United flight from Heathrow to Newark returned to London.
Flight #UA921 departed London Heathrow at 18:15 GMT for a 8 hour-flight to Newark.
The Boeing 767 climbed to 20,000 ft when the crew detected a technical issue with the left engine.
The 31 years old aircraft entered a holding pattern to burn/dump fuel West of Cardiff.
It returned to London Heathrow for a safe landing runway 27R 2 hours and 30 minutes after departure.
