How do redundancies work in aircraft systems? I've read about many aircraft systems having redundancies. For example, both Boeing's 787, and Airbus's A380 have a triple-redundant FCS. In this case, it makes sense to act upon the majority's decision. If all three components disagree, how is it decided which one wins? Is there any record of disagreement amongst redundant systems in airplanes?

The primary differences I have compared to the question about Airbus's control systems (How does the Airbus flight computer&#39;s voting system work?), is verification for non-deterministic systems, as well as optics for visibility. Given that the entire systems run in parallel up until the verification component (from sensor input, onwards), there will be some, obvious delta in their "decisions." -- Although, most of the time, this isn't perceivable, is there any visibility into what's going on with the FCS?
Triple-redundant systems have four independent members, so if one fails, a two-to-one vote of the remaining three is still possible.

One system    = No redundancy.
Two systems   = Simple redundancy.
Three systems = Double redundancy.
Four systems  = Triple redundancy.


However, often the term "triple redundant" is used when the system has only three members. And if all three disagree, two of them must be faulty already, so this is a case of multiple failures.

In case of Airbus, besides the three main FCS computers two backup systems are available to help isolate the faulty one. Inside, each has two channels which run software written by a different team, run on a processor different to the one in the other channel and which are supplied by two different suppliers.  This helps to reduce systematic errors, when all together would fail because of a common malfunction.



Airbus flight control system diagram (picture source). Only the first system of each is expanded; the boxes for system 2 and 3 look the same on the inside. Input is from the side sticks, and output goes to the hydraulic valves at the control surfaces. The voting happens first in the comparator step (between the two channels) and again in the filters (between the single systems).