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As with any system in transport category aircraft systems you want to have a redundant system in case something goes wrong. I'm only familar with the CRJ systems, so that is what I'll talk about.
On the CRJ, the captain's yoke is only directly connected to the left aileron and the left side of the elevator. The f/o's yoke is connected to the right aileron and right side elevator. However, the two control systems are linked by what is called an interconnect unit. That way whoever is flying is controlling all control surfaces.
If one side happens to get stuck, or if you have a PCU runaway (basically the hydraulic actuator gets stuck in one position), instead of not being able to control the aircraft, you can pull the disconnect button. This will disengage the interconnect unit and allow the controls to be moved independantly. While you'll only have controls of the surfaces on your side, whichever side isn't stuck will be used to control the aircraft.
In a side note, the leading theory as to what brought down the egypt air 767 in the 90s was that there was a struggle for controls in the aircraft. One pilot was trying to nose the aircraft into the water, and the other was fighting him. What is believed to have happen was that the two pilots put so much pressure on their respective yokes that the interconnect unit broke. This allowed each pilot to control their "respective" controls.
Let me know if this makes sense to you or if you have any questions.
That makes sense, I see what your saying now. Its exactly like the "Side Stick Priority" on the Airbus's. Thanks Joe!
Eh....not exactly. If there are no abnormalities with the system and you disconnect, both pilots could move their yokes in any direction, but they'd only be controlling the surfaces on their side. Wheras (I think) with the airbus one, whoever has priority gets all controls.
What IS very similar to the side stick priority is the control of the spoilerons. On the CRJ, the elevator and ailerons are controled by cables that run the hydraulic systems. The spoilerons on the other hand are done electrically. And each pilot has control of the spoilerons on the opposite side during normal ops. Should something happen they have a button to press that will automatically give them control on the on-side spoilerons as well.
Yeah my mistake, the Sidestick priority just switched one of the 5 computers (not previously used, lets say the Captain was using computer A and the First Officer B, once sidestick priority is pressed by the Captain it goes to computer C insade A or B is SNAFU'd) and you get full controll of the aircraft. Press it again and it cancells it out and bot gets controll.
Emu's dead on with the reason for the disconnect. Obviously, they're going to work differently on different aircraft.
With the Dash 8, the left control yoke moves the left elevator, and roll control spoilers on both sides. The right control yoke controls the right elevator and both ailerons.
Procedure if a jam occurs is for the flying pilot to announce the jam, either roll or pitch, and pull the appropraite disconnect. Both pilots will then try to move their yoke. The pilot with control will announce that he has the free wheel, and the other pilot will announce that he is still jammed.
Interesting thing with the Dash, is that it is possible to have a spoiler jam and both yokes still move. There are clutches in the spoiler system that will still allow the ailerons to move the airplane. It is noticable by having a detent in roll control that shouldn't be there. Procedure then would be to pull the roll control and then de-pressurize the roll control spoilers.
The airbus sidestick will produce a summation of both inputs. So if one pilot inputs a roll left input then the other pilot applies another roll left input the roll rate increases upto the limits as defined by the control law that the aeroplane is in. Now say one pilot makes a big whoops, then if the other pilot presses the sidestick priority he / she will take the other pilots input out of the control equation. There is also a call "Priority Left / Right" and an ANN illminates.
The Disconnects you see in the pictures are mechanical and are more about recovering some kind of control if the control runs become jammed by physically disconnecting various aspects of the cabling.
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