Page "Fly-by-wire" Paragraph 12

The main concern with fly-by-wire systems is reliability.

While traditional mechanical or hydraulic control systems usually fail gradually, the loss of all flight control computers could immediately render the aircraft uncontrollable.

For this reason, most fly-by-wire systems incorporate either redundant computers ( triplex, quadruplex etc.

), some kind of mechanical or hydraulic backup or a combination of both.

A " mixed " control system such as the latter is not desirable and modern FBW aircraft normally avoid it by having more independent FBW channels, thereby reducing the possibility of overall failure to minuscule levels that are acceptable to the independent regulatory and safety authority responsible for aircraft design, testing and certification before operational service.