United says Flight 821 landed safely, and passengers were deplaned normally at the gate.
Passengers will continue their trip to Mexico City on a new flight.
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Witness describes tire from United flight falling into SFO lot
The aircraft had 105 passengers and five crew members on board.
United says Airbus 320 has three hydraulic systems for redundancy purposes. They say a preliminary investigation shows there was an issue with one system on the aircraft.
This is the fourth incident involving a United jet this week -- a plane skidded off a runway in Houston Friday morning and, on Thursday, a wheel came off a plane above SFO. On Monday, flames shot from the engine of a plane over Texas.
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