Bjc local controller instructed an aircraft to enter a right base leg. The controller did not realiz...

Narrative:

I was working local 1 runway 30R (local 2 was split off due to several in the pattern). Traffic was steady moderate. Aircraft X called inbound from the north; and was instructed to enter a right base for runway 30R; which he read back. Aircraft Y was on final runway 30R; cleared to land and issued traffic which was a helicopter on final for the parallel; which he stated he had in sight. Aircraft Y reported 'going around due to the helicopter' and was told to enter right traffic to runway 30R. During this time; aircraft X reported 'left; correction right downwind for runway 30R'; but I was busy with the missed approach; and ensuring that the helicopter was in fact lined up for the correct runway. There was never any primary on my radar scope that I observed in the airspace. Aircraft Z called ready for departure; and was cleared for take-off for a north departure. My focus was northeast of the airport; ensuring that aircraft Z was rolling; as aircraft Y was on right base for runway 30R. Aircraft Z departed; and made an early turn to the north. Unknown to me; the pilot of aircraft Y had entered a downwind northwest of the field; (no primary target) not the right base that I was expecting.the pilot of aircraft Z reported that she almost had a near midair collision; and had to turn to avoid another [aircraft]. I asked her to ident; (there were no observed targets in the area that she could have possibly come into conflict with) and told her to call the tower upon landing. Aircraft Y then called and asked to depart the airspace to the north. I grabbed the binoculars; and asked the pilot if he was on final for runway 30R. He replied affirmative. I advised him that his transponder was inoperative; and told him to recycle. The pilot departed to the north; and had an intermittent primary only target on the way out.from now on; I will always require a reporting point from every pilot; and have them ident so I am fully aware of where they are.

Original NASA ASRS Text

Title: BJC Local Controller instructed an aircraft to enter a right base leg. The Controller did not realize the aircraft entered a right downwind instead. A departing aircraft made a right turn into the downwind traffic resulting in a reported NMAC.

Narrative: I was working Local 1 Runway 30R (Local 2 was split off due to several in the pattern). Traffic was steady moderate. Aircraft X called inbound from the North; and was instructed to enter a right base for Runway 30R; which he read back. Aircraft Y was on final Runway 30R; cleared to land and issued traffic which was a helicopter on final for the parallel; which he stated he had in sight. Aircraft Y reported 'going around due to the helicopter' and was told to enter right traffic to Runway 30R. During this time; Aircraft X reported 'left; correction right downwind for Runway 30R'; but I was busy with the missed approach; and ensuring that the helicopter was in fact lined up for the correct runway. There was never any primary on my radar scope that I observed in the airspace. Aircraft Z called ready for departure; and was cleared for take-off for a North departure. My focus was northeast of the airport; ensuring that Aircraft Z was rolling; as Aircraft Y was on right base for Runway 30R. Aircraft Z departed; and made an early turn to the North. Unknown to me; the pilot of Aircraft Y had entered a downwind northwest of the field; (no primary target) not the right base that I was expecting.The pilot of Aircraft Z reported that she almost had a NMAC; and had to turn to avoid another [aircraft]. I asked her to ident; (there were no observed targets in the area that she could have possibly come into conflict with) and told her to call the tower upon landing. Aircraft Y then called and asked to depart the airspace to the North. I grabbed the binoculars; and asked the pilot if he was on final for Runway 30R. He replied affirmative. I advised him that his transponder was inoperative; and told him to recycle. The pilot departed to the north; and had an intermittent primary only target on the way out.From now on; I will always require a reporting point from every pilot; and have them ident so I am fully aware of where they are.

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.

Attributes
ACN	1371651
Time
Date	201607
Local Time Of Day	0001-0600
Place
Locale Reference	BJC.Tower
State Reference	CO
Environment
Flight Conditions	VMC
Light	Daylight
Aircraft 1
Make Model Name	Small Aircraft Low Wing 1 Eng Fixed Gear
Flight Phase	Initial Approach
Route In Use	VFR Route
Aircraft 2
Make Model Name	Small Aircraft Low Wing 1 Eng Fixed Gear
Operating Under FAR Part	Part 91
Flight Phase	Final Approach
Route In Use	VFR Route
Flight Plan	VFR
Person 1
Function	Local
Qualification	Air Traffic Control Fully Certified
Experience	Air Traffic Control Time Certified In Pos 1 (yrs) 0.6
Events
Anomaly	ATC Issue All Types Conflict NMAC Deviation - Procedural Published Material / Policy

Bjc local controller instructed an aircraft to enter a right base leg. The controller did not realize the aircraft entered a right downwind instead. A departing aircraft made a right turn into the downwind traffic resulting in a reported near midair collision.