Date: 201703
Local time of day: 1201-1800
Location: PAO.Airport, CA
Flight conditions: VMC
Light: Daylight
Aircraft: Small Aircraft; High Wing; 1 Eng; Fixed Gear
Operator: Personal
Operating under: Part 91
Mission: Training
Flight phase: Initial Approach
Airspace: Class D PAO
ATC / advisory: Tower PAO
Second aircraft: Small Aircraft; High Wing; 1 Eng; Fixed Gear
Reporter organization: Government
Reporter function: Ground; Supervisor / CIC
Reporter qualification: Air Traffic Control Fully Certified
Human factors: Confusion; Situational Awareness; Communication Breakdown
Anomaly: ATC Issue All Types; Conflict NMAC; Deviation / Discrepancy - Procedural Published Material / Policy
Detector: Person Flight Crew
When detected: In-flight
Result: Air Traffic Control Issued Advisory / Alert; Flight Crew Took Evasive Action; Flight Crew Requested ATC Assistance / Clarification
Contributing factors: Aircraft; Human Factors; Procedure; Staffing; Airspace Structure
Primary problem: Human Factors

Narrative: At the time of the incident I was working Ground Control with CIC combined. Aircraft X and Aircraft Y were both flying the traffic pattern during heavy traffic. Aircraft Y was sequenced to follow Aircraft X while upwind. Aircraft Y reported the traffic in sight and was instructed to follow. Due to two other aircraft with similar sounding call signs the local controller started questioning his sequence. Aircraft Y was on downwind in trail of Aircraft X and asked his sequence. The local control replied he would call his base and the aircraft responded; but without a call sign.Due to heavy traffic at the time we had downwind extensions to about a 4 mile final to Runway 31. Aircraft X was on a wider downwind than Aircraft Y and turned base. While scanning the radar I was concerned about targets merging at the same altitude of 800 feet and asked the local control to issue a traffic advisory which he did (but did not include traffic alert). After that call Aircraft X reported someone flew right over their head. They did not report a proximity; declare a Near Mid-Air Collision; or state that they had to take evasive action at that time. Aircraft Y questioned his sequencing again; but made no mention of a NMAC to the tower. At that time I did not start filing a report based on the lack of 'Near Mid Air Collision' being stated on frequency and that Aircraft X taxied back to the runway for more pattern work without further complaint on frequency.I would note that traffic was heavy with about 8 aircraft in sequence to the runway; another 3 or 4 waiting to depart and the situation was complicated by similar sounding call signs taking each other calls and several aircraft were not answering calls in a timely manner. After Aircraft Y landed; they voluntarily called the tower to ask what happened and what their part to avoid such situations in the future. At this time I knew Aircraft Y had been in close proximity to one of the other aircraft in the pattern and was late in getting sequencing instructions as a result; but there was no mention of their proximity to Aircraft X made during this discussion. I recommended that he fly the plane in the best way to keep him safe first; then tell ATC what actions he has taken and ask for a new sequence. Not believing a NMAC had occurred; I did not take down any information. Approximately an hour after the event; the instructor aboard Aircraft X drove to the Tower and reported at the gate that they needed to talk to airport management because they were almost killed out there. At that time; the staffing was only two Controllers with no management and we were both on position working. She asked for me by name. I had previously given her a tower tour and I guess she remembered me. Traffic was light; so I gave my positions to the other Controller and went downstairs to talk with the instructor. I informed her that I was not management; but could take down the information; file a report and it would be passed up the chain of command from there. At that point she told me she was flying Aircraft X and while turning base to final she had an aircraft fly about 50 feet over her head and that she took evasive action descending to avoid a collision. I spoke with her for a while telling that safety is our main concern; I am required to file paperwork based on her story and that this will go out. I also told her that if something that like ever happens; it helps ATC to hear that you want to file a 'Near Mid-Air Collision' as soon as possible to start the paperwork. It was also discovered that the instructor had tried using her student's cell phone to call the tower; but the student's phone was not in the local area code and therefore did not connect her to the tower. At this point in time I started contacting my Supervisor on his cell phone; who was also designated acting Air Traffic Manager and informed him of what happened. I then did a quick Falcon 3 Radar review of what happened and started to informthe ROC (Regional Operation Center); who also called QC (Quality Control); to advise them I was starting to fill out a report for a NMAC that occurred earlier.I was working Ground Control and CIC (Controller in Charge) at the time of the incident. While I was doing my best to try and assist Local Control; I should have called up the Controller or Supervisor to open up Cab Coordinator. Our current SOP (Standard Operating Procedure) is being updated currently and might be worth adding guidelines for when to staff the Tower Coordinator (a position that could greatly assist local control) up. I realize this might result in times of it being staffed at times when it is not needed; but it could also take away from a CIC's trying to figure out if they need it opened or not. I am sure a balance could be struck.




Synopsis: PAO Tower Controller was notified by a pilot of a NMAC they encountered in the pattern.