PA-28 Cherokee/Archer/Dakota/Pillan/Warrior
Air Traffic Control Fully Certified
Air Traffic Control Time Certified In Pos 1 (yrs) 7
Communication Breakdown; Distraction; Human-Machine Interface; Situational Awareness
ATC Issue All Types; Conflict Ground Conflict; Critical
Air Traffic Control Issued New Clearance; Flight Crew Requested ATC Assistance / Clarification
Procedure; Human Factors
Aircraft X was doing touch and goes on runway 28R. Aircraft X Mode C was inoperative and I advised the pilot numerous times that his Mode C was inoperative. During the final approach for Aircraft X; Aircraft Y called for IFR release. I received IFR release for Aircraft Y. Aircraft X was over the displaced threshold and I cleared Aircraft Y for takeoff. As Aircraft Y was taking runway 28R for departure he asked if he was still clear for takeoff; because an aircraft landed in front of him down the runway. I cancelled Aircraft Y takeoff clearance and issued LUAW (Line Up and Wait) instructions for Aircraft Y. Aircraft X did not report/say anything; taxied to parking. I then cleared Aircraft Y for takeoff. I believe Aircraft X was obscured from my scan by a post in the Tower cab window when the event occurred.I believe that a lack of traffic lead to complacency. I think that being more vigilant and cautious in my scan could have prevented this; especially when an aircraft was having problems with the Mode C; and radar was inaccurate.
Montgomery Tower Controller reported a runway conflict after clearing an aircraft for takeoff while another aircraft was on short final.