Computer mediation between pilot input and flight control
Fly-by-wire (FBW) replaces the mechanical linkages of traditional aircraft with a computer system that interprets pilot input (stick and pedals) and translates it into commands to the control surfaces. The pilot doesn't directly move the rudder or aileron; instead, the flight control computer decides what deflection angle is appropriate and issues that command. This layer of automation allows the computer to protect the aircraft from dangerous flight regimes.
The computer implements several envelope protections: it prevents the aircraft from exceeding maximum structural loads, limits the angle of attack to prevent stalls, prevents too-steep dives, and in some cases limits banking angle or g-forces. If a pilot tries to stall the aircraft or exceed design limits, the computer simply ignores the command or moderates it, behavior impossible in a mechanically-linked system where pilot force is directly applied to surfaces.
Reliability, redundancy, and manual override
Fly-by-wire systems use redundant computers (typically three or more) that vote on control inputs, so a single computer failure doesn't result in loss of control. If one computer fails, the other two override it. Multiple independent hydraulic systems power the actuators, so loss of one system still leaves others functional. This redundancy makes FBW more reliable overall than aging mechanical systems prone to cable corrosion and wear.
In case of complete electrical failure, modern FBW aircraft fall back to a mechanical backup (like Airbus) or revert to a simplified mode with reduced protection (like Boeing). Pilots can manually override envelope protections in some modes if needed, though modern training emphasizes trusting the protection because pilot attempts to exceed safe limits historically cause most accidents. The system also improves handling characteristics: a fly-by-wire aircraft can be designed with an aerodynamically unstable wing that's inherently more efficient, with computers continuously trimming to maintain stable flight.