Date: 201903
Local time of day: 1801-2400
Location: ZZZ.Tower, US
Aircraft: Do 328 TJ (Turbojet)
Flight plan: IFR
Flight phase: Final Approach
Airspace: Class C ZZZ
ATC / advisory: Tower ZZZ
Reporter organization: Government
Reporter function: Instructor; Ground
Reporter qualification: Air Traffic Control Fully Certified
Human factors: Situational Awareness; Communication Breakdown; Training / Qualification
Anomaly: ATC Issue All Types; Conflict Ground Conflict; Less Severe; Deviation / Discrepancy - Procedural Clearance; Ground Incursion Runway
Detector: Person Flight Crew
When detected: In-flight
Result: Air Traffic Control Issued New Clearance; Flight Crew Executed Go Around / Missed Approach
Contributing factors: Human Factors
Primary problem: Human Factors

Narrative: I was providing OJT on Ground Control when this event occurred at the Local Control position. I was sitting pretty far back from the Ground Control Trainee in order to give them space and feel like they were working the position independently. However; by doing this and also having my ear piece in the ear facing the Local Controller I was not hearing their transmissions very well. I heard the Local Controller coordinating with Operations/crew about getting on to Runway XX but didn't hear how long they were going to be out there. A short while later I heard Aircraft X check on with the Local Controller and I thought they cleared them to land; but wasn't sure that was all that was said. I thought the Local Controller had a plan to have the vehicles exit prior to Aircraft X getting too close. The Runway Incursion Device (RID) was activated during the event which reinforced my thought that they were aware of the situation. I then turned my attention and focus back to my trainee. After a few minutes I heard Aircraft X say something to the Local Controller and at the same time my trainee said 'Yeah you still have vehicles on the runway.' At this point I looked up at the radar scope and Aircraft X looked to be on about a 1.5 mile final. Before I was able to say anything; the Local Controller told Aircraft X to go-around and told the vehicles that were still on the runway to exit. Aircraft X entered left traffic for Runway XY and landed without incident.I strongly believe in the Tower team concept and felt very responsible for not alerting the Local Controller to the imminent situation. In the future I am going to make several changes to prevent this from occurring again. I am going to try and be closer to the front of the cab when training; so I can scan the runways better and fully hear what the Local Controller is saying. I am going to move my ear piece to the ear opposite of the other controller in the cab to hopefully pick up on their transmissions easier. I am always going to use and teach the phraseology 'continue' for any situation that might prevent a safe landing until it is resolved. I think in this situation the Local Controller just plain made a mistake and forgot about the vehicles on the runway. The Local Controller saying 'continue' instead of 'cleared to land' may have prevented the situation. However; I don't think a phraseology/procedural change is the best solution for something that has such dire consequences. I strongly believe had there been a ground based radar system or something similar; which would have given an alarm to the Local Controller; the situation would not have escalated as far as it did.ZZZ airport is unique in the fact that we get a lot of low visibility days. Due to these conditions the controllers; pilots; and vehicles don't always have the ability to scan the runways and final. I think what really bothered me about this situation; was had it occurred in low visibility operations the outcome could have been disastrous. The controllers and management at this facility are extremely professional; detailed oriented; and work very hard to develop procedures to keep this airport as safe as possible. However; despite our best efforts mistakes still occasionally occur.With the technology that is now available and the traffic level at our airport increasing; I think it is almost negligent that we don't have some sort of ground based radar or something similar to assist controllers. We owe it to the flying public to have that extra crucial layer of safety on top of being vigilant safety professionals.




Synopsis: Tower Controller reported an aircraft go-around due to vehicles on the runway.