Date: 201701
Local time of day: 0001-0600
Location: BJC.Airport, CO
Aircraft: Any Unknown or Unlisted Aircraft Manufacturer
Flight plan: VFR
Mission: Personal
Flight phase: Initial Approach
ATC / advisory: Tower BJC
Reporter organization: Government
Reporter function: Ground; Supervisor / CIC
Reporter qualification: Air Traffic Control Fully Certified
Second reporter function: Local
Anomaly: ATC Issue All Types; Airspace Violation All Types; Conflict Airborne Conflict; Deviation / Discrepancy - Procedural Published Material / Policy
Detector: Person Air Traffic Control
Result: Air Traffic Control Issued Advisory / Alert; Air Traffic Control Provided Assistance
Contributing factors: Airport; Airspace Structure; Procedure
Primary problem: Airspace Structure

Narrative: While working Ground Control/Controller in Charge I noticed a VFR target on a 1200 code transitioning the corridor east of the Class D airspace around 7000 feet MSL. By monitoring the operation; I ensured that the Local controller saw the potential conflict this aircraft presented with another VFR inbound aircraft at 7000 feet flying a practice ILS approach. The conflict alert sounded as Local Control took action to issue traffic until the conflict passed.Though workload permitted Local Control to handle this situation effectively; if there was greater traffic volume; higher priority duties; or the inbound aircraft had not yet switched to our frequency; there is the possibility that there may not be time to issue that traffic or take action to resolve that conflict. This event is very similar to other events reported in the past. I continue to believe that action needs to be taken to mitigate conflicts in this corridor that are created by transitioning aircraft that are not receiving air traffic service. Such aircraft can cause conflict alerts and add workload to both tower and radar controllers; when those controllers must guess at what a 1200 code is doing while operating in proximity to the other traffic that is utilizing the air traffic system. Simply receiving services from TRACON or contacting our Tower to receive advisories or a Class D transition should have made this conflict a non-issue.It's essential that pilots understand that working with ATC will enhance their safety in this busy terminal area; especially as close as they are operating to both Class B and Class D controlled airspace. Controllers will provide the appropriate instructions and clearances to not only help avoid possible airspace violations; but more importantly; avoid near midair events.


Second narrative: We have been having a persistent problem of VFR aircraft encroaching into our Class D airspace; usually from the south; at about 7;500 feet. There is a VFR corridor that is located immediately east and adjacent to our airspace; whose altitude tops out at 7;500. When this corridor was created; BJC Tower's airspace only went to 7;499 feet MSL. Nowadays; it extends up to 7;999 feet MSL. This places the VFR aircraft in close conflict with our arrivals that normally come in at 7;000 feet MSL. It has been a cause for concern from a number of pilots why that corridor is in such close proximity to the airport. The aircraft flying in this area are in no way in contact with us; and on a number of occasions; the tower controller has had to take corrective action to avoid planes in this area. It particularly becomes an issue if we have IFR turboprop or turbojets departing Runway 12L. Somehow find a way to give BJC tower control over this corridor; a Class D extension; something. As our traffic increases; the potential of a serious incident will rise as more and more aircraft fly in this particular area.


Synopsis: Tower Controllers are concerned that a published VFR flight corridor allows aircraft to fly too close to aircraft in their airspace.