Large Transport
Air Traffic Control Fully Certified
Air Traffic Control Time Certified In Pos 1 (yrs) 17
Communication Breakdown; Confusion; Troubleshooting; Situational Awareness
Conflict Airborne Conflict; Deviation / Discrepancy - Procedural Clearance; Deviation / Discrepancy - Procedural Published Material / Policy
Human Factors; Procedure
I was working local east for about 23 minutes when I was asked to train on the position.  I gave a position relief briefing to the trainee and moved out of the way.  We were landing runways 34L and 34C aircraft were coming in on visual approaches.  Did not notice Aircraft X was tagged for a ILS to 34L behind Aircraft Y also landing runway 34L.Local East is located in the NW corner of the tower cab.  You have Local Assist; and Ground to the left of Local East; with a counter to the right.  When training you have to be behind the trainee and to their right.  The layout of the equipment makes it hard to see the bottom part of the STARS Scope.  With the Trainee just taking over and setting the position up to their pref sets the data tags were blocked so I could not observe them.  Also due to the other aircraft coming in on visual I assumed Aircraft X was also on one and not concerned about the collapsing behind Aircraft Y.  (I found out later that Aircraft X was in fact on a Visual Approach).  I would change the position layout of Local East so that the instructors have a better view of the STARS scope.  The procedures with approach control needs to be change currently if aircraft are tagged on a visual apch we are to assume the have the preceding aircraft or wake turbulence aircraft in sight and are providing their own separation.
SEA Tower Controller describes a situation where he does not know that the second aircraft is maintaining visual separation from the preceding aircraft.