INFLT ENCOUNTER OF LIGHT TO MODERATE ICING BY A C172RG PLT AND PAX ON V172 NEAR MLI LEADS TO A DIVERSION TO DVN; IA.

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THE INCIDENT OCCURRED ON A FLT DEPARTING FOR RFD. I WAS NOT THE PIC; BUT RATHER ACCOMPANIED ANOTHER PLT IN THE CAPACITY AS A PAX (NOT RATED IN THE COMPLEX ACFT WHICH WE FLEW). PRIOR TO MY ARR AT THE ARPT; THE PIC HAD FILED AN IFR FLT PLAN WITH FSS; AND OBTAINED A FULL WX BRIEFING. BASED ON THE WX CONDITIONS PRIOR TO DEP (31 DEGS F; OVCST 1200 FT; TOPS AT 7000+ FT); I BECAME CONCERNED ABOUT THE POSSIBILITY OF ICING. THE PIC AGAIN CALLED FSS FOR AN UPDATED WX BRIEFING. BASED ON THE INFO THAT HE RECEIVED; HE FELT CONFIDENT THAT WE WOULD SAFELY MAKE THE FLT BY OUR CRUISING AT AN ALT ABOVE THE CURRENT RPTED TOPS. ANOTHER PLT PRESENT CONCURRED WITH THIS DECISION. WE DEPARTED WITH A CLRNC FROM CID ATC FOR CLBING TO AND MAINTAIN 7000 FT. ENRTE TO 7000 FT; WE PICKED UP ICE ON BOTH THE WINDSHIELD AND THE STRUCTURE. THE TOPS WERE HIGHER THAN 7000 FT; ALTHOUGH IT APPEARED THEY WERE NOT TOO MUCH HIGHER. HOPING TO BURN OFF THE ICE; THE PIC ASCENDED SEVERAL HUNDRED FT; YET FOR ONLY A MOMENT WE BROKE OUT AND THE CLOUD LAYER AGAIN ROSE ABOVE US. HE QUICKLY DSNDED TO 7000 FT AND REQUESTED OF ATC THAT WE DSND TO A LOWER ALT. AT THAT POINT; CID ATC HANDED US OFF TO MLI ATC; AND THE PIC AGAIN MADE THE REQUEST FOR LOWER ALT. ATC FIRST OBTAINED A PIREP FROM ANOTHER SMALL PLANE IN THE AREA WHO RPTED HAVING PICKED UP ICE AT 4000 FT; BUT THAT THIS ICE CAME OFF AT 3000 FT. ATC THEN CLRED US TO 3000 FT. THROUGHOUT THIS TIME; THE PIC MAINTAINED OUR HDG ON V172. DURING THE DSCNT WE ACCUMULATED MORE ICE. AFTER A SHORT WHILE AT 3000 FT; THE ICE WAS NOT DISINTEGRATING; AND HE REQUESTED THAT WE DSND TO A LOWER ALT. ATC CLRED US TO 2400 FT; THE LOWEST ENRTE ALT POSSIBLE. IT WAS APPARENT; AFTER A SHORT WHILE; THAT THE CONDITION OF THE ICE ON THE STRUCTURE WAS NOT IMPROVING; AND MAY HAVE BEEN DETERIORATING. FURTHER; THE NAV EQUIP (VOR) WAS NOT MAINTAINING INTEGRITY AT THIS LOW ALT. ATC THEN OFFERED FOR US TO LAND AT A SMALL ARPT NEARBY (CLINTON; IA). THEY OFFERED A VARIETY OF APCHS. THE PIC WAS HAVING DIFFICULTY IN FINDING THE PAGE FOR THE ILS APCH; SO HE ACCEPTED AN NDB APCH. THE APCH WAS UNSUCCESSFUL. ATC INSTRUCTED US TO ASCEND TO 2700 FT AND PROVIDED A HDG. THEY THEN OFFERED A VOR APCH INTO ANOTHER LCL ARPT (DAVENPORT; IA); WHICH THE PIC ACCEPTED. THE APCH WAS MADE WITHOUT INCIDENT; AND WE LANDED SAFELY. HOWEVER; BASED ON THIS EXPERIENCE; I HAVE A FEW RECOMMENDATIONS FOR OTHERS FACED WITH THESE SAME CONDITIONS AND DECISIONS. I WOULD FIRST LIKE TO DISCOURAGE FLT INTO POTENTIAL ICING CONDITIONS. IN ADDITION TO INFO OBTAINED FROM WX BRIEFINGS; UTILIZE COMMON SENSE IN DETERMINING WHETHER OR NOT THE CONDITIONS ARE APPROPRIATE FOR A SAFE FLT. WHEN IN DOUBT; ERR ON THE CONSERVATIVE SIDE. SECOND; ALTHOUGH YOU MAY ONLY BE A PAX; UTILIZE YOUR KNOWLEDGE AS A PLT TO HELP GUIDE YOUR DECISIONS. DON'T GO IF YOU ARE UNCOMFORTABLE! DON'T LET THE OPINIONS OF OTHERS REPLACE COMMON SENSE! ALTHOUGH IN OUR SIT THE PLANE MAINTAINED ITS ABILITY TO FUNCTION PROPERLY; IT COULD HAVE BEEN WORSE. FINALLY; UPON RECOGNIZING THE BUILD-UP OF ICE; IT WOULD HAVE BEEN MOST PRUDENT TO TURN AROUND IMMEDIATELY AND RETURN TO THE DEP ARPT. DON'T CONTINUE HOPING THAT THE CONDITION WILL IMPROVE.