Air Traffic Control Fully Certified
Air Traffic Control Time Certified In Pos 1 (yrs) 5
Distraction; Situational Awareness
ATC Issue All Types; Conflict Ground Conflict; Critical; Deviation / Discrepancy - Procedural Clearance; Ground Incursion Runway
Air Traffic Control Separated Traffic; Flight Crew Executed Go Around / Missed Approach
Procedure; Human Factors
I had put Aircraft X in position RWY 27 behind Aircraft Y arrival. Aircraft Y exited off the runway quickly and I gave him instructions to taxi into the ramp on my frequency since it was a very short route and ground control did not have any traffic in the area. Then my mind started thinking about how much work it is for a pilot to switch frequencies rapidly when going such a short distance etc. As my mind drifted I forgot that I had put Aircraft X into position and had it in the back of my mind that the only traffic I had was still 5+ miles out.After a long moment of silence someone said 'Radio check' and I replied loud and clear then scanned the runway; that's when I realized my mistake. The ASDE-X alerted and I sent Aircraft Z around on a heading and altitude. I coordinated with TRACON while Aircraft X held in position then he departed without further incident.An ELMS on the dangers complacency? I am not really sure. I have always prided myself with always being alert and scanning most especially when it is slow and easy to let thoughts distract you from maintaining alertness. I will certainly be more cognizant of maintaining alertness during slow traffic; and keep an eye out for it with my coworkers as well.
Aircraft Z was on very short final when ASDE-X alerted and initiated go around because Aircraft X was holding in position. I was working ground and played no role in the event.
SAN Local Controller puts an aircraft into position and forgets about it while thinking about other things. ASDE-X alarms and local sends aircraft on final around.