I'm not a frequent traveler. Some years ago I found flying from west of U.S to east U.S. took 1 hour less time than return trip, I didn't find answer at that time. Recently I decided to investigate this by the help of google search.
The answer is simple: WIND . But wait, why do most people in the world find flying east takes shorter time, does the wind over Earth surface always flies east? Then I studied some flights schedule around the equator such as Singapore to Tokyo and some flights in South America, there's no significant difference in flying time between goes and come-backs. After I read some articles, below factors should be considered:
- Wind and Earth Rotation or Prevailing Winds . You know the directions of the winds in different areas of the world.
- The locations of most cities with flights. Open the world map, you can find the much more cities are located between lattitude 30 and 60, in this area, the direction of wind is from the west to the east.
- You should consider long distance flights (>3 hours), otherwise the airplane may not spend most of time at cruise speed, and many other factors (such as reserving time for air traffic control) can affect the flying time.
- Coriolis effect. Yes, it affects the direction of the wind and it also affects the trails of airplanes, i.e. when the airplane flies from low lattitude to high lattitude, the plane will be deflected to the east. However, Coriolis itself nearly does not affect the differences of flying time between goes and come-backs. For example, if you fly northwest from Singapore to North Eurpoe, the real flying distance may be longer than you imagine from the map, the return trip is also longer.
Am I right? I'm not 100% sure. I'm just trying to explain the reason :)