The same route regularly takes an hour longer in one direction than the other. The difference comes from fast-moving bands of high-altitude wind that aircraft either ride or avoid.
An aircraft moves through air, not over ground
An aircraft's speed through the surrounding air is fairly constant in cruise, but the air itself is moving, sometimes very quickly.
Speed over the ground is the sum of the two. A strong tailwind adds directly to progress, while an equivalent headwind subtracts from it.
At cruising altitude these winds can be strong enough to change a long-haul journey time substantially, which is why the outbound and return legs differ.
Eastbound and westbound are not symmetric
The dominant high-altitude flow in the mid-latitudes runs broadly west to east, so eastbound flights gain from it and westbound flights fight it.
Dispatchers plan eastbound routes to sit inside the fastest part of that flow and westbound routes to skirt around it, often flying a longer distance to do so.
The longer track is chosen because time and fuel matter more than mileage. A detour that avoids a strong headwind can arrive earlier than the direct line.
Fuel loading responds to the forecast
Fuel is planned against the specific winds forecast for that day rather than an average, since carrying surplus fuel itself costs fuel to lift.
A forecast that proves wrong can force a technical stop, which is why marginal ultra-long routes are the first to be affected in unusual wind conditions.
On the tightest routes the payload may be reduced instead, limiting cargo or occasionally passengers so the aircraft can carry the fuel it needs.
Schedules are built around arrival windows
Overnight timings are not chosen purely for sleep. They exist so the aircraft arrives when the destination airport opens and connecting banks are ready to receive passengers.
Night curfews at many airports narrow the acceptable arrival window further, which fixes the departure time more rigidly than demand alone would.
The wind pattern then has to fit inside that window, which is part of why eastbound overnight flights feel so compressed.
Why the return leg is scheduled longer
Published journey times are built from historical averages for that direction, so the westbound leg carries a longer scheduled block time as standard.
This is also why a westbound flight can land early in unusual conditions while the eastbound equivalent rarely does. The schedule already assumes the tailwind.
Seasonal shifts in the wind band move these times slightly through the year, which is visible in timetables that change block times between summer and winter.