Skylane 182/RG Turbo Skylane/RG
Air Traffic Control Fully Certified
Other / Unknown
Conflict NMAC; Deviation / Discrepancy - Procedural Published Material / Policy
General None Reported / Taken
Human Factors; Procedure
A C182; departed Boulder airport and called BJC tower; with the current ATIS; to request a transition through the airspace from NW to SE at 6;500 MSL. This request was approved as requested. Shortly after; a C172; was inbound from the north with a left base entry instruction for runway 11L at 7;400 MSL; descending. I exchanged traffic between the two aircraft. The C182 pilot reported that he had the C172 in sight; and that it would be 'no problem.' I informed the C172 pilot that the C182 pilot had him in sight. The C182 pilot told me two or three more times in the span of a few minutes that he still had the C172 in sight. After the two aircraft passed; the C172 reported only 200 feet separation. The two aircraft pilots began arguing on frequency about who should have given way; or gone behind. The C172  landed and the C182 continued SE bound. These were two VFR aircraft in VMC conditions. If one aircraft had given the other one a wider berth; it may have prevented the C172 pilot from getting surprised.
BJC Tower Controller describes an NMAC between a C182 transitioning through the Class D at pattern elevation and a C172 on left downwind for Runway 11L. The C182 pilot had reported the C172 in sight several times; but apparently chose to pass with minimal separation; resulting in a verbal exchange on the Tower frequency.