Question: On the sidesticks of Airbus aircraft, there is a Priority Takeover button. [Wikipedia](http://en.wikipedia.org/wiki/Side-stick) has this to say:

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> 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.
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On US flight 1549, [the CVR transcript](https://www.ntsb.gov/investigations/AccidentReports/Reports/AAR1003.pdf) shows that Sully hit the Priority T/O button, after the co-pilot (Skiles) handed over control of the aircraft:

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> *15:27:23.2* - **Sully:** My aircraft.
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> *15:27:24.0* - **Skiles:** Your aircraft.
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> *15:27:26.5* - **FWC:** Priority left.
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I'm curious as to why there's a need for this button at all?

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

Answer: 
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).
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](http://en.wikipedia.org/wiki/Air_France_Flight_447#Final_report) 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.