The poster, which was printed by Reading-based VGL, is positioned on the roundabout into terminals 1, 2 and 3 and measures 37x3.8m.
It is expected to be seen by more than 100m passengers during 2009.
The job was turned around in seven days on self-adhesive vinyl using the VGL's recently installed Durst Rho 320R wide-format printer, and overlaminated to give weathering and UV protection.
Bmi sales and marketing director Katherine Gershon said: "London Heathrow has been the home of Bmi for over 40 years and this sign symbolises our long-term presence."
The advert was designed by advertising agency M&C Saatchi and the space was negotiated by MPG with the site contracted for 12 months.
MPG chief executive Marc Mendoza said: "It is the most noticeable site at the world's busiest airport. The image can't be missed."
Bmi outdoor ad set to be seen by 100m passengers at Heathrow
Business airline Bmi has secured a "prime slot" at London Heathrow for a year-long run of its printed outdoor advert.
