The Boeing 747 had been descending through 3.500 ft approaching at London Heathrow runway 27L on June 6th.

The crew clearly saw a "yellow and orange quadcopter" pass down the left side of the aircraft, with a vertical separation of just 20 ft and horizontal separation of 40 ft.

Investigators of the Airprox Board classified the incident as high-risk.

The drone operator has not been identified by the investigators.

Source: FlightGlobal