BLR-FRA-SFO Path with ETOPS 120 min rule timeMaps generated by the Great Circle Mapper - copyright © Karl L. Swartz.
Ever wondered why airplanes take weird paths from one city to another? There are multiple reasons, the primary one being that Earth is not flat and is more of an "oblate spheroid" or "ellipsoid" in shape. So the shortest distance between two points on Earth is not a straight line like in planar geometry and is a "Great Circle" (if considered a spherical) or a geodesic path. The other reasons include regulations stating that twin engined aircraft have to be able to fly to an alternative airport on failure of one of the engines within a specified period of time, taking advantage of tail wind etc.
See the ETOPS (Extended-range Twin-engine Operational Performance Standards) page in Wikipedia for more information on regulations governing flight paths for twin engine commercial aircraft.
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