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The plane had to abandon plans to head to Aberdeen Airportand emergency crews "lined the runway" as the plane came in to land at Heathrow. "It was very dramatic," the source added.
Firefighters were reportedly seen waiting to board the plane as emergency services monitored the situation.
The incident appeared less stressful than an event in August 2019, when smoke filled the cabin of a BA plane shortly before landing in Valencia. Passengers reported feeling terrified as they had to evacuate down emergency chutes.
A statement from the airline said the Airbus A321 had "experienced a technical issue" as it approached Valencia from London Heathrow. Three passengers were taken to hospital and were since discharged.