Narrative:

I was traveling sbound along the east side of pwk airport following I294 south on routine traffic patrol. A cessna 172 was departing runway 34. We were both in contact with the control tower, and we both had visual on one another. The cessna informed the tower he would be taking an eastbound heading, and the tower cleared him, but told him to maintain visual contact with me, and I with him, which we both acknowledged. I had already been cleared to continue my sbound heading so I maintained my flight path. Because the cessna was departing from a heading of 340 degrees, and because of our proximity, I expected him to climb out and then make his eastward turn behind me. As I continued to maintain visual contact, the airplane made an unexpected steep right turn in front of me. I immediately and aggressively slowed down, and made a steep right descending turn to avoid hitting the cessna. My passenger and I estimate that we avoided a midair collision by approximately 100 ft. Because my path had remained unchanged, I expected the cessna to maintain his heading until I had passed and continue his turnout behind me. Had the cessna announced his intentions to turn in front of me, I would have had more time to alter my course. Additionally, had the tower given instructions for me to give way or given the cessna direct instructions to turn behind me, this near accident most likely would not have occurred.

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Original NASA ASRS Text

Title: B06 TRANSITING SBOUND THROUGH PWK CTL ZONE TAKES EVASIVE ACTION TO AVOID DEPARTING C172, EVEN THOUGH BOTH ACFT RPTED THE OTHER IN SIGHT.

Narrative: I WAS TRAVELING SBOUND ALONG THE E SIDE OF PWK ARPT FOLLOWING I294 S ON ROUTINE TFC PATROL. A CESSNA 172 WAS DEPARTING RWY 34. WE WERE BOTH IN CONTACT WITH THE CTL TWR, AND WE BOTH HAD VISUAL ON ONE ANOTHER. THE CESSNA INFORMED THE TWR HE WOULD BE TAKING AN EBOUND HDG, AND THE TWR CLRED HIM, BUT TOLD HIM TO MAINTAIN VISUAL CONTACT WITH ME, AND I WITH HIM, WHICH WE BOTH ACKNOWLEDGED. I HAD ALREADY BEEN CLRED TO CONTINUE MY SBOUND HDG SO I MAINTAINED MY FLT PATH. BECAUSE THE CESSNA WAS DEPARTING FROM A HDG OF 340 DEGS, AND BECAUSE OF OUR PROX, I EXPECTED HIM TO CLBOUT AND THEN MAKE HIS EASTWARD TURN BEHIND ME. AS I CONTINUED TO MAINTAIN VISUAL CONTACT, THE AIRPLANE MADE AN UNEXPECTED STEEP R TURN IN FRONT OF ME. I IMMEDIATELY AND AGGRESSIVELY SLOWED DOWN, AND MADE A STEEP R DSNDING TURN TO AVOID HITTING THE CESSNA. MY PAX AND I ESTIMATE THAT WE AVOIDED A MIDAIR COLLISION BY APPROX 100 FT. BECAUSE MY PATH HAD REMAINED UNCHANGED, I EXPECTED THE CESSNA TO MAINTAIN HIS HDG UNTIL I HAD PASSED AND CONTINUE HIS TURNOUT BEHIND ME. HAD THE CESSNA ANNOUNCED HIS INTENTIONS TO TURN IN FRONT OF ME, I WOULD HAVE HAD MORE TIME TO ALTER MY COURSE. ADDITIONALLY, HAD THE TWR GIVEN INSTRUCTIONS FOR ME TO GIVE WAY OR GIVEN THE CESSNA DIRECT INSTRUCTIONS TO TURN BEHIND ME, THIS NEAR ACCIDENT MOST LIKELY WOULD NOT HAVE OCCURRED.

Data retrieved from NASA's ASRS site as of July 2007 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.