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I have seen that flights will make a turn, after traveling some large distance (say after some kilometers), but that can be done before.

enter image description here
Why not flying directly like in 2?

I am not expecting a passenger flight to do like fighter jets and perform nearly vertical take-offs, or some other manoeuvrers.

Are these flights restricted to do so, or is there any reason to behave so? Or did I misunderstand something?

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    $\begingroup$ for one - imagine an arrow at the end of each line - your plane would be heading in a different direction at the target point. $\endgroup$
    – user1804
    Jun 22, 2016 at 5:46
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    $\begingroup$ If the question is Why an aircraft doesn't turn as soon as possible and at once towards its destination, to fly the shortest route? (better drawing), then I think @Federico 's answer is good. Is there something missing? $\endgroup$
    – mins
    Jun 22, 2016 at 6:32
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    $\begingroup$ Think about how "inefficient" your car route might look to someone who doesn't know you have to drive it on roads :) $\endgroup$
    – Lightness Races in Orbit
    Jun 22, 2016 at 14:59
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    $\begingroup$ Possible duplicate of Why do pilots use airways instead of just "flying direct" every time? $\endgroup$
    – Jay Carr
    Jun 22, 2016 at 17:25
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    $\begingroup$ @NANDA - I hope the edits have all been in the spirit of your original question. The answers are all good, but I think they may have biased the edits (including mine) to your original question. Come on back and give some feedback to ensure we're answering what you're asking, and if one of the answers answers the question best for you, feel free to click the check mark next to it. If not, please clarify and we'll try a different approach. $\endgroup$
    – FreeMan
    Jun 23, 2016 at 12:14

5 Answers 5

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It is mostly standardized procedures.

I take as example Amsterdam airport (AMS), but the following is applicable to most, if not all, major commercial airports.

Let's look at the departure charts, in particular this one:

enter image description here

as you can see, if an airplane is departing via that runway, it MUST follow one of the two specified paths to exit the airspace of the airport through the "ANDIK" waypoint. It has different paths for the other exit waypoints, but they are similarly defined and limited in number.

In the link you will find similar charts for each of the runways.

Within restrictions due to factors like terrain, noise abatement over populated areas (as Dan's mentions in his answer), areas restricted due to security and safety (over city centres, government and military installations, nuclear power plants and such) or used for other purposes (military operations, space launch and such), these routes are defined so they don't intersect (much).

This ensures arriving and departing aircraft don't come close to each other, which greatly reduces the risk of collision and allows responsibility for different routes to be split among several controllers who can each handle their assigned routes mostly independently of the others.

Weather conditions and other factors will dictate which runways are used, but the runway will dictate how the airplane will exit the airport's airspace, after the appropriate exit waypoint has been selected (you generally would not use a waypoint on the north side if you have to go south).

So, to summarize:

  1. you have a limited amount of exit waypoints
  2. you have a limited amount of runways
  3. weather will restrict your choice (well, the airport's tower will do this for you) of runway
  4. your destination will restrict your choice of exit waypoint
  5. for each runway-waypoint pair you will have one or two possible routes

This results in the kind of behaviour you have observed.

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    $\begingroup$ Well, the most important property of the procedures is that the paths don't intersect (too much), so the responsibility for the aircraft can be split between multiple controllers and each can work, within the assigned routes, mostly independently of the others. $\endgroup$
    – Jan Hudec
    Jun 21, 2016 at 18:07
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    $\begingroup$ Another reason might be the Earth's curvature, making routes near the poles appear as big curves going away from Equator's line in sites like flighradar24. But these need a lot more than some kilometers to be visible... $\endgroup$
    – ricardomenzer
    Jun 21, 2016 at 18:25
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    $\begingroup$ And even with just one controller if the paths don't intersect (too much), the controller can be confident that the arriving and departing traffic won't come anywhere close to each other when using the standard routes, significantly reducing their workload and risk of collision. $\endgroup$
    – Jan Hudec
    Jun 21, 2016 at 21:19
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    $\begingroup$ The 2 particular routes shown are west and south of Amsterdam, definitely noise-related. The topological part here is that the climb from Pampus is over open water (IJsselmeer), and is most of the climb after Spijkerboor. $\endgroup$
    – MSalters
    Jun 22, 2016 at 12:42
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    $\begingroup$ @Federico, added. $\endgroup$
    – Jan Hudec
    Jun 23, 2016 at 8:13
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Aircraft are routed based on all manner of different things. Just off the top of my head, here are some of the more common reasons:

