Narrative:

Aircraft X was on an instrument approach to maintain visual separation with preceding traffic (4.5 miles ahead). The adjacent airport appeared to have quite a bit of VFR traffic during my session on local control; so I was scanning that vicinity often. Aircraft X was roughly inside a 3 mile final and had not checked in with tower and I observed aircraft Y off the adjacent airport turning west (fairly normal procedure).I called TRACON to ask if they could try switching the aircraft to me again. I reached out for aircraft X again when he was on a 2 mile final and we had stepped on each other. As I unkeyed; that is when I realized that we had stepped on each other; and went right back and issued the traffic that was ahead and to his left; west bound; and altitude indicating 900 feet. Aircraft X was at 1100 feet and descending. By then; the aircraft Y was passing behind aircraft X. Another aircraft then checked on that was further out on final. As I was keying up to clear them to land; the other airport tower called to issue a point-out on the aircraft that was traffic for aircraft X (the aircraft was on a 1.5 mile final by then). No complaints were made that I am aware of.it is my recommendation that the adjacent airport change their departure procedure for their west downwind departure. This departure is commonly used; but does not appear to be adhered to by the pilots nearly as much as it should with our arrivals going right over top of their airport. This departure is known for causing TCAS/ras for our arrivals and causing go arounds on the regular. While pilots can be tagged for deviations; it would appear that the safest change would be to change the departure.

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Original NASA ASRS Text

Title: An aircraft departing an airport was issued incorrect VFR departure instructions. The aircraft went into the Class B airspace of the adjacent airport.

Narrative: Aircraft X was on an instrument approach to maintain visual separation with preceding traffic (4.5 miles ahead). The adjacent airport appeared to have quite a bit of VFR traffic during my session on Local Control; so I was scanning that vicinity often. Aircraft X was roughly inside a 3 mile final and had not checked in with tower and I observed Aircraft Y off the adjacent airport turning west (fairly normal procedure).I called TRACON to ask if they could try switching the aircraft to me again. I reached out for Aircraft X again when he was on a 2 mile final and we had stepped on each other. As I unkeyed; that is when I realized that we had stepped on each other; and went right back and issued the traffic that was ahead and to his left; west bound; and altitude indicating 900 feet. Aircraft X was at 1100 feet and descending. By then; the Aircraft Y was passing behind Aircraft X. Another aircraft then checked on that was further out on final. As I was keying up to clear them to land; the other airport tower called to issue a point-out on the aircraft that was traffic for Aircraft X (the aircraft was on a 1.5 mile final by then). No complaints were made that I am aware of.It is my recommendation that the adjacent airport change their departure procedure for their west downwind departure. This departure is commonly used; but does not appear to be adhered to by the pilots nearly as much as it should with our arrivals going right over top of their airport. This departure is known for causing TCAS/RAs for our arrivals and causing go arounds on the regular. While pilots can be tagged for deviations; it would appear that the safest change would be to change the departure.

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.