From Halopedia, the Halo wiki
Flyover Country is an achievement in Halo: The Master Chief Collection. It is unlocked by beating the 20 minute par time on Halo. The achievement is worth 10 Gamerscore and can be unlocked on any difficulty.
The achievement's name is a reference to a pejorative Americanism referring to the parts of the United States between the East and West coasts which are often only experienced from high above while flying between the country's largest urban centres.