Date: 201601
Local time of day: 1201-1800
Location: SFO.Tower, CA
Flight conditions: IMC
Weather elements / visibility: Cloudy; Turbulence; Rain; Windshear
Light: Dusk
Aircraft: Small Transport
Operator: Corporate
Operating under: Part 91
Flight plan: IFR
Mission: Passenger
Flight phase: Descent
Airspace: Class B SFO
Second aircraft: Widebody; Low Wing; 2 Turbojet Eng
Reporter organization: Government
Reporter function: Instructor; Local
Reporter qualification: Air Traffic Control Fully Certified
Human factors: Training / Qualification; Situational Awareness; Communication Breakdown; Workload
Anomaly: Deviation / Discrepancy - Procedural Clearance; Deviation / Discrepancy - Procedural Landing Without Clearance; Ground Incursion Runway; Inflight Event / Encounter Weather / Turbulence
Detector: Person Air Traffic Control
When detected: In-flight
Result: Air Traffic Control Issued Advisory / Alert; Air Traffic Control Provided Assistance; Air Traffic Control Separated Traffic; Flight Crew Landed in Emergency Condition
Contributing factors: Airspace Structure; Procedure; Aircraft; Weather
Primary problem: Aircraft

Narrative: I was an Instructor training a trainee on Local Control during IFR Southeast configuration of Landing Runways 19 and Departing Runways 19 and 10. Weather was extremely bad all morning with wind heavy out of the South gusting above 30 knots; heavy rain and low visibility in all directions especially directly North of the field on final. We had reports of visibility on final of 1/2 to 1 1/2 miles which makes for the possibility of a missed approach more likely. There were reports of moderate chop and turbulence and wind shear on the upwind almost all morning long which had affected the climb out of many aircraft we departed.  One aircraft departed turning southwest slightly on departure when they should have been turning southeast due to chop on the upwind which made it difficult to fly their aircraft. We were informed from our Radar Coordinator that NCT had lost contact with Aircraft X who was en-route to ZZZ and was located somewhere west southwest of SFO at an unknown altitude. NCT promptly put a hold on all our departures not knowing the intentions of the NORDO aircraft but did advise us that he might be proceeding direct to ZZZ as that was his intended destination and where he would have been headed in the event of loss of communication with ATC.  We were able to monitor the radar target for Aircraft X on the radar display but it was omitting the altitude of the aircraft. The target was south and west of SFO at all times. At one point he flew a few miles west of the field through our Runways 28 Departure corridor and appeared to be heading west on a 280 degree heading to the coast. We continued to allow aircraft to land Runways 19 as Aircraft X was not an issue at this time for any arrival aircraft based on what we were observing.  Aircraft X had tracked a course from about 6-7 miles directly west of the field to a few miles west-southwest of the field where it appeared he began circling over the city of San Bruno as observed on the radar. At this time my Manager who was in the tower advised that we should taxi the aircraft off runways 19 & 10 in the event Aircraft X was more than just a NORDO aircraft and was an actual emergency and tried to attempt to land at SFO. We had both 19R and 10L loaded with aircraft holding in position awaiting release for departure from NCT. My trainee issued the instructions to both aircraft that were holding in position to exit the runway. The tower team had been searching out the windows to the West trying to get Aircraft X in sight as no one knew the exact altitude of the aircraft and last we were informed from NCT that he might be anywhere up to 4000 feet.During this time NCT continued to run ILS approaches to Runway 19L. Aircraft Y was on approach to Runway 19. At some point in time exactly when I can't recall someone in the tower said they saw Aircraft X out the window. We all had been searching and were trying to visually find him in which I saw him briefly. It looked like he was making a circle to the northwest at a very low altitude and then quickly vanished back into the bad weather. I can't recall the exact location of Aircraft Y on final at this time as a lot of focus went toward locating Aircraft X. I believe I was able to locate him visually again but briefly and very faint as he blended into the rain and clouds. He appeared to be heading Southbound towards the 380/101 freeway maybe near Tanforan Mall where I honestly thought that his intentions might be to follow the freeway southbound towards SQL. Not knowing the intentions of Aircraft X's intended flight path; actual altitude if he was climbing or descending; if he had got a visual reference to the ground or is still IMC; I found it in the best interest to allow Aircraft Y to land Runway 19L as we did not have him insight until he was short final and if a go around was issued could potentially led to a mid-air collision in the clouds with Aircraft X.Aircraft X was not in sight until he made an evasive maneuver out of the clouds and attempted to land his aircraft Northbound on Runway 1L opposite direction to the traffic flow of the airport. Aircraft Y landed safely on Runway 19L and was rolling out as Aircraft X was still airborne attempting to land Runway 1L with a heavy tailwind over 20 knots. Aircraft X managed to avoid aircraft taxing on Taxiway B which is slightly west of the runway while landing his aircraft safely on Runway 1L. He exited the runway at the approach end of Runway 19R. Let me remind you that this all happened very quickly and from the point when Aircraft X was visually observed making an evasive maneuver south of the airport which appeared to me to be an attempt to either land his plane or in all honestly I thought he might be crashing somewhere on the freeway or possibly on the airfield there was nothing we could do to help him as we were never had communication with the aircraft and his intentions and nature of emergency were unknown. After reviewing the event I believe the only thing I could have done to make this situation a little safer was to send Aircraft Y around on initial contact anticipating this outcome which in all honesty no one knew was coming. As a reminder there was not a lot of time from the point Aircraft X was first seen to the time he attempted his landing on Runway 1L. All this happened very quickly and I believe that everyone in the Tower did their job and provided the best services to the aircraft that we had communication and control of. I would like to say our Manager deserves a lot of the attention for his help and quick thinking in predicting that this NORDO situation could quickly turn into much more. His instructions to have the aircraft in position on 19R exit the runway saved a potential collision with Aircraft X. I would like to make a note that this is our job to be able to make quick last second decisions and to provide safe instructions to aircraft in the event of an unforeseen or unlikely situation and we did our job.




Synopsis: A NORDO aircraft landed at a different airport than its flight plan destination opposite direction from the runway in use.