It was the second United flight in two days to make an emergency return to Newark Liberty International Airport. An other United flight returned to Houston after experiencing mechanical issues on Sunday.

United flight 409 to Seattle landed safely at Newark at 9:15 a.m. EDT on Sunday, FAA spokeswoman Kathleen Bergen said.

"Preliminary indications are a possible problem with the air conditioning system," she said.

On Saturday, United flight 96 returned safely to Newark after experiencing problems with its left engine. That flight landed at about 8 p.m. on Saturday, about two hours after taking off for Berlin.

There were unconfirmed reports that debris from a blown tire might have been sucked into the engine and caused the problem.

Flights 409 and 96 were twin-engine Boeing 757s.

Another United flight, 1124 to Boston, returned to Houston "out of an abundance of caution" on Sunday after experiencing mechanical problems with an engine on the Boeing 737, airline spokesman Joe Micucci said.

"We are still conducting a thorough review," he said.

United Airlines is a unit of United Continental Holdings Inc (UAL.N).

(Reporting by Jane Sutton; Editing by Eric Beech)

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