Two passengers were injured on a United Airlines flight from New Jersey that was heading to San Francisco last week after the pilots responded to an onboard warning of a potential midair collision. It happened when the plane was about 70 miles north of San Francisco. The flight was at 31,000 feet when the pilots responded to an alert from the Traffic Collision Avoidance System, according to the FAA. That requires a maneuver that alters the plane’s altitude. Flight data showed two other flights were within three thousand feet; they were below the United flight.