A United Airlines Boeing 777-200 suffered a technical incident at London Heathrow.
The Boeing 777-200ER arrived yesterday on flight #UA880 from Houston to London Heathrow.
Today, the aircraft (reg. N78003) was supposed to take off from London to Washington at 12:15 BST.
But an incident is delaying the flight. White smoke was seen from the APU when the aircraft was towed to stand stand 235.
An Auxiliary Power Unit or APU allows an aircraft to operate autonomously without reliance on ground support equipment such as a ground power unit, an external air-conditioning unit or a high pressure air start cart.
The APU is a small jet engine which is normally located in the tail cone of the aircraft but, in some cases, is located in an engine nacelle or in the wheel well.
The fight #UA919 to Washington is delayed. The plane is 23 years old.