Date: 202011
Local time of day: 1801-2400
Location: ZZZ.Tower, US
Aircraft: Commercial Fixed Wing
Operating under: Part 121
Flight plan: VFR
Airspace: Class C ZZZ
ATC / advisory: Tower ZZZ
Second aircraft: Light Transport; Low Wing; 2 Turbojet Eng
Reporter organization: Government
Reporter function: Local; Trainee
Reporter qualification: Air Traffic Control Developmental
Human factors: Workload; Training / Qualification; Situational Awareness; Distraction; Communication Breakdown; Time Pressure
Second reporter function: Instructor; Local
Anomaly: ATC Issue All Types; Conflict Ground Conflict; Less Severe; Deviation / Discrepancy - Procedural Clearance
Detector: Person Air Traffic Control
When detected: In-flight
Result: Air Traffic Control Issued Advisory / Alert; Air Traffic Control Issued New Clearance; Flight Crew Returned To Clearance
Contributing factors: Aircraft; Procedure; Human Factors; Environment - Non Weather Related
Primary problem: Human Factors

Narrative: While I was training on Local during the busiest session since returning to training aswell as it being my trainer's first time training on local in more than 6 months thetraffic volume was enough that required us to split Local and Ground. For most of theyear our airport had been on a straight runway configuration due to construction and we had only been back to 2 runway operations for about a week. We had Aircraft Zdoing full stop taxi backs and Aircraft A doing stop and go's with afternoon push ofarrivals and departures. I had cleared Aircraft Y to land on runway YY (IFR arrival) andAircraft X was ready to go on runway XX (VFR departure). I asked Aircraft X if heneeded time on the runway or if he was ready to go and the pilot replied that he wasready to go. When I cleared Aircraft X for takeoff the Aircraft Z was on 2.5 mile finaland traffic was exchanged. After takeoff clearance I went back and told Aircraft X toclear for immediate takeoff and the pilot read it back. After 30 seconds the departingaircraft had not began its takeoff roll yet and my instructor told them to start roll now.At this time the arrival was at 1 mile final runway YY. When the arrival was at thethreshold the departure had barely cleared the intersection due to the delay it madeon the runway.To prevent this situation I could have waited until the arrival had landed to clear thedeparture for takeoff. I relied on expectation bias that the departure was going tobegin their takeoff roll immediately and not take time on the runway. I could havealso told the Aircraft Y to reduce to final approach speed to create a bigger hole forthe departure. Knowing when to slow down during busy sessions is also somethingthat could have prevented this situation.


Second narrative: (Training in progress on Local Control on one of the busier sessions seen by all inmonths. Trainer's first time training on LC in over 9 months)Ground had been split off a couple minutes before due to increased traffic andproposed traffic. In addition to multiple arrivals and departures; there was Aircraft Zdoing multiple take offs with full stop taxi backs; and Aircraft A doing stop & go's.Aircraft X taxied out for departure contacted tower holding short of Runway XX. Trafficwas within a mile final for Runway XX so they were waiting to depart. Because therewas another aircraft inbound to Runway YY that we were going to try to depart insideof; Local Trainee was instructed to ask Aircraft X if they would need time on therunway or if they would be ready to go-they said they would be ready. Once the Stopand Go traffic was rolling and we would have our separation; Aircraft X was cleared fortakeoff and told about traffic on final for crossing runway. No hustle from aircraft.Trainee then told Aircraft X cleared for immediate takeoff. He still sat on the runway.Trainer keyed up and told Aircraft X to start their roll.The runway separation was extremely close to call and in the moment the trainerdecided that it was safer to let the arrival land than issue a go around and potentiallycause an evasive maneuver at a critical stage of flight with another departure.We should have just waited to clear Aircraft X for takeoff due to their tendency to beslow and spend time on the runway. Once they did not move with an immediatetakeoff; take off clearance should have been canceled.I do believe that traffic volume and complexity added and distracted the tower-teamfrom checking every aspect of the situation (speeds; timing; rolling of aircraft); but Ido not believe we should refrain from training on busy sessions like these.


Synopsis: ATC Developmental and Instructor reported an aircraft that did not depart as anticipated with an arrival on final.