A spokesperson for British Airways said there was a technical issue and that the aircraft later landed safely.
A British Airways flight dropped about 27,000 feet in 10 minutes while en route from London to Chicago on Monday, according to flight data.
The plane started the descent from 36,000 feet to 9,000 feet about 30 minutes after it departed London, tracking data from FlightAware shows. It appeared to be a controlled descent.
A spokesperson for British Airways said there was a technical issue and that the aircraft later landed safely.
Flight data shows the plane turned around over Ireland and circled for a few hours, apparently to burn off fuel.
It lfinally landed back at Heathrow Airport Monday evening local time. Video of the landing was captured by an automatic tracking camera streaming live to the Flightradar24 YouTube channel.
"Customers disembarked the aircraft normally and our teams are working to get their travel plans back on track as quickly as possible," the airline spokesperson said.
Amanda Forssell and Sarah Ploss contributed to this report.