A flight path drawn on a screen frequently curves far north or south of the straight line between two cities. Most of that apparent detour is not a detour at all.
Flat maps distort what is short
The shortest path between two points on a sphere is a great circle, and projecting a sphere onto a rectangle stretches those curves. A northern arc on the screen is often the direct line.
The effect is strongest on long routes at higher latitudes. Flights between continents in the northern hemisphere pass much closer to the pole than a flat map suggests.
Passengers reading a moving map see this as an inefficiency. The aircraft is following the shortest available track.
Winds are worth chasing
Strong, persistent winds sit in narrow bands at cruising altitude. Flying with them saves fuel and time, and flying against them costs both.
Flight planning systems calculate the fastest track rather than the shortest, weighing distance against forecast wind. The chosen route can be substantially longer in miles and shorter in time.
This is why the same route differs in duration by direction. The eastbound leg on many long routes is consistently quicker than the westbound one.
Airspace is not uniformly open
Some airspace is closed to particular operators for political reasons, and some is closed to everyone because of conflict. Closures force reroutes that can add hours.
Military exercise areas and restricted zones create smaller diversions that come and go. Traffic flow management also assigns routes to keep aircraft separated across busy regions.
Overflight fees are charged by the countries crossed, and route choice takes them into account. A slightly longer path avoiding an expensive corridor can be cheaper overall.
Twin-engine rules shape ocean crossings
Aircraft with two engines operate under rules limiting how far they may be from a suitable diversion airport, expressed as flying time on one engine. The permitted distance depends on the aircraft and the operator's approval.
On ocean crossings that constraint bends the route toward island and coastal airports. The path connects a chain of possible diversions rather than cutting straight across.
Approvals have widened over time, which is why direct routes across large ocean areas became viable. The rule still shapes planning where suitable airports are sparse.
Weather and terrain move the final track
Thunderstorms are avoided laterally, sometimes by a wide margin, and crews request deviations in flight. These are small changes that accumulate visibly on a tracking map.
Volcanic ash is avoided entirely because it damages engines, and an ash cloud can close a corridor at short notice.
Terrain matters at the ends of the flight. Departure and arrival paths near mountains follow procedures designed around obstacle clearance rather than around the direct line.