Date: 201910
Local time of day: 0001-0600
Location: MYF.Airport, CA
Aircraft: Cheetah; Tiger; Traveler AA5 Series
Operator: Personal
Operating under: Part 91
Flight plan: VFR
Mission: Personal
Flight phase: Initial Approach
Airspace: Class D MYF
ATC / advisory: Tower MYF
Second aircraft: Skyhawk 172/Cutlass 172
Reporter organization: Government
Reporter function: Local
Reporter qualification: Air Traffic Control Fully Certified
Human factors: Confusion; Communication Breakdown; Situational Awareness
Anomaly: ATC Issue All Types; Conflict Airborne Conflict; Deviation / Discrepancy - Procedural Clearance
Detector: Person Air Traffic Control
When detected: In-flight
Result: Air Traffic Control Issued New Clearance; Air Traffic Control Separated Traffic
Contributing factors: Human Factors
Primary problem: Human Factors

Narrative: I was working Local 1 for one runway another Controller was working Local 2 for the parallel right runway. I had 4 aircraft inbound and sequenced with landing clearances issued when I noticed there was a target gaining on my number 4 aircraft; approximately 7 miles on final at 1;700 ft. The target was at a point where they would normally check in on my frequency as it is listed as the Tower primary frequency. I asked Controller if he was in communication; Controller responded negative. I reached out probably 4 times to establish communication with this target to no avail. The target crossed the Class D boundary and I tagged it up as a violator. I informed my waiting departures that there was a violator in the airspace and their departure would be delayed. The Ground Controller and Controller in Charge took initiative to grab the light gun and pull the window shade. The target came into visual range and we determined it was a low wing; probably P28A or AA5; and was lining up for the right side runway. The CIC and myself both informed Controller B that the unknown aircraft was going to land on the Right Runway and there was a serious conflict with it and Aircraft X converging on the final approach course. They would be meeting at the threshold at the same time. It wasn't until the CIC yelled at Controller B to turnout Aircraft X that he took action to prevent the collision. When the unknown aircraft was on landing rollout; the CIC and myself determined the callsign to be Aircraft Y which the CIC immediately told Controller B he had been talking to the aircraft the entire time. Controller B's response was 'well he wasn't where he said he was'.Upon reviewing the tapes for both positions; I discovered that Aircraft Y checked in with Controller B stating he was 9 miles east which was precise. Controller B issued straight in the Right Runway and traffic 3 mile final. One minute later; Controller B issued landing clearance number 2. Controller B appeared to have completely forgotten about Aircraft Y after that; issued 'direct the numbers' to Aircraft X who was the tie aircraft; and cleared another aircraft to land number 2 behind Aircraft X. He made only one transmission to attempt to identify the unknown aircraft; Aircraft Y; when the aircraft was about a 4 mile final and had already received a landing clearance. It is my interpretation that Controller B issued a landing clearance to Aircraft Y without having any idea of the position of the aircraft.




Synopsis: Tower Controller reported that another controller forgot about an aircraft that had been cleared to land; resulting in a NMAC.