SMA PLT ON VFR FLT PLAN ENCOUNTERS IMC CONDITIONS AND INADVERTENTLY PENETRATES CLASS D AIRSPACE.

Narrative

FLT FROM OKC TO DEN; DEST BJC ARPT. AT ABOUT 30 NM FROM DEN VOR NEAR VICTOR 366; CLOUD LAYER SEEMED TO BE LOWERING SO I DSNDED TO 7000 FT MSL (NO LOWER DUE TO TWRS BTWN 20-30 NM FROM DEN VOR. WHEN TWRS SIGHTED OFF L WING; DSNDED TO 6500 FT MSL. MEANWHILE; APA AND BJC ATIS WERE GIVING CEILINGS AT 2500 FT AND VISIBILITY 10 MI. USING GPS/MOVING MAP (IFR CERTIFIED GPS) AND DME; PLANNED TO SKIRT APA/CENTENNIAL AIRSPACE TO THE S THEN TURN NORTHWARD TOWARD BJC/JEFFCO. VISIBILITY WAS APPROX 3-5 MI; CEILING ESTIMATED AT 7000 FT MSL. WITH NO WARNING; FLEW INTO A MODERATE HVY RAINSHOWER. DUE TO RAIN ON WINDSCREEN; FORWARD VISIBILITY DROPPED SIGNIFICANTLY. SKY WAS LIGHTER TO THE NW SO TURNED IN THAT DIRECTION; MAINTAINING ALT USING SIDE VISIBILITY DUE TO LIMITED FORWARD VISIBILITY. TRIED TO STAY CLR OF APA/CENTENNIAL'S CLASS D AIRSPACE BUT WHENEVER I TURNED AWAY; VISIBILITY FORWARD WAS WORSE. ENTERED APA (CENTENNIAL) AIRSPACE BY 1-2 NM AND UNABLE TO GET AWAY UNTIL WNW OF ARPT. AT THAT POINT RAIN STOPPED AND VISIBILITY IMPROVED.