Date: 201510
Local time of day: 0601-1200
Location: PAO.Airport, CA
Flight conditions: IMC
Weather elements / visibility: Fog
Light: Daylight
Aircraft: Baron 58/58TC
Operator: Personal
Operating under: Part 91
Mission: Personal
Flight phase: Final Approach
Airspace: Class D PAO
Reporter organization: Government
Reporter function: Supervisor / CIC
Reporter qualification: Air Traffic Control Fully Certified
Human factors: Situational Awareness; Distraction; Confusion; Workload
Anomaly: Deviation / Discrepancy - Procedural Clearance; Deviation / Discrepancy - Procedural Published Material / Policy; Deviation / Discrepancy - Procedural FAR; Inflight Event / Encounter VFR In IMC; Inflight Event / Encounter Weather / Turbulence
Detector: Person Air Traffic Control
When detected: In-flight
Result: Flight Crew Exited Penetrated Airspace; Flight Crew Executed Go Around / Missed Approach
Contributing factors: Airspace Structure; Human Factors
Primary problem: Ambiguous

Narrative: Shortly after arriving at the tower cab and signing in on the Controller in charge computer to start the morning checklist I heard an aircraft fly by the tower. This was unexpected as the weather was foggy with such poor visibility I could not see past the first turn off; and no aircraft made a broadcast on the CTAF. Looking at the radar scope I observed a 1200 target over the airport at 200 feet and continued watching it while working on the morning checklist. I watched the target fly towards the final approach fix for the GPS approach. The aircraft was below the 2;000 foot crossing minimum for the GPS approach and then turned inbound towards the airport; while squawking 1200 the entire time. As I was working on the weather observation; which included visibility 1/8 SM and VV001; I was distracted watching the aircraft continue inbound; flying the profile of the GPS approach; but well below the minimums for the approach. I made an error not recording the obscuration FG in the METAR and ATIS broadcast as I was concerned watching aircraft on the radar scope. The aircraft continued inbound about 1 mile from the field the aircraft descended to 000 indicated and the radar had difficulty picking up the aircraft. I did not see the aircraft outside the window. I then heard the aircraft as it sounded to be powering up. For a brief instant I observed the aircraft through the fog; very low level and not aligned to the runway. As the aircraft went around a second time I observed it depart to the North East. I think I could avoid making future mistakes by making sure that one person works on the weather while the other opener focused on watching the aircraft. The pilot of the aircraft should have been on an IFR clearance to the airport and following appropriate minimums for the approach. The pilot should also have been announcing his actions on the CTAF.




Synopsis: Control Tower observed an aircraft on radar squawking a VFR code following the path of the GPS Approach to the airport. Airport weather was IFR at the time with very limited visibility due to fog. The aircraft descended to a very low level and the Controller briefly observed the aircraft fly by through the fog. The Controller then observed the aircraft depart the area to the northeast. While monitoring this aircraft the Controller failed to include 'FG' for fog in the METAR and ATIS broadcast for the airport.