DA40 Diamond Star
Air Traffic Control Developmental
Distraction; Situational Awareness; Time Pressure; Workload
ATC Issue All Types; Conflict Airborne Conflict; Deviation / Discrepancy - Procedural Landing Without Clearance; Ground Incursion Runway
Flight Crew Executed Go Around / Missed Approach
Human Factors; Procedure
I was working local. Aircraft Y and Aircraft X were both on frequency and sequenced to parallel runways; Aircraft Y on 18R and Aircraft X on 18L. Both were issued traffic just prior to them turning base leg. I knew Aircraft Y pilot was good; so I cleared them right away; Aircraft X acted hesitant; so I waited to clear him; so I could check and see if he was lined up for the right runway.Aircraft X appeared to be lined up for 18L when he turned final and; just before I was going to clear him; Aircraft Z who was unfamiliar with the area called up. I thought I had time to acknowledge Aircraft Z because the parallel runway thresholds are offset. By the time I got back to Aircraft X he had landed on 18R without clearance and Aircraft Y had to go around.When Aircraft X showed hesitation I should have said the name of the runway Aircraft Y was landing on instead of calling it 'the parallel runway.' I also should have waited until after I cleared Aircraft X to acknowledge Aircraft Z; even though I was sure Aircraft X was lined up for the right runway.
APC Tower Controller reported an aircraft landing on the wrong runway; causing another aircraft to go-around.