Two passengers were taken to hospital injured on a United flight after pilots responded to a collision warning

The Federal Aviation Administration has opened an investigation into the jolt on a United Airlines flight that injured two passengers as the pilots responded to a mid-air warning from the onboard collision system.

United Airlines said Flight 2428 from Newark, New Jersey, “slowed its descent to compensate for another aircraft at a lower altitude” while approaching San Francisco International Airport last Thursday. Both injured passengers were taken to the hospital, United said in a statement.

According to an FAA incident report, one passenger suffered serious injuries and another suffered minor injuries last Thursday when pilots responded to a traffic accident avoidance system, or TCAS RA, resolution recommendation.

Collision resolution advisories are treated as extreme warnings of a possible collision with another aircraft and pilots are urged to act immediately.

Data from FlightRadar24 shows the incident occurred at an altitude of 31,000 feet while the flight was near Lake Berryessa, about 70 miles north of San Francisco. According to FlightRadar24, a Southwest Airlines flight was 3,000 feet below the United flight and a SkyWest flight was 1,000 feet below.

In a statement, the FAA – which is responsible for controlling air traffic – said there was “no loss of safe separation.”

Audio recordings obtained by LiveATC.net showed the pilots declaring a medical emergency and telling air traffic controllers in Oakland that “someone may have broken an ankle and there are passengers who were injured when we had an RA.”

United Airlines told CNN that at the time of the incident, the seat belt sign was in place and one of the injured passengers had left his seat.

The National Transportation Safety Board has not yet launched an investigation, but a spokesperson told CNN that the agency is gathering preliminary information.

Thursday's in-flight incident is the latest possible near-collision incident this month. On September 12, an Alaska Airlines flight had to abort its takeoff from Nashville because a Southwest flight crossed the same runway. Two days earlier, the wingtip of a Delta Air Lines Airbus A350 struck a smaller regional aircraft while both aircraft were taxiing, causing the regional aircraft's tail to fall off.

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