A British Airways flight descended more than 27,000 feet within minutes after a "technical issue" caused it to declare an emergency and return to London.
According to flight tracking data, the service to Chicago took off from Heathrow Airport at around 12.15pm on Monday.
It declared a mid-air emergency and made a steep descent while flying over Ireland around half an hour later.
Flight tracking data shows the plane had reached its cruising altitude of 36,000 feet before dropping sharply to an altitude of 9,000 feet.
The plane then turned back and spent around four hours zigzagging over the Bristol Channel, before landing back at Heathrow at just after 5pm.
In a statement, British Airways said: "The aircraft landed safely after our highly-trained pilots returned to Heathrow following a technical issue.
"Customers disembarked the aircraft normally and our teams are working to get their travel plans back on track as quickly as possible."
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It is likely passengers will be provided with hotel rooms by the airline until another flight is made available, as stipulated by the UK Civil Aviation Authority.
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