The Airbus A320 took off from San Francisco International Airport with 110 people on board, headed to Mexico City, and was diverted to L.A. "due to an issue with the aircraft's hydraulic system," United said in a statement.
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The plane landed at Los Angeles International Airport just before 4:30 p.m.
"The flight landed safely and passengers were deplaned normally at the gate," the airline said, adding that passengers will be taken to Mexico City on a new aircraft that was scheduled to depart Friday later.
"This aircraft type has three hydraulic systems for redundancy purposes. Preliminary information shows there was only an issue with one system on this aircraft."
No injuries were reported and the FAA has since launched an investigation.
This is the fourth incident involving United aircraft this week.
United plane rolls off runway in Houston
Earlier this morning, passengers were evacuated from a United flight at Houston's George Bush Intercontinental Airport after the plane rolled off a runway and got stuck in the grass.
United said the Boeing 737 MAX 8 was coming in from Memphis with 160 passengers and six crew members on board.
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In video from the airport, it appears the plane was tilted to the side at one point and the left wing was touching the ground, off the runway. The airline did not immediately say what caused the plane to veer off the runway.
United plane loses tire after takeoff
On Thursday, a tire fell off a United plane after it took off from SFO, initially bound for Japan.
The tire debris landed in one of SFO's employee parking lots. United said that flight had 235 passengers, 10 flight attendants and four pilots -- for a total 249 people onboard.
Tire falls off United flight after takeoff from San Francisco; diverted plane lands safely at LAX
Video from AIR7 HD showed the plane touching down on a runway at LAX as fire engines and other emergency vehicles were standing by. It was then towed away. No one onboard the plane was hurt.
Bubble wrap sucked into United plane engine
A United flight returned to Houston's Bush Airport shortly after takeoff Monday after a flight engine reportedly ingested bubble wrap, the airline said.
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The incident, which was caught on camera, happened shortly after the Boeing 737 took off from IAH, headed toward Fort Myers, Florida.
It's unclear how it happened, but the airline said employees are always looking for litter on tarmacs.
"The plane was nosediving, and the pilot was bringing the plane back up," a passenger told our sister station in Houston. "The plane was also rocking back and forth a lot."
United said a new aircraft took those passengers to Fort Myers later that evening and the aircraft involved was back in service soon after.
United responds
In an earlier statement, United said it was investigating the recent incidents to "understand what happened and learn from them."
"Much of this work is conducted together with Boeing, the FAA, and the NTSB as well as with the manufacturers of individual components. While this work is ongoing, each of these events is distinct and unrelated to one another. Safety is our top priority, and we'll continue to do everything we can to keep our customers and employees safe."