MKC88FA059

The plt rcvd a wx briefing bfr starting a ngt cargo opn in known icing conds. Bfr the flt, a line serviceman noticed ice on the acft's wings & suggested its removal, but the plt declined. Aprx 45 min after tkof, the plt rprtd to ARTCC that a de-ice boot had 'blown' & he was having difficulty ctlg the acft. He said the boot was 'going back and forth' & that full rgt aileron was required to keep the wings level. Also, he rprtd a continuous loss of alt & said that full rgtaileron would not keep the acft fm turning to the left. Subsequently, the acft crashed on descending terrain while in a slightly nose down, left wing low attitude. Much of the acft (including much of the left wing) was demolished by impact & fire. Prop slash marks indcd the engs were providing pwr. Maint records had no info of replacement, repair or modification of the de-ice boots in the prev 10 yrs. The plt's wife indcd he had previously been pressured to fly acft with discrepancies. Sigmets golf 3 & 4 warned of ocnl severe mixed icing in clouds & precip.