Narrative:

I was working local control; and observed a VFR target northbound near alike around 075. It's not unusual to see VFR aircraft transitioning in a corridor just east of the airspace; and I didn't think much of it at first. I turned my attention to a helicopter operating on the airport; and the next thing I remember was hearing the ca going off. I saw on the racd an IFR target causing the ca; and transmitted in the blind to see if the other VFR aircraft '4 northeast of bjc at 076' was 'on' (frequency). There was a brief pause; and right as I was about to transmit again; a 'cessna at 7;600' responded. I immediately issued a traffic alert for a BE20 that was southbound at 080; less than a mile away and less than 500 feet above him. The cessna who appeared to have drifted into D airspace northwest of alike; and he reported the traffic was in sight. I then approved the cessna for transition through D airspace; and he continued northbound without further incident. Recommendation; this event highlights an area of concern regarding VFR pilots transitioning just outside of controlled airspace to the east. The area between eik and downtown denver; with den to the east and bjc to the west; can present hazards to a VFR pilots due to traffic volume; especially when pilots are not transitioning the area with D01 or bjc ATCT. Denver class B airspace is close to this corridor to the east; and arrivals to both bjc and apa can present traffic conflicts as well; even if traffic is level at 080. Possible conflicts are greater if conflicting aircraft inbound to bjc are descending from 080 for visual approaches and/or turning final at 070 for their instrument approaches. While this pilot was following a good suggested technique by at least having our frequency tuned in and monitoring; the traffic advisory/alert could have been relayed more quickly if the pilot had requested ATC services to begin with; while in the vicinity of class B and class D airspace. The message I'd like to give to the pilot communities: ATC is here for a reason; so rather than circumvent controlled airspace (or risk a pilot deviation if you don't!); talk to us and take advantage of ATC services. Even if you believe you'll pass just clear or just above D airspace; as long as the frequency isn't too busy; check in: having your VFR aircraft on frequency enhances safety. From the tower perspective; worst case we might have is to transition outside the airspace if we're too busy or we'll simply acknowledge your presence if you're passing outside our airspace; and that's it. Getting ATC advisories could save you (and us) a scare from a near midair; or even save your life!

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Original NASA ASRS Text

Title: BJC Controller described a conflict event between an IFR aircraft and a VFR aircraft that had drifted into Class B airspace; the Controller suggesting that VFR aircraft request ATC services in the subject airspace.

Narrative: I was working LC; and observed a VFR target northbound near ALIKE around 075. It's not unusual to see VFR aircraft transitioning in a corridor just east of the airspace; and I didn't think much of it at first. I turned my attention to a helicopter operating on the airport; and the next thing I remember was hearing the CA going off. I saw on the RACD an IFR target causing the CA; and transmitted in the blind to see if the other VFR aircraft '4 NE of BJC at 076' was 'on' (frequency). There was a brief pause; and right as I was about to transmit again; a 'Cessna at 7;600' responded. I immediately issued a traffic alert for a BE20 that was southbound at 080; less than a mile away and less than 500 feet above him. The Cessna who appeared to have drifted into D airspace northwest of ALIKE; and he reported the traffic was in sight. I then approved the Cessna for transition through D Airspace; and he continued northbound without further incident. Recommendation; this event highlights an area of concern regarding VFR pilots transitioning just outside of controlled airspace to the east. The area between EIK and downtown Denver; with DEN to the East and BJC to the West; can present hazards to a VFR pilots due to traffic volume; especially when pilots are not transitioning the area with D01 or BJC ATCT. Denver Class B airspace is close to this corridor to the east; and arrivals to both BJC and APA can present traffic conflicts as well; even if traffic is level at 080. Possible conflicts are greater if conflicting aircraft inbound to BJC are descending from 080 for visual approaches and/or turning final at 070 for their instrument approaches. While this pilot was following a good suggested technique by at least having our frequency tuned in and monitoring; the traffic advisory/alert could have been relayed more quickly if the pilot had requested ATC services to begin with; while in the vicinity of Class B and Class D airspace. The message I'd like to give to the pilot communities: ATC is here for a reason; so rather than circumvent controlled airspace (or risk a Pilot Deviation if you don't!); talk to us and take advantage of ATC services. Even if you believe you'll pass just clear or just above D airspace; as long as the frequency isn't too busy; check in: having your VFR aircraft on frequency enhances safety. From the Tower perspective; worst case we might have is to transition outside the airspace if we're too busy or we'll simply acknowledge your presence if you're passing outside our airspace; and that's it. Getting ATC advisories could save you (and us) a scare from a near midair; or even save your life!

Data retrieved from NASA's ASRS site as of July 2013 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.