United flight 718 turned around after the captain reported that the fuel levels from one of the engines indicated an imbalance in fuel pressure.
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Since the plane was full of fuel when it landed, fire engines were on standby as the plane landed on the tarmac.
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"We're really sorry about this. Soon after we took off, it was pretty obvious the fuel wasn't flowing in a balanced way. So the fuel tanks were quickly becoming lopsided in weight so we needed to get to Mexico City as soon as possible," the pilot said in a video captured on a cell phone.
Because of the extra fuel and weight on landing, the pilot told passengers once the plane landed, that there was a potential for the landing gear to overheat and that the fire trucks were on standby as a precaution.
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No injuries have been reported. A spokesperson for United Airlines tells ABC7 News that the flight "returned to Mexico City out of an abundance of caution due to a mechanical issue."
The airline rebooked most passengers on another aircraft later in the evening and booked others on alternative flights.