Flyover Country
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.
Tips
When you're supposed to save the three groups of marines, kill the group of marines in the building using grenades then abandon M. Fitzgerald in the valley. If done successfully, Cortana will mention being unable to save any marines. Next, drive to the area where marines are hiding in the hills and try to get the sniper and another marine into the warthog, then run over whoever is left and drive off. Cortana will tell you to wait then report on Captain Keyes being captured by the Covenant if done correctly. Finally head to the marines hiding in the rocks and drive around the area, having the two marines take out all the waves of enemies.
Trivia
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.
