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On the sidesticks of Airbus aircraft, there is a Priority Takeover button. Wikipedia has this to say:

In typical Airbus side-stick implementations, the sticks are independent. The plane's computer either aggregates multiple inputs or a pilot can press a "priority button" to lock out inputs from the other side-stick.

On US flight 1549, the CVR transcript shows that Sully hit the Priority T/O button, after the co-pilot (Skiles) handed over control of the aircraft:

15:27:23.2 - Sully: My aircraft.

15:27:24.0 - Skiles: Your aircraft.

15:27:26.5 - FWC: Priority left.

I'm curious as to why there's a need for this button at all?

Here's a short video on YouTube, demonstrating this:

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It's been several years since I worked at United's flight training center, but if my memory serves me, it is there in case the system is receiving input that the pilot wants to exclude. As was explained to me, an example of this would be the other pilot becoming incapacitated, with their body leaning against the sidestick. In this case, one would want to use the Priority Takeover button to eliminate the other pilot's input.

In Captain Sullenberger's case, it is likely that he wanted to ensure that First Officer Skiles wasn't providing any control inputs that would work against his control inputs.

Now, it is also my understanding that such functionality is not useful in a "hostile takeover" event, since the other pilot can also attempt a priority takeover (or use physical force, due to the close proximity of the two pilots).

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    $\begingroup$ The "Priority Takeover" override capability is particularly important in the Airbus control implementation because the sidesticks aren't linked: The pilots can't know if the other seat is giving a conflicting control input by feeling a force on the stick, so using the button ensures that there is no conflict: the stick with priority is the one the system is listening to. $\endgroup$ – voretaq7 Apr 18 '14 at 4:50
  • $\begingroup$ That is correct. One of the unique aspects of Airbus flight controls is that they provide no feedback (other than self-centering). $\endgroup$ – newmanth Apr 18 '14 at 5:00
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    $\begingroup$ I believe the priority takeover also gives priority over autopilot, which was most likely on already. $\endgroup$ – Jan Hudec Apr 22 '14 at 22:23
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    $\begingroup$ @DannyBeckett: In that we would agree, yes (I don't know the exact procedure, just that it disconnects autopilot). And most likely here the reason to use it in case of US1549 was to make sure autopilot was disconnected. $\endgroup$ – Jan Hudec Apr 22 '14 at 22:48
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    $\begingroup$ @JanHudec, yes the button is used for both autopilot disconnect and priority takeover. Note that one merely has to tap the button to disconnect autopilot, but it must be held down for priority takeover. It is not possible to perform a priority takeover without disconnecting the autopilot first; there is no "control wheel steering" functionality on the Airbus. Additionally, autotrottle is disengaged separately via the disconnect buttons located on the throttle handles themselves. $\endgroup$ – newmanth Apr 23 '14 at 4:10
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I'm not sure it exactly answers your question, but there is an excellent example of why this feature ought to always be used in an emergency situation.

Air France Flight 447 was an Airbus A330, I'm not entirely certain if it had this feature (I assume so?) Anyway, it went into a stall somewhere near cruise altitude and the co-pilot, for some unknown reason, kept pulling back on the right stick. Eventually the plane was at a 40 degree incline and losing altitude quickly. The pilot on the left stick noticed the the stall and tried to push the plane forward to correct, but found the controls to be unresponsive, he was apparently unaware that the right stick was pulled fully back. The plane stayed in that stall all the way until it crashed into the Atlantic Ocean.

In an emergency, it's really really important to have a single input or a "single source of truth", not matter what industry you're in (I hear it all the time in software development anyway.) I'd argue that this is the main reason they have that override switch.

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    $\begingroup$ In this particular example, the PIC did recognize the situation and told the FO to stop, but it was already too late. $\endgroup$ – Rhino Driver Nov 21 '14 at 0:03
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    $\begingroup$ Eventually, yes, the PIC came in and figured out what was going on. $\endgroup$ – Jay Carr Nov 21 '14 at 15:11
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A prime reason for this is the fact that the Airbus sidesticks are not connected mechanically, unlike the center control columns of other aircraft. In the event of conflicting inputs from both pilots, such a condition would not be readily apparent. With mechanically connected columns, conflicting input is not possible, without both pilots being aware that the other was trying to do something different.

As Jay Carr mentioned, this was one of the reasons that led to the AF447 crash - conflicting stick inputs, and apparently neither pilot was aware of that situation, as they would have been with mechanically connected columns. That was an avoidable accident, on several levels.

It's surprising that Airbus came up with dual disconnected sidesticks and didn't consider the possibility of conflicting inputs, certainly didn't do a very good job of alerting the pilots that such a condition existed. In retrospect, the computer should be screaming bloody murder at the pilots if it detects both sidesticks in use simultaneously - a situation that should never happen.

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As far as I recall the priority button suppresses the other stick and autopilot. Since it is right under the thumb on the side-stick, that makes it quicker way to disconnect autopilot than navigating the other hand to the flight management panel.

The overall idea is that when the pilot needs to make manual input quickly they simply push that button and it will ensure that input will be obeyed, so they don't need to think what state the system was in.

Note, that the priority take-over button will disconnect autopilot immediately when pressed, but it only disables the other side-stick while held down, or permanently when held down for more than 40 seconds. The last case is primarily for situations when the other pilot is incapacitated holding their side-stick deflected or when the other side-stick fails.

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