PLT OF A CESSNA SUPER SKYMASTER P337; ON A VFR FLT; PENETRATED CLASS D AIRSPACE DURING DSCNT TO HOME BASE. HE RECOGNIZED HIS MISTAKE AND DEPARTED OUT OF IT AT ONCE.

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MVFR. RETURNING FROM PAO TO O15; SAN JOAQUIN VALLEY FOG; HAZE. USING LORAN TO NAV AND PILOTAGE; REQUESTED FLT FOLLOWING FROM STOCKTON APCH AND HAD BEEN MONITORING STOCKTON APCH AND MODESTO TWR. WHEN I CALLED STOCKTON AND RPTED MY LOCATION -- 180 DEG RADIAL AND 4 MI DME -- I THEN REALIZED I HAD ENTERED MODESTO'S CLASS D AIRSPACE AND IMMEDIATELY TURNED S TO GET OUT OF THEIR AIRSPACE AND RPTED I WAS 5 DME MODESTO. (I HAD RPTED 4 DME TO STOCKTON.) THERE WAS NO TFC CONFLICT AS I WAS MONITORING MODESTO TWR AND STOCKTON. I COULD NOT SEE THE MODESTO ARPT ENVIRONMENT DUE TO THE MVFR. PURE PLT ERROR AND INATTN AS I DID HAVE #1 NAV IN THE RMI MODE AND JUST DID NOT RECOGNIZE 'IT' TILL I HAD ENTERED THEIR AIRSPACE. THIS IS A COMMON RTE FOR ME -- O15 DIRECT PAO AND RETURN. I WAS JUST TOO COMPLACENT; AS I USUALLY AM OUTSIDE MODESTO'S AIRSPACE BY 1 MI. I AM EMBARRASSED AND DISAPPOINTED IN THIS. I ALSO JUST COMPLETED PHASE 3 OF THE WINGS PROGRAM. INSTEAD OF FLT FOLLOWING; I WILL FILE IFR AND USE THE SYS AND BE 'MONITORED' ALL THE WAY.