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  • Vortex separation ?

    Is there defined separation for landing and departing traffic ?
    [photoid=5629316]
    Or is it purely pilot discretion. This one looked close and I have seen go arounds with what seemed greater separation.

    Jid

  • #2
    Hi Jid, it's known as 'Wake Turbulence Separation' and it varies according to local legislation.

    In the case of your (lovely) photo very little risk exists as the peak of the 747's wake turbulence is at the point of rotation, by which stage the Airbus will be firmly on the runway. In Japan they would have cleared the Airbus to land with the words 'caution wake turbulence from departing 747, cleared to land'. Again legislation varies, some countries require the departure to be over the upwind end of the runway before issuing a landing clearance, others simply airborne, or, like here, that the controller is satisfied that there is no chance of collision.

    The major risk in your shot is if the next departure was just airborne in the region of the 747's rotation point, that is why generally up to three minutes are required after a heavy departure to allow the wake to dissapate.

    It's a really complex issue, one that has cost lives and airframes, including a famous DC-9 accident behind a DC-10, many lost airframes behind 757s and the loss of an American A300 due to overcontrolling during a wake turbulence encounter. I have experienced wake in many types and places, including after passing 1,000 feet below a SQ 777 at FL360....wow, what a thump!

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    • #3
      Thanks for that AJ, I think the clearance given in this instance was "cleared to land after the departing 747". In the states I have heard landing clearance given when the departing aircraft has still been rolling, presumably past their V1 point.

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      • #4
        I believe the minimum seperation between a lightweight (in this case the A320) and a heavy is around five miles?

        Correct me if I'm wrong.

        Dale

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        • #5
          Dale, I have heard MAN ATC clear heavies for take off with landing traffic under three miles but as AJ says I think it is a down to local controllers, as well as the landing pilots !

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          • #6
            On my flight from Chicago to Toronto earlier this year on United. They had Ch9 tuned to the controller. They were alternating departures and landings on the same runway, and my flight was cleared to takeoff at least a good 5 secs before the arriving aircraft touched ground.

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            • #7
              Would it not really matter that much as the vortex's sink and the 747 is one the ground?

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              • #8
                I've landed behind an A321 before in a 172. I came in high (we're told to stay above the jet's approach path and land after his landing point). I've also landed behind a 757 before, which is a little nerve wracking, since they're the ones with the worst wake. But as always, if I'm coming in tight behind a jet, ATC always says "Cessna XXXX Caution wake Turbulance Follow the 737 on Final. Cleared to land Runway 23" I usually always read back the wake call too so I'd say "Caution the wake, I've got the 737 in sight and cleared to land on 23, Cessna XXXX" God I love being a pilot ! Always fun to see an MD80 nose-up out the you know what landing right in front of you when you're holding short!

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                • #9
                  Originally posted by Crism
                  I've landed behind an A321 before in a 172. I came in high (we're told to stay above the jet's approach path and land after his landing point). I've also landed behind a 757 before, which is a little nerve wracking, since they're the ones with the worst wake. But as always, if I'm coming in tight behind a jet, ATC always says "Cessna XXXX Caution wake Turbulance Follow the 737 on Final. Cleared to land Runway 23" I usually always read back the wake call too so I'd say "Caution the wake, I've got the 737 in sight and cleared to land on 23, Cessna XXXX" God I love being a pilot ! Always fun to see an MD80 nose-up out the you know what landing right in front of you when you're holding short!
                  http://www.caa.co.uk/docs/299/Flying...20CTR_2005.pdf

                  Thats what i was reading, somewhere in there it says about landing deaper in the runway.

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                  • #10
                    Originally posted by AJ
                    I have experienced wake in many types and places, including after passing 1,000 feet below a SQ 777 at FL360....wow, what a thump!
                    Can you imagine what "fun" a WW2 Me109 pilot must have had buzzing a "swarm" of B17's
                    RobB

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                    • #11
                      Steve's got the right idea.

                      In the US, General Aviation pilots are taught the general rule of thumb to:

                      Fly above the jets approach path to touchdown BEYOND the jets touchdown point

                      &

                      Rotate before and climbout above the departure path of the heavy aircraft before you.

                      Theoretically, you wouldnt have to wait at all as long as you followed these two rules because wake turbulence vortices have the tendancy to sink about 200-400fpm. BUT, just cause windy conditions can move the wake around.. its best to wait a couple minutes.

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                      • #12
                        This is an interesting topic. After reading AJ's answer, I was looking on the internet to learn some more about wake turbulence and found an interesting website: http://www.faa.gov/ATpubs/AIM/Chap7/aim0703.html. It explains how the vortex is generated (pressure differential over the wing surface). After reading this I wondered how it was possible that the highest risk for wake turbulence is at the point of rotation. Why not when the plane is speeding over the runway? Is there some one around here that can explain these physics? Has it something to do with maximum lift?

                        Thanks, Pamela

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                        • #13
                          Originally posted by pdeboer
                          This is an interesting topic. After reading AJ's answer, I was looking on the internet to learn some more about wake turbulence and found an interesting website: http://www.faa.gov/ATpubs/AIM/Chap7/aim0703.html. It explains how the vortex is generated (pressure differential over the wing surface). After reading this I wondered how it was possible that the highest risk for wake turbulence is at the point of rotation. Why not when the plane is speeding over the runway? Is there some one around here that can explain these physics? Has it something to do with maximum lift?

                          Thanks, Pamela
                          Good question Pamela, the reason for it is when an airplane is at a slower speed, in order to create the amount of lift required to keep it airborne, the wing needs to be going through the air at a higher angle. This increases the amount of lift a wing is creating by making the difference between the pressure on the top and the bottom greater. When the plane is going faster, there is more air flowing over the wing for a given time, so this increase in pitch isn't needed. Hope that made sense, I'll try to find a picture to hep illustrate what I'm talking about.


                          A graph showing how as you increase the angle of attack (the angle of the wing vs the relative wind) lift increases up to a certain point where it stalls and the lift created decreases rapidly
                          Last edited by screaming_emu; 2005-11-17, 20:27.

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                          • #14
                            Thanks for explaining , it does help a lot although I have to admit I had to watch the figure some while before I understood. It's clearly all in the angle of the wing. That now also explains to me why the turbulence is very low after the point of landing (wake ends).

                            cheers, Pamela

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                            • #15
                              I just realized what I posted was the lift vs drag curve instead of the angle of attack vs lift curve, but it does look pretty much the same and still demonstrates what I'm trying to say.

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