Narrative:

I noticed a 1200 code transitioning from north to south east of the class D. Aircraft X was inbound for 30R and I was trying to determine where aircraft X would fit inbound with my other traffic. As aircraft X approached the airport; it appeared they needed to descend out of 080; but there was traffic near them on the approach at 075. Eventually; after it appeared aircraft X had to level off before turning inbound; the approach controller was able to switch the inbound to me; and he checked on reporting that he had the traffic in his proximity in sight. Out of curiosity; I asked if the pilot could tell what type of aircraft it was and he said it looked like a cherokee.when traffic permitted; I was able to check and determine from ads-B that the callsign of the 1200 target was aircraft Y. A few minutes later; the approach controller called with a request to turn aircraft Z into the class D to avoid that northbound aircraft as well. I approved the request and aircraft Y appeared to continue out of the area.this is another instance of an aircraft flying perfectly legally just outside of controlled airspace; yet causing traffic conflicts with aircraft utilizing the air traffic system. The air traffic system worked as intended (collisions avoided); but it would have been safer and more efficient if the transitioning aircraft was in contact with air traffic control. They could have received flight following from approach or contacted our tower to either transition or advise their intentions.my recommendation is to continue to encourage pilots to take advantage of air traffic control services; even if it is to simply advise as a courtesy that they are transitioning nearby controlled airspace. It may be helpful to remind pilots at local events; through online materials; and even provide suggestions on aviation charts (eg. Frequencies for traffic advisories from approach or towers). In hindsight; I wish I had called in the blind to see if the aircraft was on frequency. However; I recall having other duties to perform at the time (traffic in the pattern or arriving); and did not want to get distracted with the transitioning aircraft.

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Original NASA ASRS Text

Title: BJC Tower Controller reported an unsafe but legal operation involving VFR and IFR aircraft.

Narrative: I noticed a 1200 code transitioning from north to south east of the Class D. Aircraft X was inbound for 30R and I was trying to determine where Aircraft X would fit inbound with my other traffic. As Aircraft X approached the airport; it appeared they needed to descend out of 080; but there was traffic near them on the approach at 075. Eventually; after it appeared Aircraft X had to level off before turning inbound; the approach controller was able to switch the inbound to me; and he checked on reporting that he had the traffic in his proximity in sight. Out of curiosity; I asked if the pilot could tell what type of aircraft it was and he said it looked like a Cherokee.When traffic permitted; I was able to check and determine from ADS-B that the callsign of the 1200 target was Aircraft Y. A few minutes later; the approach controller called with a request to turn Aircraft Z into the Class D to avoid that northbound aircraft as well. I approved the request and Aircraft Y appeared to continue out of the area.This is another instance of an aircraft flying perfectly legally just outside of controlled airspace; yet causing traffic conflicts with aircraft utilizing the air traffic system. The air traffic system worked as intended (collisions avoided); but it would have been safer and more efficient if the transitioning aircraft was in contact with air traffic control. They could have received flight following from approach or contacted our tower to either transition or advise their intentions.My recommendation is to continue to encourage pilots to take advantage of air traffic control services; even if it is to simply advise as a courtesy that they are transitioning nearby controlled airspace. It may be helpful to remind pilots at local events; through online materials; and even provide suggestions on aviation charts (eg. frequencies for traffic advisories from approach or towers). In hindsight; I wish I had called in the blind to see if the aircraft was on frequency. However; I recall having other duties to perform at the time (traffic in the pattern or arriving); and did not want to get distracted with the transitioning aircraft.

Data retrieved from NASA's ASRS site 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.