  • Standardised routing. It's much easier for controllers to manage aircraft if everyone is following a similar route, and so there are generic routes defined. This is especially true in and out of airports - imagine if dozens of planes were simply arriving in any direction they like, it would be very chaotic to deal with.

  • Noise Abatement. Many areas have noise abatement procedures, where aircraft are either restricted from flying over or the number or flights is limited. It's not uncommon for aircraft to have to fly around certain places.

  • Weather Avoidance. For comfort and safety, aircraft will sometimes be routed around certain types of weather.

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This isn't an answer, just an addition to @federico's answer.

The following image shows departing aircraft from international airports near my location, Düsseldorf (red) and Cologne (blue). The data was recorded over 24h from flightradar24 and put on a map together with some beacons. The dotted lines indicate the angles mentioned below.

enter image description here

Looking into the departure charts for Düsseldorf, one finds for take-offs to south-west this, in layman’s words:

  • Turn hard right and fly away from NOR at 0°
  • Turn right to LMA (Roughly, there are more points not shown in the map, and more than one route), then depending on destination turn left to 203° or 209°.
  • Turn left and fly to NOR at 174°, then turn left to GMH at 86°

Finally, there's a chart showing all this together, though this is more about noise: enter image description here

For cologne it's obvious that there are similar rules. It's also clear that the flights from Düsseldorf to south-east are guided around Cologne by flying a little towards GMH first.

So, not an answer, just to show that aircraft fly as they are intended to do, though there are a few exemptions.

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    $\begingroup$ You could have edited Federico's answer, but I think yours does a pretty good job of standing on its own. $\endgroup$
    – FreeMan
    Jun 23, 2016 at 12:15
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Just like you can't drive your car straight from point A to point B because there might not be a direct road between the points, aircraft can't necessarily fly from point A to point B because there might not be airways that directly connect the points.

Also, most traffic between major airports will follows established procedures. These are either Standard Information Departure (SID) - for flights departing or Standard Terminal Arrival Route(s) (STAR or STARS), for flights arriving.

These standard departure and arrivals procedures are put in place based on the geography, weather or other restrictions at or near the airport and don't have to coincide with the flight's arrival or departure direction.

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    $\begingroup$ Not everyone (especially one asking a question like this) is familiar with SID and STARS. Your answer would be improved by defining those within your answer or including links to Wiki, Skybrary or other sources of definitions. (Sure, someone could Google them, but remember, we're here to answer questions, not create more work!) $\endgroup$
    – FreeMan
    Jun 21, 2016 at 12:04
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    $\begingroup$ @FreeMan - fixed. $\endgroup$
    – Burhan Khalid
    Jun 22, 2016 at 6:57
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    $\begingroup$ Thanks, @BurhanKhalid, I could have done that as well, I was hoping our newer member would come back & take care of it. ;) $\endgroup$
    – FreeMan
    Jun 22, 2016 at 12:02
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Just one clear answer why it is also done on short trips (so not depending on the earths curvature that much): Airplanes have the allowance to fly on a flight level (FL), given in feet / 100 AND refering to an imaginary surface level where the standard pressure of 101325Pa is located. So FL 130 is 13000 feet above the level with 101325Pa ambient pressure. Some turns are only allowed at that FL to avoid collisions with other airplanes.

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