Firstly we should have the clarity of certain terms like FAULT and FAULT TOLERANCE ARCHITECTURE.
FAULT: This is the characteristic of a software system whose occurrence can result to a system error.
FAULT TOLERANCE: The system is designed in such a way that system failure does not occur by detecting the faults and unexpected system behaviour at runtime during executing a program.
The fault tolerance system architecture are used whenever there is a requirement of high level of system availability and reliability for example AIRBUS FLIGHT CONTROL SYSTEM.
In AIRCRAFT FLIGHT CONTROL SYSTEM computations are carried out in parallel by each of the flight control computers while using same inputs. There are hardware filters connected to the outputs which can detect if a fault condition occurs. If a fault in one computer occurs output can be taken from other alternative system without disturbing he operation.