Newark Liberty International Airport, seen here in July 2021, saw an emergency landing on Thursday morning.
CNN  — 

A mechanical problem on a United Airlines flight prompted an emergency landing early Thursday at New Jersey’s Newark Liberty International Airport.

“After our aircraft experienced a mechanical issue shortly after takeoff, it remained in the air to burn fuel and then landed safely,” United said in a statement. “Passengers deplaned at the gate and a new aircraft is scheduled to depart this morning.”

An initial maintenance inspection suggests an issue with a hydraulic pump on the aircraft, which had 256 passengers on board for the flight, United Airlines said.

Sparks flew off the plane as it climbed after takeoff, according to a video posted online that purports to show the flight.

The Federal Aviation Administration is investigating what happened with Flight 149, which departed Newark airport late Wednesday night, then declared an emergency. The Boeing 777-200 aircraft had been headed to Sao Paulo, Brazil, before returning to Newark, the FAA said.

The hydraulic system on the Boeing 777 runs key systems, such as the landing gear, flaps and brakes. There are three redundant hydraulic systems on the plane, meaning two can fail and the airplane can still operate normally.

The flight circled over the Atlantic Ocean to burn fuel before returning to the airport about two hours after departure, flight tracking data shows.

CNN has reached out to the Port Authority of New York and New Jersey for comment.