTED IN AN AIRSPACE CONFLICT. WE SHOULD NOT HAVE RELIED SO HEAVILY ON THE GARMIN 530 AS A FLIGHT PLANNING AIRSPACE BOUNDARY TOOL. WE WERE TOO CONFIDENT IN OUR SIMPLIFIED METHOD OF FLIGHT PLANNING SINCE WE HAD RECENTLY COMPLETED MANY VFR CROSS-COUNTRY FLIGHTS AT ALTITUDES OF 10500 FT TO 12500 FT MSL ALONG VICTOR AIRWAYS WITHOUT INCIDENT.