NMAC. 2 SMALL ACFT CLBING OUT FROM ARPT. TWR ASKED SECOND ACFT IF IT SAW THE RPTR AND RESPONSE WAS AFFIRMATIVE; BUT APPARENTLY THEY DID NOT BECAUSE IT CLBED INTO RPTR ACFT AND ALMOST HIT IT.

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THE COMPANY I WORK FOR OPERATES A CESSNA 172 UNDER CONTRACT TO RPT TFC IN THE CHICAGO AREA. I WAS FLYING ON THIS DAY WITH ONE TFC RPTR ON BOARD THE ACFT. THE RPTR AND I ARE IN COM WITH EACH OTHER BY AN INTERCOM AND HEADSETS. I WAS FLYING FROM THE L SEAT OF THE AIRPLANE; AND THE RPTR WAS SITTING IN THE R REAR SEAT. THE WX WAS GOOD AND IFR CONDITIONS HAD JUST CLRED AN HR EARLIER. I TOLD GND CTL THAT I WAS REQUESTING A NE DEP AND I HAD ATIS. GND CTL ADVISED THAT I WAS TO USE RWY 34 FOR DEP. AFTER THE BEFORE TKOF CHKLIST WAS COMPLETE; I ADVISED TWR THAT I WAS READY FOR TKOF. I WAS CLRED FOR TKOF ON RWY 34 WITH A R TURNOUT. AFTER MY DEP ANOTHER ACFT; CESSNA X WAS CLRED FOR TKOF ON RWY 30R ALSO FOR A NE DEP. AFTER X TURNED NE; THEY WERE ADVISED THAT THEY HAD TFC AHEAD (I WAS THAT TFC). X TOLD TWR THAT TFC WAS IN SIGHT. I WAS NOT ADVISED OF THE TFC BEHIND ME BY TWR. I WAS CLBING TO 2000 FT; AND I BELIEVE THAT X WAS ALSO CLBING. AS I BEGAN LEVELING OFF (1-2 MI NE OF PWK) MY RPTR SUDDENLY YELLED THAT TFC WAS OFF TO MY R SIDE AND BEHIND. I LOOKED TO THE R; AND SAW X JUST SLIGHTLY BELOW AND TO THE R; COMING RIGHT AT ME. I IMMEDIATELY TOOK EVASIVE ACTION BY PULLING UP TO CLB. X'S DISTANCE WAS NO MORE THAN A WINGSPAN AWAY FROM ME AT THE CLOSEST POINT. AFTER CATCHING MY BREATH; I ASKED TWR IF THEY KNEW OF THE TFC IN MY VICINITY; THEY ADVISED ME THAT X TOLD THEM THEY HAD ME IN SIGHT. THE TWR CTLR THEN ASKED X IF THEY HAD ME IN SIGHT BECAUSE THEY CAME WITHIN 20 FT OF ME. X RESPONDED THEY THOUGHT THEY DID. MY FLT THIS DAY WAS 20 FT FROM DISASTER. I DO NOT UNDERSTAND HOW X DID NOT SEE ME UNLESS THEY JUST WERE NOT LOOKING OUTSIDE. IF IT WAS NOT FOR MY RPTR LOOKING OUT THE SIDE WINDOW; THE OUTCOME MIGHT HAVE BEEN DIFFERENT. THE RPTR FLYING WITH ME THAT DAY HAD BEEN RPTING FROM OUR AIRPLANE FOR OVER 2 YRS. HE IS VERY HELPFUL IN LOOKING FOR OTHER AIRPLANES. I FEEL THAT IT IS IMPORTANT FOR ALL GA PAX TO ASSIST IN LOOKING FOR OTHER AIR TFC THAT MAY BE UNKNOWN TO THE PLT. I ALSO FEEL THAT PLTS GET TOO COMFORTABLE WITH ATC ADVISORIES. JUST BECAUSE ATC IS THERE; PLTS CANNOT STOP LOOKING FOR OTHER AIRPLANES; AND I SOMETIMES AM JUST AS GUILTY OF THIS TOO. I WILL NOW TAKE A MORE ACTIVE ROLE IN LOOKING FOR OTHER AIR TFC IN ANY AIRSPACE AND WHILE IFR IN VMC. SINCE THE ACFT WAS BEHIND ME; I DO NOT THINK I COULD HAVE SEEN X EVEN WITH A BETTER SCAN FOR TFC; BUT IF ALL PLTS WERE TO LOOK MORE ACTIVELY; MIDAIRS WOULD BE FEW AND FAR BTWN.