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As part of a project I'm doing I plotted the flight paths of about 2000 flights around the globe, I noticed over both the North Atlantic and Pacific that there is a pattern to the flight paths:
You should be able to see Europe on the right and the USA on the left. Why is there such a common pattern in the flight paths? They all seem to cross every 10 degrees longitude and 1 degree latitude. Is there any explanation for this?
Over the North Atlantic, there are no predefined low-level or high-level airways as there are over the continents. In order for aircraft to get a tailwind for eastbound flights and get out of the wind for westbound flights, Canadian and UK air traffic control set tracks based on the expected wind conditions twice a day.
The lines you are seeing are called an 'Organized Track System'; is this part of the world, they're called 'NAT' (North Atlantic) Tracks. Five to six roughly-parallel tracks will be defined and move around every day, but they will generally go between whole degrees of latitude and 10° increments of longitude.
For example, tonight's (Dec 31/13) track BRAVO ('B') will go from a intersection in the UK called...
ERAKA (at 58°N/10°W) to
60°N/20°W to
61°N/30°W to
61°N/40°W to
60°N/50°W to
MOATT (at 58.025°N/59.928°W) to
LOMTA (at 57.203°N/62.619°W).
TADEX 55/10 55/15 55/20 56/30 56/40 55/50 OYSTR STEAM. TADEX, OYSTR and STEAM are VOR fixes at the entry and exit of the track and 55/10, 55/15 are the grid fixes. These are entered into the FMC literally as you see them, 55 north, 10 west and the autopilot flies from one to the next, hence the patterns you see.
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Historically, when flying over land, aircraft follow jet routes or airways (same thing, different altitude), which are pre-defined routes from one land-based navigational station (such as VOR) to another. Variations in terrain (such as mountains), location of the VOR, and other airspace considerations all affect the location of routes available to aircraft in flight.
Over water, however, there are no terrain features, navigation is not conducted primarily with VOR, and there are far fewer airspace concerns in general. Therefore, waypoints are placed in a grid (why not?), which encourages uniformity in flight paths over time, which is what you have discovered.
It's not that the autopilot forces the airplane over certain coordinates. The autopilot follows a programmed flight plan (that is typically entered by the pilot or the dispatcher). There is a huge database of standard reporting points that the flight plan is made up of, which is why you see similar routes. So in answer to your question, it's not the autopilot, but rather there are published reporting point and standard routes that most pilots program their autopilot to follow. The autopilot is the servant, not the master :)
This helps with search and rescue (heaven forbid!) efforts if an airplane is lost, then it can give SAR a good starting point.
GPS is supposed to help create more direct routes, but total reliance on GPS-aided navigation isn't fully implemented world-wide.
This map should help explain it.
The navigation points are spaced out evenly, and the planes aim for those points.
