Air transport enjoy greater freedom of route choice than most other modes. Yet while it is true that the mode is less restricted than land transport to specific rights of way, it is nevertheless much more constrained than what might be supposed but have physical obstacles such as the Rocky Mountains and the great gap of the North Atlantic limited the articulation of air transport networks. While those limitations have fallen, physical geography still affects the geography of intercity air transportation. Aircraft seek for instance, to exploit (or avoid) upper atmospheric winds, in particular the jet stream, to enhance speed and reduce fuel consumption. Volcanic eruptions may also impede air travel by releasing ash in the atmosphere, which can damage and even shot down turbofan engines.