This question already has an answer here:
Please give a basic explanation of how commercial aviation traffic is routed for readers without any foreknowledge:
- Can commercial aircraft only follow airways like cars on highways ?
- Or are airlines free to choose a straight line between airports (excluding probhibited and restricted airspace) ?
- Or is it a mix of both ? If so:
- When must airliners follow airways ?
- And when are they permitted to leave them (absent emergency) ?
- What differences (if any) are there between traffic over sea and over land ?