Narrative:

While on a VFR flight en route to mmv from rnt; I was flying direct to hio to pass over the class D airspace. As I flew near hio; the ATIS at hio reported scattered at 3000 ft; overcast 5000 ft. Observed conditions in-flight were in accordance with that report. There was an airmet for icing in washington and oregon with freezing levels starting at 3500 ft MSL and up to 4500 ft MSL. I descended from en route cruising altitude of 3000 ft MSL to get under the cloud layer at hio. I left flight following from portland center and had not switched over to hio tower. I descended in the aircraft to around 2300 ft MSL and scanned to observe that I was clear of the cloud layer as well as clouds in the immediate vicinity of my aircraft. Continuing my scan out the front window of the airplane; I observed that hio was a short distance ahead. At this time I realized that I was low enough to be in hio airspace. I quickly checked the GPS; and observed that I was inside hio class D surface airspace. I immediately turned approximately 90 degrees to my right to head away from the direction of the airport. On rollout; I observed an unidentifiable single engine aircraft flying directly towards my position under the cloud layer as well. I proceeded to turn the aircraft to the right to evade the aircraft per right-of-way rules. I then rolled out on a northerly heading and proceeded out of hio class D airspace. Human performance considerations: the space between the scattered layer and the overcast layer could have been sufficiently clear enough to fly between the layers around 4000-4500 ft without causing icing conditions. This would have also kept the airplane out of hio airspace; and could have still retained VFR flight following.

Related reports:

Google
 

Original NASA ASRS Text

Title: A C172RG PLT EXPERIENCED AN NMAC WHILE MANEUVERING BELOW AN OVCST AND ICING CONDITIONS NEAR HIO.

Narrative: WHILE ON A VFR FLT ENRTE TO MMV FROM RNT; I WAS FLYING DIRECT TO HIO TO PASS OVER THE CLASS D AIRSPACE. AS I FLEW NEAR HIO; THE ATIS AT HIO RPTED SCATTERED AT 3000 FT; OVCST 5000 FT. OBSERVED CONDITIONS INFLT WERE IN ACCORDANCE WITH THAT RPT. THERE WAS AN AIRMET FOR ICING IN WASHINGTON AND OREGON WITH FREEZING LEVELS STARTING AT 3500 FT MSL AND UP TO 4500 FT MSL. I DSNDED FROM ENRTE CRUISING ALT OF 3000 FT MSL TO GET UNDER THE CLOUD LAYER AT HIO. I LEFT FLT FOLLOWING FROM PORTLAND CTR AND HAD NOT SWITCHED OVER TO HIO TWR. I DSNDED IN THE ACFT TO AROUND 2300 FT MSL AND SCANNED TO OBSERVE THAT I WAS CLR OF THE CLOUD LAYER AS WELL AS CLOUDS IN THE IMMEDIATE VICINITY OF MY ACFT. CONTINUING MY SCAN OUT THE FRONT WINDOW OF THE AIRPLANE; I OBSERVED THAT HIO WAS A SHORT DISTANCE AHEAD. AT THIS TIME I REALIZED THAT I WAS LOW ENOUGH TO BE IN HIO AIRSPACE. I QUICKLY CHKED THE GPS; AND OBSERVED THAT I WAS INSIDE HIO CLASS D SURFACE AIRSPACE. I IMMEDIATELY TURNED APPROX 90 DEGS TO MY R TO HEAD AWAY FROM THE DIRECTION OF THE ARPT. ON ROLLOUT; I OBSERVED AN UNIDENTIFIABLE SINGLE ENG ACFT FLYING DIRECTLY TOWARDS MY POS UNDER THE CLOUD LAYER AS WELL. I PROCEEDED TO TURN THE ACFT TO THE R TO EVADE THE ACFT PER RIGHT-OF-WAY RULES. I THEN ROLLED OUT ON A NORTHERLY HDG AND PROCEEDED OUT OF HIO CLASS D AIRSPACE. HUMAN PERFORMANCE CONSIDERATIONS: THE SPACE BTWN THE SCATTERED LAYER AND THE OVCST LAYER COULD HAVE BEEN SUFFICIENTLY CLR ENOUGH TO FLY BTWN THE LAYERS AROUND 4000-4500 FT WITHOUT CAUSING ICING CONDITIONS. THIS WOULD HAVE ALSO KEPT THE AIRPLANE OUT OF HIO AIRSPACE; AND COULD HAVE STILL RETAINED VFR FLT FOLLOWING.

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.