PA31 ON ASR RWY 22 APCH TO PIA WITH SPI APCH HAS AN NMAC WITH A BE90 IN THE PIA RWY 22 TFC PATTERN.

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I WAS OPERATING VFR AFTER BREAKING OUT OF THE CLOUDS AT APPROX 600 FT ON AN ILS APCH WHICH WAS CHANGED TO CIRCLE TO LAND. I UNDERSTAND THAT THE RULE IS SEE AND BE SEEN IN VFR; BUT WHEN OPERATING AT APPROX 600 FT AGL; RIGHT AT THE BASE OF CLOUDS AND IN A TIGHT CIRCLE TO LAND CLRNC; ALL OF THE PLT'S ATTN IS CTRED ON KEEPING THE RWY IN SIGHT AND ON THE ACTUAL FLYING OF THE ACFT AT MINIMUM AIRSPDS. THE ARPT WAS IFR. IT WAS MY EXPECTATION THAT; WHEN I WAS CLRED TO LAND; I WOULD BE THE ONLY ACFT IN THE VICINITY OF THE APCH. I BELIEVE THAT THIS IS A REASONABLE EXPECTATION AND I CANNOT THINK HOW I WOULD HAVE CHANGED THE SIT HAD I TO DO IT OVER AGAIN. MY CAUSAL BELIEVE IN THIS INCIDENT IS THAT THERE WERE 2 ACFT CLRED TO LAND ON THE SAME RWY AT THE SAME TIME. POST DISCUSSION WITH THE OTHER ACFT PLT REVEALED THAT HE WAS ON A SPRINGFIELD ATC SURVEILLANCE APCH; WHILE I WAS BEING CTLED BY THE PEORIA TWR. I BELIEVE THAT THE 2 FACILITIES WERE EITHER MISCOMMUNICATING WITH EACH OTHER; OR NOT COMMUNICATING AT ALL IN THIS SIT. IN THE ALTERNATIVE; THERE COULD HAVE BEEN A HUMAN ERROR MISTAKE BY THE TWR CTLR WHO CLRED ME TO CIRCLE TO RWY 22; CLRED TO LAND. IN ANY CASE; IF MY COPLT HAD NOT SEEN THE IMPENDING COLLISION AND PUSHED THE YOKE FORWARD; AND; IF THE PIPER NAVAJO HAD NOT INITIATED AN IMMEDIATE CLB; I BELIEVE THE OUTCOME WOULD HAVE BEEN DISASTROUS. WE WERE ON A HALF MI FINAL TO RWY 31 WHEN PIA TWR ISSUED INSTRUCTION TO FLY RWY HEADING AND CLB TO 3000 FT. PIA TWR STATED THAT THERE WAS DEBRIS ON RWY 31; BUT STATED THAT RWY 22 WAS AVAILABLE IF I WANTED TO ACCEPT A CIRCLE TO LAND. I TOLD THE TWR I WOULD ACCEPT THE CIRCLE TO LAND ON RWY 22. THE TWR GAVE INSTRUCTION TO CIRCLE TO LAND ON RWY 22; CLRED TO LAND. THE NAVAJO LATER TOLD US THAT HE SAW US AND IMMEDIATELY INITIATED A CLB. PIC ONLY SAW THE TAIL OF NAVAJO AS IT PASSED TO THE L OF THE PIC'S SIDE WINDOW.