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Coriolis…Earth’s Rotation…and the airplane, oh my!

Asked by: 31796 views ,
General Aviation

I just read the following statement:

"Pilots need to correct their flight path based on the earth rotating under the airplane, which is the Coriolis effect.  Take for example a flight leaving from San Francisco that is heading to New York. If the earth did not rotate, there would be no Coriolis effect and thus the pilot could fly in a straight path to the east. However, due to Coriolis effect, the pilot has to constantly correct for the earth's movement beneath plane. Without this correction, the plane would land somewhere in the southern portion of the United States."

I'm a beginner pilot, but this certainly does not seem correct!  The plane is in the Earth's atmosphere which moves with the Earth as it rotates.  No correction should be needed for the Earth's rotation.  If correction WAS needed, I suppose a plane moving in the opposite direction of the Earth's rotation would get to a destination faster than a plane moving with the Earth's rotation.  But I've NEVER heard of having to correct for rotation of the Earth...

Secondly, the Coriolis effect and wind.

The Coriolis effect is an apparent motion/deflection (dependent on the position of the observer), deflection of moving objects when they are viewed from a rotating reference frame.

When it comes to winds, the direction of deflection is always to the right in the Northern Hemisphere and to the left in the Southern Hemisphere.  However, are winds really moving in a straight line...but only SEEM to be deflected left or right due to Coriolis?  Or are the actually being DEFLECTED because of Coriolis?

And winds move counter-clockwise around low pressure and clockwise around high pressure (in Northern Hemisphere).  Are they TRULY moving in these directions due to pressure gradient force...or merely seem to move in these directions due to Coriolis?  Perhaps pressure gradient force and Coriolis both have an effect on the winds.

Confusing and rather broad topics, I know.  However, any elaboration would be appreciated.

 

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12 Answers



  1. MaggotCFII on Feb 01, 2011

    Hello Anonymous: Your quoted source please.

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  2. VFR Student on Feb 01, 2011

    My quoted source is:
    http://geography.about.com/od/physicalgeography/a/coriolis.htm
    Surely, this is not true!

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  3. Steve Pomroy on Feb 01, 2011

    Hi VFR Student.
    That’s an interesting question.  The short answer:  “In theory, yes.  In practice, no.”.
     
    The Coriolis Effect is a result of the conservation of angular momentum.  As you move along the Earth’s surface, your angular momentum (in the absence of any outside torques) must be conserved.  So as you move closer to (or further away from) the axis of rotation (which passes through the North and South Poles), you tend to be deflected off your path.  We see on weather maps the effect of this deflection:  cyclones and anti-cyclones rotate according to the rules of Coriolis.
     
    The effect is very slight over short distances and even slighter at low speeds.  So even though the correction is necessary in principle, it tends to get lost in all of the other corrections we make as pilots.  In other words, the effect is so small that it gets lost in the background noise of everything else going on.  The most important influence of Coriolis on pilots is the effect it has on wind direction.  By the time you account for winds, changing winds, varying tracks due to ATC clearances, minor errors and corrections made by the flight crew, etc., Coriolis doesn’t even rank a mention as something to be corrected for — even on long-range flights.
     
    As a more concrete example of why we don’t need to worry about it, refer back to my earlier statement, which included, “in the absence of any outside torques“.  As we fly, the aircraft is acted upon by the wind.  The forces imposed by the winds produce torques around the axis of the Earth (where a torque is a force multiplied by the moment arm).  These torques negate the pure, unmodified effect of the conservation of angular momentum. So, in effect, by the time we’ve corrected for the winds, we’ve also corrected for the Coriolis effect without thinking about it.
     
    Cheers,
    Steve
    http://www.flightwriter.com

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  4. Koehn on Feb 01, 2011

    Agree with Steve 100%. You do compensate for the Coriolis Effect; it’s called the Wind Correction Angle.

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  5. Anonymous on Feb 01, 2011

    So, the winds are actually BEING deflected…and not truly just moving in a straight line (with the apparent APPEARANCE of being deflected?)
    I’m thinking of a ball being tossed on a merry-go-round.  The ball is really moving in a straight line.  However, it’s the merry-go-round that’s moving underneath the ball thats makes the ball appear as if it’s being deflected.

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  6. MaggotCFII on Feb 01, 2011

    Adding;
    “Coriolis Effect” deflects the weather.  Take a look in FAA AC 00-6A, Chapter 4, pages 25-29, here is the link:
     
    http://rgl.faa.gov/Regulatory_and_Guidance_Library/rgAdvisoryCircular.nsf/0/c2df8d9d7471617786256a020078083a/$FILE/AC%2000-6A%20Chap%204-6%20-%20Aviation%20Weather,%201975.pdf
    If the link does not work AC 00-6A is available on the FAA.gov site.
    We are always flying in some sort of weather system, benign thru horrid, which is being influenced by the “Coriolis”.
     
