Date: 201707
Local time of day: 0001-0600
Location: IAD.Tower, DC
Flight conditions: Marginal
Weather elements / visibility: Windshear
Light: Night
Aircraft: B737 Next Generation Undifferentiated
Operator: Air Carrier
Operating under: Part 121
Flight plan: IFR
Mission: Passenger
Flight phase: Taxi
ATC / advisory: Tower IAD
Second aircraft: B737 Next Generation Undifferentiated
Reporter organization: Government
Reporter function: Local
Reporter qualification: Air Traffic Control Fully Certified
Human factors: Human-Machine Interface; Fatigue; Distraction; Communication Breakdown; Situational Awareness; Time Pressure; Workload
Anomaly: ATC Issue All Types; Conflict Ground Conflict; Less Severe; Deviation / Discrepancy - Procedural Clearance; Deviation / Discrepancy - Procedural Published Material / Policy; Ground Incursion Runway; Inflight Event / Encounter Weather / Turbulence
Detector: Person Air Traffic Control; Person Flight Crew
When detected: Taxi
Result: Air Traffic Control Issued Advisory / Alert; Flight Crew Returned To Clearance
Contributing factors: Weather; Human Factors; Procedure; Staffing
Primary problem: Procedure

Narrative: I was working LC1 [Local1]; LC2; and LC3 combined at the time of the event. The position was combined at LC3 which faces the west side of the airport. This configuration makes working the east side of the airport (RWY1R/19L) difficult. I also had planes on 2 separate frequencies 120.1 which is LC1 frequency and 134.42 which is LC3 frequency. This also creates confusion and potential for transmissions being stepped on.There was weather in the area with a thunderstorm over the airport. This added complexity and extra focus necessary to issue weather updates; wind shear alerts; and RVR [Runway Visual Range] readings. I have received many briefings in the last year stressing how important weather dissemination is. This was my major focus during this event which could have lead me to get tunnel vision.I had Aircraft X number 1 for RWY 1R and Aircraft Y number 2 for RWY 1R. Both planes were cleared to land and given weather advisories. I was working a departure in position on RWY 30 that was given weather information and was holding in position looking at the weather and deciding if they were able to depart. My attention was on the weather and trying to give accurate information and watching the TDWR [Terminal Doppler Weather Radar] for new wind shear readings because it was continuously changing. Aircraft X had safely landed and missed the last high speed exit (K2) so I instructed them to turn left J1.At this time Aircraft Y was on a 1.5 mile final and with the speed of Aircraft X continuing down the runway I did not think timing would be a factor. The visibility was drastically reduced and the end of the runway was not visible from the tower. I was relying now on the ASDE-X [Airport Surface Detection Equipment] to insure that Aircraft X had cleared the runway. I saw what I believed to be Aircraft X committed to the K1 exit while Aircraft Y was short final and I allowed them to continue. The Controller in Charge saw Aircraft X's target still on the ASDE-X now at J1 and instructed me to send Aircraft Y around.I issued go around to Aircraft Y and it was too late he had already crossed the landing threshold and responded that they 'had already touched down'. I then realized my mistake of issuing Aircraft X to turn off the runway at J1 instead of K1 which caused confusion to the pilot because J1 is 500 feet farther down the runway. I believe Aircraft X was exiting the runway at K1 and realized he made the wrong turn and then turned back to comply with my exiting instructions of J1.This event happened because of an incorrect runway exiting instruction; weather at the airport; the position being combined due to lack of staffing and not recognizing a developing situation due to fatigue.I recommend that when weather is a factor and adds complexity to workload that LC1 be separate from LC2 and LC3 and worked from the proper position in the tower. The tower was improperly staffed and I believe with holdover overtime this event could have been prevented.




Synopsis: IAD Tower Controller reported that a flight crew missed the taxiway turnoff; turned around to exit; causing aircraft on final to be sent around. Instruction was too late and aircraft landed on occupied runway.