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On April 4, 2012, El Al flight 4 from New York to Tel Aviv flew on an almost-a-rhumb-line route eastward rather than on the usual almost-a-great-circle line.

Why might an airline choose to have one of its airplanes do such a thing?

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What do you mean, it actually crossed the Atlantic from NY to France? –  littleadv Apr 18 '12 at 4:05
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@littleadv, not even! It flew over Portugal and Spain, never flying over France at all. –  msh210 Apr 18 '12 at 15:13
    
El Al never cease to surprise me... Hope you had a good flight nevertheless...:-) –  littleadv Apr 18 '12 at 16:43
    
What makes you think it was a rhumb line and not just some other curve that deviated somewhat from a great circle, and happened to look somewhat like a rhumb line? –  Ben Crowell Apr 12 '13 at 4:00
    
@BenCrowell, I didn't say it was a rhumb line. How do I know it was almost a rhumb line, do you mean? A map of the flight's progress was available to me throughout the flight. –  msh210 Apr 12 '13 at 6:31
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2 Answers 2

There's a number of reasons why a flight might fly a different route, with Weather on-route being the most common.

However for this specific flight, there's probably a far less common reason - Solar Flares!

You may recall that around the 2nd of April this year there were a number of major solar flares, which have the potential to impact a number of aspects of flight, including communications. As the impact of the flares is greater around the poles, for the few days following a large number of flights took more southerly routes, and a few flights that rely on taking a very northerly route were actually canceled as a result.

The more direct route results in using more fuel, but presuming that they had planned for that route they would have taken on extra fuel, and thus other than the potential for the flight time being increased there's no real impact due to the different route.

You can view the first part of the flight path this flight took, along with the full flight plan on FlightAware (Currently viewable without a FlightAware account, but by the time you view it you might need a free account if it's more than 2 weeks old)

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+1 and thanks. But the path on that map doesn't match the map shown to passengers in-flight, which shows we flew over Portugal and Spain, south of France. (See also comments on the question, above.) –  msh210 Apr 18 '12 at 15:16
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The dotted part of the line in FlightAware is the planned path, for areas where FlightAware doesn't have the actual path. If the flight actually took a different path it means that the path was changed after takeoff, which isn't that uncommon, normally for weather/winds, although in this case it could have just been a general desire to stay further south due to the solar flares. –  Doc Apr 18 '12 at 15:23
    
Curious, then, that the planned path was already modified significantly from the great-circle path, and that the actual path was modified still further. Seems almost like they were trying to avoid most of western Europe (first planning to fly north of it, then flying south of it). –  msh210 Apr 18 '12 at 16:32
    
This map doesn't make sense at all, why would they fly around the whole of Europe? Usually the path is from Iceland, between Scotland and Ireland, England, Germany, Greece, Cyprus, and Israel. They would actually stay closer to the poles and for longer, with the path they had on FlightAware... Your explanation matches the OP's described path, though. –  littleadv Apr 18 '12 at 16:44
    
Good idea, but it doesn't seem to be corroborated by evidence. Today's path also crosses northern Spain. I suspect westerly winds are the main explanation. –  Gilles Apr 18 '12 at 18:53
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Wikipedia has a thought on why this might occur:

Over longer distances and/or at higher latitudes the great circle route is significantly shorter than the rhumb line between the same two points. However the inconvenience of having to continuously change bearings while travelling a great circle route makes rhumb line navigation appealing in certain instances

Of course, with computer controlled flight, any such inconvenience vanishes.

So then we move to the Airlines.net forums:

Several years ago it was quite common for us to be offered a vector 3/4 of the way across the country during the climb out of the departure airport. Eventually flight ops sent out a memo telling us not to accept those clearances anymore because they were rhumb-line direct and actually were longer than the filed route.

However they then go on to say:

Of course current winds aloft must be considered and it does not take much wind to wipe out the difference between great circle and some other logical routing.

So possibly due to prevailing winds, it made less sense to use a great circle line, and to use another nearly-as-convenient method of plotting course, and in this case went for a rhumb-line track?

Naturally without actually asking the airline, we'll never know in your flight's case, but these are some of the possible reasons.

Either that, or perhaps the pilot wanted to test it for a software reason. Or possibly they knew the software was having problems with great circle paths. I can think of a few other 'pettier' reasons, but again, we'd just be guessing.

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