Why bother
The advantages over a conventional system are the syllabus's trio:
weight — wiring replaces cable runs, pulleys, and much of the mechanical gearing, and the computers replace shelves of dedicated black boxes;
pilot workload — the computers stabilise the aircraft continuously, trim it automatically and hold what the pilot last asked for, turning gusty hand-flying into request-making;
flight-envelope protection — a computer that sits between hand and surface can refuse to exceed the aircraft's limits, whatever is asked of it.
The aerodynamic side of those protections was the 081 story; here it is an architecture feature: protection is possible precisely because the connection is computed.
Always irreversible
An FBW system is always irreversible, and more deeply than the powered controls of the primary-controls lesson: there, a mechanical run existed and the trapped fluid refused to be back-driven; here there is no mechanical path at all on which a hinge moment could travel back to the pilot. A wire carries signals in one direction and forces in none. Everything the pilot feels at a sidestick is therefore artificial by construction, usually simple spring feel, and every cue about speed and load must arrive through instruments and protections rather than through the hands.
Laws, and losing them
The modes of operation describe how much of the computed intelligence is available:
Normal operation (normal law or normal mode) — the full design: the stick commands a result (a load factor, a roll rate), the computers achieve and hold it, and the envelope protections stand guard at the limits.
Downgraded operation (alternate law or secondary mode) — after failures (sensors, computers, hydraulics), the system still computes but with reduced authority and reduced or lost protections: warnings replace hard limits.
Direct law — the last computed rung: stick deflection commands surface deflection, proportionally, like a conventional aircraft with the computers reduced to signal converters. No protections, no auto-trim, conventional handling.
The implications of mode degradation run in one direction: pilot workload rises as flight-envelope protection falls. In normal law the aircraft refuses to stall however the stick is held; in alternate law it will stall while a warning shouts; in direct law it flies like the conventional aeroplane the type rating assumed the pilot could handle, gusts untamed, trim manual, limits guarded by nobody but the crew. Degradation does not make the aircraft uncontrollable; it makes the pilot the protection system.
Two sidesticks, no linkage
On sidestick aircraft the two sticks are not mechanically connected, and the syllabus asks for both implications:
Dual input — if both pilots move their sticks at once, neither can feel it, so the computers sum the two commands (capped at a single stick's authority) and announce the condition with lights and an aural warning, because a summed input is almost never an intended one.
The takeover facility — a pushbutton on each stick gives its holder priority, deactivating the other stick, latching with a sustained press, and announcing who has control.
The etiquette is procedural because it cannot be tactile: on an aircraft where the sticks cannot argue through the linkage, the crew must argue out loud.
Dissimilar, multiple, voted
Several, dissimilar computers — redundancy against ordinary failure demands several computers; dissimilarity (different processors, different software written by different teams, often different suppliers) defends against the failure redundancy cannot touch: a common design error that would take out every identical copy at the same instant.
Several surfaces on every axis — an FBW aircraft splits its surfaces (multiple spoiler pairs, split elevators, multiple actuators) so that computers can reconfigure around a failed or jammed surface: the jam answer of the primary-controls lesson, now managed by software.
Several sensors on critical parameters — the computers' picture of the aircraft is only as good as its sensors, so critical parameters (airspeed, angle of attack, attitude) are measured by several independent sources, and the computers vote: two agreeing sources outvote a liar, and a parameter that loses its majority takes its protections down with it, which is exactly how normal law is most often lost.