Date: 201604
Local time of day: 1201-1800
Location: PRC.Tower, AZ
Flight conditions: VMC
Light: Daylight
Aircraft: Skyhawk 172/Cutlass 172
Operator: FBO
Operating under: Part 91
Flight plan: VFR
Mission: Training
Flight phase: Taxi
ATC / advisory: Tower PRC
Second aircraft: Trinidad TB-20
Reporter organization: Government
Reporter function: Local
Reporter qualification: Air Traffic Control Fully Certified
Human factors: Communication Breakdown; Training / Qualification; Situational Awareness
Second reporter function: Supervisor / CIC
Anomaly: ATC Issue All Types; Conflict Ground Conflict; Less Severe; Deviation / Discrepancy - Procedural Clearance; Ground Incursion Runway
Detector: Person Air Traffic Control
When detected: In-flight
Result: Air Traffic Control Issued New Clearance; Air Traffic Control Provided Assistance; Flight Crew Executed Go Around / Missed Approach; Flight Crew Requested ATC Assistance / Clarification
Contributing factors: Procedure; Human Factors; Airport
Primary problem: Human Factors

Narrative: Aircraft X; a student solo; called up ready for departure. I had Aircraft Y on a 2.5 mile final. I asked Aircraft X if they could accept a no delay departure. Aircraft X responded; 'Say again?' I instructed Aircraft X to hold short. I received a clear read back of hold short instructions. Then I instructed another aircraft on the downwind to extend downwind; with traffic holding in position phraseology. His reply was garbled; but I understood it on position. As the downwind aircraft had a bad radio all flight; I paid no extra attention to the garbled read back. Aircraft Y then asked; 'Is there an aircraft on the runway?' At that time I saw Aircraft X on the runway and sent Aircraft Y around.


Second narrative: I was working the CIC position; primarily monitoring ground control frequency and also listening to the local controller's transmissions in the cab. I heard the local controller ask Aircraft X a student solo; if they could accept a 'no-delay?' this caught my attention because it is a well-known bad practice at this airport to ask a student solo to depart without delay into a small window.I observed an aircraft on about a 2 mile final and glanced at the radar to see their groundspeed. Then I heard the local controller instruct Aircraft X to hold short. Then I heard the local controller issue an instruction to 'continue; traffic holding in position.' moments later; ground control coordinated a runway crossing at a taxiway that local approved; however local did not include a local requirement to advise the ground controller that traffic was holding in position. This also caught my attention because at the time the crossing was taking place I observed Aircraft X on the runway holding in position and the aircraft on final inside 2 mile final indicating 120 knots.After the crossing was complete I didn't hear the local controller clear Aircraft X for takeoff and began to focus my attention on the traffic on approach. The local controller appeared unaware that there was an aircraft holding in position; looking at the taxiway crossing and the radar display. Aircraft X's strip was not moved to indicate that a line up and wait clearance (LUAW) had been issued.I observed Aircraft Y on inside of 1 mile final now and aircraft x still not rolling. I advised the local controller out loud that 'this isn't going to work.' the local controller initially seemed surprised and began to transmit the call-sign of Aircraft Y on final; then unkeyed; then issued go around instructions to Aircraft Y. Then the local controller instructed Aircraft X to exit the runway and contact ground control. My initial reasoning of why this may have occurred was that the local controller didn't move the strip to indicate the runway was occupied with Aircraft X holding in position and forgot they were LUAW. When asked if Aircraft X was given a LUAW clearance the local controller wasn't sure if they were or were not. I filed an MOR (Mandatory Occurrence Report) and Aircraft X taxied back to the runway; completed their first solo flight remaining in the pattern. After landing Aircraft X was given the brasher warning by me who had now relieved the ground controller from the position. The pilot called and advised that it was their first time flying alone and that they didn't understand some of the things the local controller was saying. After being relieved from position I reviewed the tapes to determine if the local controller issued and subsequently forgot about the LUAW aircraft or if Aircraft X taxied into position without authorization. From the tapes I discovered that Aircraft X was not given a LUAW clearance. After the local controller asked the solo if they could accept 'no-delay' the student took a while to respond with 'say again?' and then the local controller issued hold instructions. An aircraft on left downwind was previously instructed to follow Aircraft Y on 2 mile final but the plan was changed and the local controller issued the following to the downwind aircraft. 'Runway ... Continue; traffic holding in position; tower will call base.'After unkeying; the response was garbled and unreadable with two aircraft transmitting at once. Then the local controller issued a touch and go clearance to an aircraft. The response was also garbled and unreadable. Then ground control coordinates the runway crossing and pilots continue to transmit simultaneously causing a breakdown in communication/read backs. During one garbled read back you can make out the Aircraft Y on final asking tower if there was an aircraft on the runway?I believe Aircraft X heard the transmission to the downwind aircraft and then taxied onto the runway. I don't think the local controller ever saw it; or questioned the garbled and unreadable read backs after their transmissions.I recommend a better runway scan and requests for clarification when read-backs are unreadable. Also issue similar sounding call-sign advisory.


Synopsis: A Controller told an aircraft to hold short of a runway; but did not notice when the aircraft taxied onto the runway. The Controller had an arrival less than two mile final and had allowed another aircraft to cross the runway from an intersection.