    So for practical purposes I would argue that – since the “Coriolis” effects the weather systems in which we fly,  a “Coriolis” correction has already taken place before we do our flight planning.
    So you can’t blame “Coriolis” if you get lost on a cross country!  Enjoy your flight training!
     
     
     
     
     
     
     

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  7. Wesley Beard on Feb 01, 2011

    Yes.  The winds are actually being deflected.  I never really liked the merry go round example.  To me, it implies that the ball is outside the “jurisdiction” of the merry go round and we (who are in the rotation with the merry go round) see it as a curved path.  I think, perhaps, it’s used to illustrate the winds want to equalize the easiest way possible and that is a straight line.
    I much prefer the old style record player example.  In this example, a turntable with a record is rotating around the center.  Draw a straight line on the record while it is rotating and then stop the turntable.  The end result of the “Straight” line is a curved path in the direction of rotation.  To me, this is more plausible as it makes the wind a part of the rotation of the Earth.  If you were to draw a straight line on the bottom of the record while it is rotating you will get a curved path in the opposite direction.
     
    +1 for Steve.  He answered the question very well I thought.

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  8. skyboyCFI on Feb 01, 2011

    VFR Pilot,

    The article u linked is correct.

    http://geography.about.com/od/physicalgeography/a/coriolis.htm

    Desribes coriolis effect very well. I’m gonna print that and use it in teaching. Thanks for the link.

    BTW, the toilet water rotation myth is always a funny topic.

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  9. Andrew K on Feb 06, 2011

    The winds are traveling in a different path due to coriolis.  This will happen to any “free moving” objects, hence objects that cannot steer themselves like the wind, a thrown rock, or another projectile.  Snipers and artillery gunners have to compensate for coriolis effect when making really long shots. A plane is not really a “free object” because it has control over it’s direction after it’s set in motion and can fly to its destination as it pleases and negates coriolis while doing so.
    Understanding the coriolis effect does have a lot to do with understanding apparent motion of the wind from the reference/perspective of standing on earth and vice versa.
    Take a look at these two youtube videos, they visualize the concept really well.
    http://www.youtube.com/watch?v=mcPs_OdQOYU
    http://www.youtube.com/watch?v=49JwbrXcPjc
    Imagine multiple objects (from the video) converging to one point, that would be the equivalent to air in low pressure systems.  Multiple objects diverging from one point would be like high pressure systems.

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  10. MaggotCFII on Mar 06, 2011

    Nice article in the March 2011 issue of the AOPA Pilot, page 91, “Zooming through the jet cores”.
    Speaks to Coriolis force and pressure gradient force.
    Interesting read.

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  11. Micah on Mar 07, 2011


    This is a very good question and a very interesting discussion. My first discussion on this topic was with an uncle who is an engineer who asked, “Why do jets fly faster from West to East than from East to West?” My answer was jetstream and his offer was the Earth’s rotation. I thought my answer was more correct than his but, more importantly, it wasn’t an issue to argue over with him. Later I realized that these answers are almost identical (even if offered in ignorance of the other) because jetstream is a byproduct of the Earth’s rotation (much related to the Coriolis effect). 
     
    Here are some thoughts on the four statements in the referenced quote. Hope these are interesting and helpful (and fully correct, or at least close enough.)
     
    “Pilots need to correct their flight path based on the earth rotating under the airplane, which is the Coriolis effect.” 
     
    Like Steve said very well (and others have added), Coriolis affects the weather and you, the pilot, flies (in) the weather. If there were no weather (purely hypothetical suggestion) then it seems possible that you may need to correct for Coriolis, but…
     
    “Take for example a flight leaving from San Francisco that is heading to New York. If the earth did not rotate, there would be no Coriolis effect and thus the pilot could fly in a straight path to the east.”
     
    False, or at least mostly false. This language presumes a “flat-earth” path between San Francisco and New York. Of course, we know that the earth is not flat but instead round. A “straight line” path may be your true course between San Francisco and New York, but this “straight line” path is not one single heading for the entire distance. Because the Earth’s surface is curved, this straight line path is actually a curved path (that’s why the heading changes). If you need more explanation on that, the sources below may help.
     
    “However, due to Coriolis effect, the pilot has to constantly correct for the earth’s movement beneath plane.”
     
    Again, see the responses above.
     
    “Without this correction, the plane would land somewhere in the southern portion of the United States.”
     
    I’m not going to answer this with absolute certainty, but I believe this presumes that you fly a single heading instead of a straight path or using some (erroneous) “straight line” projection from a flat map. Remember that a straight line path is actually a curved line…
     
    Some interesting links:
    http://opinionator.blogs.nytimes.com/2010/03/21/think-globally/
    http://en.wikipedia.org/wiki/Geodesic
    http://en.wikipedia.org/wiki/Great_circle
    http://www.mapfolds.com/2010/11/19/flight-paths-on-canvas/

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  12. tvc on Mar 24, 2014

    THE WINDS are a reflection of the Coriolis force …. if not also the reaction of air particles moving in a rotating coordinate system.

    We are not on a gold standard any more. Gold is valued in dollars not the other way around.

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