Narrative:

I was tower controller in charge. We were in limited radar coverage due to radar outages. Aircraft targets below 9000 feet MSL near the airport are typically out of radar coverage. Aircraft X was on an 8 mile final for runway 17L. Aircraft X asked tower about traffic that they observed on TCAS ahead and 800 feet below. The local controller advised his only known traffic was on the runway; but to maneuver as necessary to avoid traffic they might observe. At the point I began to scan for traffic in that area with binoculars. I could easily observe aircraft X but initially didn't observe the traffic. The local controller called departure radar to inquire about who it may have been. The aircraft was in the coast/suspend list for local east. Local east is combined with local west 99% of the time; but the east and west tags (which notate sector position) are used to identify assigned runway. As they were coordinating or shortly thereafter I observed the aircraft and aircraft X initiated a go around on their own. Shortly after aircraft X went around aircraft Y checked in on final for runway 17L. Aircraft X went around and aircraft Y landed without further incident.after further research it appeared that aircraft Y was tagged up for runway 17L and instructed to make a straight in. Shortly thereafter it dropped below radar coverage between 15-17 miles northeast of the airport. At that point the tag dropped off and went into the local east coast suspend list. A position relief briefing then occurred on departure radar. It is unknown to me if that traffic was mentioned in the briefing; however I would suspect it was forgotten about as departure radar had no visual indication of the traffic at that point. Aircraft X was then cleared for the visual approach to runway 17L and switched to tower.scheduled work to the radar should be done on mid shift hours when traffic is slow; and all positions are worked from local west. Slow traffic is obvious; and by one position being worked; there is less of a chance of a disconnect between transferring of control between controllers during abnormal operation. Radar is often worked on during the day rather than in the evening for cost and convenience. Also; being on multi mode we are using center radars. It was chosen to not include denver TRACON's denver and pueblo radars into cos radar for cost reasons. Had that radar data been available aircraft Y may have not dropped below radar coverage.the radar data tag may have not been forgotten about had it been in the coast/suspend list on departure radar rather than local east; or if the tags would not drop off the scope due to lost radar coverage. Stars should be programmed to keep the tags on the scope the controller initiates tag termination.

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Original NASA ASRS Text

Title: COS Tower CIC reported an airborne conflict when an arriving aircraft on final approach disappeared from the controller's radar display due to a scheduled radar outage; limiting radar coverage.

Narrative: I was tower CIC. We were in limited radar coverage due to radar outages. Aircraft targets below 9000 feet MSL near the airport are typically out of radar coverage. Aircraft X was on an 8 mile final for Runway 17L. Aircraft X asked tower about traffic that they observed on TCAS ahead and 800 feet below. The Local Controller advised his only known traffic was on the runway; but to maneuver as necessary to avoid traffic they might observe. At the point I began to scan for traffic in that area with binoculars. I could easily observe Aircraft X but initially didn't observe the traffic. The Local Controller called Departure Radar to inquire about who it may have been. The aircraft was in the Coast/Suspend list for Local East. Local East is combined with Local West 99% of the time; but the E and W tags (which notate sector position) are used to identify assigned runway. As they were coordinating or shortly thereafter I observed the aircraft and Aircraft X initiated a go around on their own. Shortly after Aircraft X went around Aircraft Y checked in on final for Runway 17L. Aircraft X went around and Aircraft Y landed without further incident.After further research it appeared that Aircraft Y was tagged up for Runway 17L and instructed to make a straight in. Shortly thereafter it dropped below radar coverage between 15-17 miles northeast of the airport. At that point the tag dropped off and went into the Local East coast suspend list. A position relief briefing then occurred on Departure Radar. It is unknown to me if that traffic was mentioned in the briefing; however I would suspect it was forgotten about as Departure Radar had no visual indication of the traffic at that point. Aircraft X was then cleared for the Visual Approach to Runway 17L and switched to tower.Scheduled work to the radar should be done on mid shift hours when traffic is slow; and all positions are worked from Local West. Slow traffic is obvious; and by one position being worked; there is less of a chance of a disconnect between transferring of control between controllers during abnormal operation. Radar is often worked on during the day rather than in the evening for cost and convenience. Also; being on multi mode we are using Center Radars. It was chosen to not include Denver TRACON's Denver and Pueblo radars into COS radar for cost reasons. Had that radar data been available Aircraft Y may have not dropped below radar coverage.The Radar Data Tag may have not been forgotten about had it been in the Coast/Suspend list on Departure Radar rather than Local East; or if the tags would not drop off the scope due to lost radar coverage. STARS should be programmed to keep the tags on the scope the controller initiates tag termination.

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.