A London-bound passenger plane had to divert to Canada while halfway across the Atlantic after passengers reported that the aircraft "dipped like a ferry".
Cabin crew "ran down the aisles" as the United Airlines' Boeing 757 started losing height, said passenger Lizzie Laming, 22, of Sittingbourne, Kent.
The plane, which had set off from Washington DC on Sunday night, turned round and flew two hours back west before landing safely on Monday morning at St John's in Newfoundland, Canada.
Passengers are being taken on to London on other flights.
Ms Laming, accompanied by her friend Ellie Cauldwell, 22, from Bristol, said she had gone to sleep when she was awakened by the movement of the plane.
She went on: "The plane dropped and it felt like being on a ferry when it goes up and down.
"Most of the cabin crew were running up the aisles and we were not told anything for a while. We could tell from the on-screen maps that we had lost height - from around 40,000ft to 20,000ft - and that we had turned round. We gathered there was something wrong with one of the engines."
Ms Laming and Ms Cauldwell, who both attended Exeter University, were on a travel tour which included the US, having just completed their degree courses.
Both were put on a flight from St John's to Newark, New Jersey, US, and were being flown back to London from there.
A United Airlines spokesman in London said: "UA130, a Boeing 757-200 which departed Washington Dulles airport at 7.06pm on Sunday to Heathrow was diverted to St John's, Newfoundland, and landed safely at 2.55am today. We are re-accommodating customers on alternative flights."