Following 2 recent Fume Events on this Aircraft, the last one on 15th November 2017 (shown on a recent update), the aircraft was taken out of service for 3 nights and was allegedly fitted with a new APU (Auxiliary Power Unit). It departed 18 November as Flight BA 49 London Heathrow/Seattle & 18 November as Flight BA 48 Seattle/London Heathrow arriving this morning - 19 November 2017. The APU is located in the tail end of commercial aircraft. Large engines are heavy and require a lot of torque to spin up, more than a battery can generate so, the battery spins up a much smaller APU jet engine. High speed BLEED AIR from the APU is used to spin up the main engine. Once the engines are started, the APU is no longer required, but it does provide a couple of secondary functions: . It provides cabin air and electric power before the engines are started (saving battery power). . It provides an emergency source of electric power in the event of engine failure. . It can start the engines mid-flight in an emergency. If you or any of your friends and family are involved in a Fume Event - please report it. Use the FUME REPORTING APP.