Narrative:

Established contact with den TRACON 20 NM east of den, altitude 6500 ft AGL, heading 241 degrees magnetic and requested transition through B airspace with arrival information. Radar contact was confirmed. Frequency changed to den tower 12 NM for clearance into surface B airspace and instructed to deviate about 20 degrees right to overfly control tower and tower would call a turn to the southwest on course. At about 10 NM east of the control tower, I spotted a white cessna 310 emerging from blind spot of the right window support at same altitude, sbound, no more than 400 ft distance on a positive collision course. I placed the aircraft in a very steep dive, called the aircraft position and estimate we passed under by 50-150 ft vertical with no lateral separation. The passenger estimated the cessna passed 50 ft directly overhead. The aircraft was observed out the left side in a gentle descent after we completed a 300 ft altitude deviation at 1/4 mi at same altitude. If evasive action had not been initiated, the result would have been a midair. I reported the near miss and evasive action to tower which did not have the aircraft on radar. Moments later, he reported a primary target on the boundary of the surface B airspace. I was directed to call the tower supervisor to provide them with additional information to track the cessna. He reported the aircraft was 'tagged,' but provided only momentary primary radar returns with no altitude readouts. The return was eventually lost in traffic. I was advised that all returns were on the border of the surface B airspace 'and those pilots know where the line is when transitioning south.' additionally, he informed me the tower operator may have been distracted because he was in the middle of a runway change. I did advise him that I would be filing a report with my company.

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Original NASA ASRS Text

Title: BHT407 PLT HAS NMAC DURING XSIT OF DEN CLASS B AIRSPACE.

Narrative: ESTABLISHED CONTACT WITH DEN TRACON 20 NM E OF DEN, ALT 6500 FT AGL, HEADING 241 DEGS MAGNETIC AND REQUESTED TRANSITION THROUGH B AIRSPACE WITH ARR INFO. RADAR CONTACT WAS CONFIRMED. FREQ CHANGED TO DEN TWR 12 NM FOR CLRNC INTO SURFACE B AIRSPACE AND INSTRUCTED TO DEVIATE ABOUT 20 DEGS R TO OVERFLY CTL TWR AND TWR WOULD CALL A TURN TO THE SW ON COURSE. AT ABOUT 10 NM E OF THE CTL TWR, I SPOTTED A WHITE CESSNA 310 EMERGING FROM BLIND SPOT OF THE R WINDOW SUPPORT AT SAME ALT, SBOUND, NO MORE THAN 400 FT DISTANCE ON A POSITIVE COLLISION COURSE. I PLACED THE ACFT IN A VERY STEEP DIVE, CALLED THE ACFT POS AND ESTIMATE WE PASSED UNDER BY 50-150 FT VERT WITH NO LATERAL SEPARATION. THE PAX ESTIMATED THE CESSNA PASSED 50 FT DIRECTLY OVERHEAD. THE ACFT WAS OBSERVED OUT THE L SIDE IN A GENTLE DSCNT AFTER WE COMPLETED A 300 FT ALTDEV AT 1/4 MI AT SAME ALT. IF EVASIVE ACTION HAD NOT BEEN INITIATED, THE RESULT WOULD HAVE BEEN A MIDAIR. I RPTED THE NEAR MISS AND EVASIVE ACTION TO TWR WHICH DID NOT HAVE THE ACFT ON RADAR. MOMENTS LATER, HE RPTED A PRIMARY TARGET ON THE BOUNDARY OF THE SURFACE B AIRSPACE. I WAS DIRECTED TO CALL THE TWR SUPVR TO PROVIDE THEM WITH ADDITIONAL INFO TO TRACK THE CESSNA. HE RPTED THE ACFT WAS 'TAGGED,' BUT PROVIDED ONLY MOMENTARY PRIMARY RADAR RETURNS WITH NO ALT READOUTS. THE RETURN WAS EVENTUALLY LOST IN TFC. I WAS ADVISED THAT ALL RETURNS WERE ON THE BORDER OF THE SURFACE B AIRSPACE 'AND THOSE PLTS KNOW WHERE THE LINE IS WHEN TRANSITIONING S.' ADDITIONALLY, HE INFORMED ME THE TWR OPERATOR MAY HAVE BEEN DISTRACTED BECAUSE HE WAS IN THE MIDDLE OF A RWY CHANGE. I DID ADVISE HIM THAT I WOULD BE FILING A RPT WITH MY COMPANY.

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.