A United Airlines Boeing 787 Dreamliner about to cross the Atlantic was forced to make an emergency landing on September 26 after the cockpit computer displays suddenly went black and the flight management system entered a “degraded mode.”

The nine-year-old Boeing 787 (registration: N27958) had departed Los Angeles International Airport around 1:30 p.m. for a routine 12-hour flight to London Heathrow, but was eventually diverted to United’s main hub in Chicago O’Hare.

According to Canada’s Transportation Safety Board, the plane was flying at a cruising altitude of 35,000 feet in a remote region over Hudson Bay with up to 257 passengers on board and the cockpit computer displays on the captain’s side of the plane suddenly went black.

At the same time, both flight management computers went into emergency mode, which limited the aircraft’s capabilities and, among other things, disabled lateral navigation.

The pilots of Flight UA231 had to declare a mayday and then began to divert to Chicago O’Hare, where United has a large maintenance base. It took more than two hours for the plane to reach Chicago, but the pilots managed to get the plane to the ground without incident.

The flight to London Heathrow then had to be canceled and passengers had to book alternative flights while technicians worked to fix the system problems. The plane was grounded for more than a day but has been back in service since September 28th.

The incident came just days before the Federal Aviation Administration (FAA) confirmed it was closing a safety investigation into United Airlines after concluding there were no “significant” concerns that needed to be addressed by the agency .

The FAA launched its rare safety investigation into United in March after a series of mishaps, including an incident in which the landing gear of a United 737MAX collapsed after the plane rolled into the grass at the end of the runway at Houston Intercontinental Airport in March was 8.

In a brief statement in early October, the FAA said it was concluding its investigation after finding no “significant safety issues” at the airline.

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