Ultimate back-up

Despite all these precautions, a mechanical redundancy has been retained on the A320 to A340. This mechanical system is connected to the horizontal stabilizer, which allows pitch control and to the rudder resulting in direct yaw control and indirect roll control. 

On the A3 80 and the subsequent aircraft, the final stage was reached the mechanical connection between the controller and the PHR cyhnder is removed. The ultimate rescue redundancy is similar to that of the A340-600, but it ensures the control of the rudder, a pair of elevators, and a pair of ailerons, on the basis of yoke side-stick and pedal orders. The technology is now analog. 

The safety objectives for the FEW part of the system (PRIM plus SEC) have been defined without benefiting from this ultimate back-up. However, on the A400M, the back-up is fidly capable of flight and safe landing, and this must be demonstrated for mihtaiy qualification. This results in a decreased need for dissimilarity between PRIM and SEC they share a common type of hardware (but different functions and software). Tolerance to a design or hardware manufacturing error is ensmed by the functional dissimilarity between PRIM and SEC on the one hand, and the total hardware dissimilarity between PRIM/SEC and the ultimate rescue redundancy, on the other. 

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