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  • Rudder question

    Hi,
    I just joined this interesting forum and I have a few questions regarding aircraft.I visited my home country Germany last year and spent many hours watching aircraft in Hanover airport.I was watching in particular this Airbus A320 which was sitting idle on the ground.Now suddenly the rudder moved all by itself.In fact many aircraft seem to use the rudder when idle on the ground.What is the reason behind that?I thought it only gets used in the air.

  • #2
    I doubt if it moved "all by itself". The crew were likely doing the control check, which includes checking the rudder function.

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    • #3
      The rudder is moved (powered) by hydraulic actuators. When there is no hydraulic power (pressure) on the aircraft, like when is sitting idle the wind can move the rudder. So what you saw was normal.

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      • #4
        Originally posted by 474218 View Post
        The rudder is moved (powered) by hydraulic actuators. When there is no hydraulic power (pressure) on the aircraft, like when is sitting idle the wind can move the rudder. So what you saw was normal.
        I've never seen a transport-category aircraft, whose rudder could "move in the wind". When hydraulic pressure is removed, servojacks or actuators (depending on aircraft type) are designed to lock the respective control surface in either the neutral position or in whatever position it was when pressure was removed. This is why sometimes you may see an airplane at the gate with its rudder deflected fully, but you won't see it just flapping in the breeze. A control surface that heavy could do damage if allowed to just move.

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        • #5
          Originally posted by Spad13 View Post
          I've never seen a transport-category aircraft, whose rudder could "move in the wind". When hydraulic pressure is removed, servojacks or actuators (depending on aircraft type) are designed to lock the respective control surface in either the neutral position or in whatever position it was when pressure was removed. This is why sometimes you may see an airplane at the gate with its rudder deflected fully, but you won't see it just flapping in the breeze. A control surface that heavy could do damage if allowed to just move.
          I've never seen one that didn't "move in the wind".

          When hydraulic power is removed ailerons and elevators droop and the rudder is free to move. There are no locks to keep them in neutral. If there were locks the control surfaces they would not have to be balanced. Balancing keeps the control surface from fluttering when the damping of the hydraulic pressure is removed during flight.

          Check out pictures of parked airliners (Boeing/Airbus/Douglas/Lockheed) if they have hydraulicly powered controls, you will see the drooped ailerons and elevators and the rudder moved where ever the wind takes it.

          Maybe you have heard of "gust locks"? They are sometimes installed to prevent control surface movement during high wind conditions.

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          • #6
            Originally posted by 474218 View Post
            I've never seen one that didn't "move in the wind".

            When hydraulic power is removed ailerons and elevators droop and the rudder is free to move. There are no locks to keep them in neutral. If there were locks the control surfaces they would not have to be balanced. Balancing keeps the control surface from fluttering when the damping of the hydraulic pressure is removed during flight.

            Check out pictures of parked airliners (Boeing/Airbus/Douglas/Lockheed) if they have hydraulicly powered controls, you will see the drooped ailerons and elevators and the rudder moved where ever the wind takes it.

            Maybe you have heard of "gust locks"? They are sometimes installed to prevent control surface movement during high wind conditions.
            I'm sorry, but you are being too general.

            Yes, I've seen drooped ailerons, but I've never seen them flap in the wind. I for sure have never seen rudders flapping in the breeze.

            On the airplane I fly, the flutter dampers you mentioned act as control locks when pressure is removed. This removes the need for gust locks for the yokes, the surfaces won't move at all until pressure comes back.

            I also thought there were other reasons for control surface balancing, but I could be wrong.

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            • #7
              Originally posted by Spad13 View Post
              I'm sorry, but you are being too general.

              Yes, I've seen drooped ailerons, but I've never seen them flap in the wind. I for sure have never seen rudders flapping in the breeze.

              On the airplane I fly, the flutter dampers you mentioned act as control locks when pressure is removed. This removes the need for gust locks for the yokes, the surfaces won't move at all until pressure comes back.

              I also thought there were other reasons for control surface balancing, but I could be wrong.
              Wind does move flight controls. No not to where they are "flapping" but they do move. Just moving the plane around the flight line the rudder will move. (Not sure about an Airbus. Never had any experience with a FBW aircraft.)

              One of the biggest New Pilot gripes, is "I can't pull the rudder boost handle out(handle used to disconnect boost packages in the event of hydraulic failure, also in the case of the rudder limits it's throw.)... For our aircraft the rudder needed to be pretty close to center for the boost handle to be pulled out. Quite a bit, there is enough of a wind to put pressure on the rudder to keep it from going to the near center position to allow the boost handle to be pulled, causing a false gripe of a binding boost handle.
              -Not an Airbus or Boeing guy here.
              -20 year veteran on the USN Lockheed P-3 Orion.

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              • #8
                Originally posted by Spad13 View Post
                I'm sorry, but you are being too general.

                Yes, I've seen drooped ailerons, but I've never seen them flap in the wind. I for sure have never seen rudders flapping in the breeze.

                On the airplane I fly, the flutter dampers you mentioned act as control locks when pressure is removed. This removes the need for gust locks for the yokes, the surfaces won't move at all until pressure comes back.

                I also thought there were other reasons for control surface balancing, but I could be wrong.
                I never said the ailerons would move in the wind I said the rudder can be moved by the wind.

                On aircraft with fully powered flight controls there is no physical connection between the flight station controls (column/joy stick/peddles) and the control surface. Pilot inputs go to a servo, the servo then sends hydraulic fluid to actuators that move the surface. If hydraulic pressure is lost to the servo it can't supply any fluid to the actuators and control surface is free to move. Ailerons and elevator submit to gravity and droop the rudder has such a large surface area that even a slight cross wind will move it.

                I don't know what type of aircraft you fly but flutter dampers are basically just shock absobers will not prevent the surface movement. They do exactly what there name implys they dampen the surface movement.

                Gust locks are a maintenance tools installed on parked aircraft to prevent surface movement in the wind. They have nothing to do with the yoke, because as I pointed out earlier there is no physical connection between the yoke and the surface.

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                • #9
                  On the B767 all control surfaces except the spoilers and outboard ailerons can be moved by the wind.

                  Gravity normally wins on the elevators and inboard ailerons so they simply 'sag'. That being said I've seen the 767 in windy Wellington with both inboards fully deflected up, one elevator up, one down and the rudder fully deflected. When we pressurised the hydraulics the whole aircraft shook!

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                  • #10
                    I tried....

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