Narrative:

I was working the dr control position. Aircraft X was VFR in the east practice area. I heard aircraft Y call inbound for cos. Thinking it was aircraft X in the east practice area I told the aircraft to enter a left base runway 17R and advise the current ATIS. After subsequent transmissions with other aircraft I noticed aircraft X did not appear to be inbound. Just as I was about to ask why; aircraft Y asked for the GPS runway 17R from hidan. I assumed that aircraft X changed his mind and decided they wanted the GPS approach and then the full stop on runway 17R. I told the aircraft to proceed to hidan. Sometime later aircraft Y asked me if he should switch to tower. I said negative; I thought you wanted to go to hidan first (aircraft X was about 15 miles northeast of cos northbound). Aircraft Y said negative he was told to enter and was now on a left base to runway 17R. It was at this time I realized I was talking to two aircraft and only knew about one. I asked for the full callsign of the aircraft on base and was told aircraft Y. I asked my aircraft going to hidan to say full callsign and was told aircraft X. I noticed a V tag in tower's airspace and asked dra to give tower a heads up on what was going on. I put in a new code for aircraft Y and asked to them to identify. Sure enough they were already in tower's airspace and starting to turn final for runway 17R. There is a procedure at cos to abbreviated callsigns (especially local VFR) to the last three digits. This is not always ideal when similar sounding callsigns come into play as I learned the hard way during this session. In this particular case the tower was not busy and the miscommunication did not lead to any errors or loss of separation. But I feel the possibility for a huge safety error exists. While trying to identify the two different aircraft I fell behind with some of my other traffic. Not to mention the aircraft that flew into tower's airspace unidentified squawking VFR and I had no idea until after the fact.I think everyone is aware of the dangers that can arise because of miscommunication as a result of similar sounding callsigns. At cos there is an aircraft Z and an aircraft a both locally based. With those two aircraft we never abbreviate their callsigns. Obviously the same rule needs to apply to aircraft X and aircraft Y. I have seen both aircraft before and remember thinking that something adverse could happen if controllers abbreviate either callsign. I would take it one step further and question the safety ramifications of using abbreviated call signs at all. It was seriously disturbing for me to realize that I let an aircraft go unidentified into tower's airspace and I had no idea until it was too late.

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Original NASA ASRS Text

Title: COS Approach Controller describes a situation where two aircraft have the last three numbers/letter and the confusion it caused her.

Narrative: I was working the DR control position. Aircraft X was VFR in the east practice area. I heard Aircraft Y call inbound for COS. Thinking it was Aircraft X in the east practice area I told the aircraft to enter a left base Runway 17R and advise the current ATIS. After subsequent transmissions with other aircraft I noticed Aircraft X did not appear to be inbound. Just as I was about to ask why; Aircraft Y asked for the GPS Runway 17R from HIDAN. I assumed that Aircraft X changed his mind and decided they wanted the GPS approach and then the full stop on Runway 17R. I told the aircraft to proceed to HIDAN. Sometime later Aircraft Y asked me if he should switch to tower. I said negative; I thought you wanted to go to HIDAN first (Aircraft X was about 15 miles NE of COS northbound). Aircraft Y said negative he was told to enter and was now on a left base to Runway 17R. It was at this time I realized I was talking to two aircraft and only knew about one. I asked for the full callsign of the aircraft on base and was told Aircraft Y. I asked my aircraft going to HIDAN to say full callsign and was told Aircraft X. I noticed a V tag in tower's airspace and asked DRA to give tower a heads up on what was going on. I put in a new code for Aircraft Y and asked to them to IDENT. Sure enough they were already in tower's airspace and starting to turn final for Runway 17R. There is a procedure at COS to abbreviated callsigns (especially local VFR) to the last three digits. This is not always ideal when similar sounding callsigns come into play as I learned the hard way during this session. In this particular case the tower was not busy and the miscommunication did not lead to any errors or loss of separation. But I feel the possibility for a huge safety error exists. While trying to identify the two different aircraft I fell behind with some of my other traffic. Not to mention the aircraft that flew into tower's airspace unidentified squawking VFR and I had no idea until after the fact.I think everyone is aware of the dangers that can arise because of miscommunication as a result of similar sounding callsigns. At COS there is an Aircraft Z and an Aircraft A both locally based. With those two aircraft we never abbreviate their callsigns. Obviously the same rule needs to apply to Aircraft X and Aircraft Y. I have seen both aircraft before and remember thinking that something adverse could happen if controllers abbreviate either callsign. I would take it one step further and question the safety ramifications of using abbreviated call signs at all. It was seriously disturbing for me to realize that I let an aircraft go unidentified into tower's airspace and I had no idea until it was too late.

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.