Narrative:

A cessna 140; was inbound from the south on left downwind. I had issued the C140 runway 26R; and he read back runway 26R. I issued traffic to the C140 on a biplane that was on a close-in base for runway 26L. A few moments later; the biplane was over the numbers [for] runway 26L; and I told the C140 that traffic was no factor; and approved a short approach and gave clearance to land on runway 26R. The C140 read back that he was cleared to land on runway 26R. An aircraft checked in on the ILS practice approach; and was in conflict alert with an unknown aircraft; I issued a traffic alert. The targets were overlapping; and I was having trouble finding the altitude of the unknown aircraft. After this situation was resolved I got an IFR release on citation; who was waiting to depart runway 26L. I scanned runway 26L; and cleared the citation for takeoff. Ground control then shouted 'what's that cessna doing?!' I saw a cessna crossing the threshold of runway 26L; and realizing that it was the C140 I sent him around telling him he lined up for the wrong runway. The citation had not moved forward; and was stationary holding short of runway 26L. The C140 went around; and the citation departed without further incident. Apparently my scan failed me in this instance. I did scan runway 26L prior to issuing a takeoff clearance; and did look at short final; but I failed to see the C140 at that time. This is an instance where the tower team concept yet again proved its worth. I must practice to improve my scan to spot aircraft on short final runway 26L. It is the farthest point from the tower; and can be difficult to quickly spot aircraft at this point; extra vigilance is required.

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Original NASA ASRS Text

Title: Tower Controller described a required go-around event when he/she failed to properly scan for arrivals before issuing a takeoff clearance with traffic on short final.

Narrative: A Cessna 140; was inbound from the south on left downwind. I had issued the C140 Runway 26R; and he read back Runway 26R. I issued traffic to the C140 on a biplane that was on a close-in base for Runway 26L. A few moments later; the biplane was over the numbers [for] Runway 26L; and I told the C140 that traffic was no factor; and approved a short approach and gave clearance to land on Runway 26R. The C140 read back that he was cleared to land on Runway 26R. An aircraft checked in on the ILS practice approach; and was in Conflict Alert with an unknown aircraft; I issued a Traffic Alert. The targets were overlapping; and I was having trouble finding the altitude of the unknown aircraft. After this situation was resolved I got an IFR release on Citation; who was waiting to depart Runway 26L. I scanned Runway 26L; and cleared the Citation for takeoff. Ground Control then shouted 'what's that Cessna doing?!' I saw a Cessna crossing the threshold of Runway 26L; and realizing that it was the C140 I sent him around telling him he lined up for the wrong runway. The Citation had not moved forward; and was stationary holding short of Runway 26L. The C140 went around; and the Citation departed without further incident. Apparently my scan failed me in this instance. I did scan Runway 26L prior to issuing a takeoff clearance; and did look at short final; but I failed to see the C140 at that time. This is an instance where the Tower team concept yet again proved its worth. I must practice to improve my scan to spot aircraft on short final Runway 26L. It is the farthest point from the Tower; and can be difficult to quickly spot aircraft at this point; extra vigilance is required.

Data retrieved from NASA's ASRS site as of July 2013 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.