16.1 Fly-By-Wire Flight Control

16.1 Fly-By-Wire Flight Control

Current Appendix I gives only the broad topic 5.15 heading at level 1. The pre-12 June 2024 detailed description named Fly-By-Wire (FBW) among its typical systems, so this section retains FBW as a representative study example. Knowledge level 1 applies for categories A, B3, B1 and B2/B2L: familiarisation with the general arrangement and associated Built-In Test Equipment (BITE). This section explains how flight-control computers, electrical signalling and powered actuators replace a continuous mechanical cable run from the cockpit inceptors to the primary surfaces, how flight-control laws shape the relationship between sidestick or column and the aeroplane, and why independence and redundancy are part of the architecture. Airbus names such as ELAC, SEC and FAC, and Boeing Flight Control Computer (FCC) / Actuator Control Electronics (ACE) names, are classroom examples of typical installations, not a claim that every type uses the same boxes.

From mechanical signalling to electrical command

On a conventional mechanically signalled aeroplane, the captain's control column and rudder pedals move cables, quadrants and pulleys that position hydraulic servo valves. Artificial feel comes from springs, bob-weights and, on larger types, a Q-feel unit. FBW keeps the powered actuators but replaces the signalling path with electrical commands. Position transducers on the sidestick or control column, and on the pedals, send analogue or digital demand to flight-control computers. Those computers output commands to actuator servo valves or to Actuator Control Electronics. Hydraulic, electro-hydrostatic or electrical actuators then move the elevators, ailerons, spoilers and rudder.

Three maintenance-relevant consequences follow. First, a broken cable is no longer the typical jam: a failed computer, a lost electrical bus, a failed actuator transducer or a hydraulic shut-off becomes the typical fault. Second, the relationship between inceptor deflection and surface angle is software-defined, so the same physical stick movement can mean load-factor demand in one law and a direct surface command in another. Third, there is no single mechanical path that a technician can trace from stick to surface with a torch: isolation uses BITE, computer reset, hydraulic status and actuator position feedback.

Computers and actuators — typical arrangements

Classroom teaching uses two transport families as the general arrangement.

Airbus types (A320 family as the usual example) place multiple dissimilar computers between the sidesticks and the surfaces. Typical names are two Elevator Aileron Computers (ELAC), three Spoiler Elevator Computers (SEC) and two Flight Augmentation Computers (FAC). ELACs normally compute elevator and aileron orders and autotrim of the Trimmable Horizontal Stabiliser (THS). SECs compute spoiler orders and provide standby elevator command. FACs compute rudder trim, yaw damper and characteristic speeds. Later Airbus types keep the same idea with different names (PRIM/SEC). Each computer is a Line Replaceable Unit (LRU) with its own power supply, data-bus connections and BITE.

Boeing fly-by-wire types (777 and 787 as the usual examples) split computation from actuation electronics. Primary Flight Computers (PFC) or Flight Control Computers (FCC) compute the control laws. Actuator Control Electronics (ACE) convert those digital commands into analogue drive for the hydraulic actuators and return position feedback. If the PFCs are lost, the ACEs can provide a direct electrical path from the column transducers to the actuators so that the aeroplane remains controllable. The conventional control column is retained.

Actuators are typically dual-chamber hydraulic units, or later electro-hydrostatic actuators, fed from independent hydraulic systems (Airbus Green/Blue/Yellow teaching, or Boeing Left/Centre/Right). Position is closed-loop: the computer or ACE compares commanded versus actual surface position. Uncommanded movement is detected by monitoring, dual transducers or comparison between computers, and the offending actuator is bypassed or shut off.

Flight-control laws (typical Airbus teaching)

A control law is the computed relationship between inceptor demand, aircraft motion and surface command. Airbus classroom teaching uses three electrical laws plus a mechanical backup. These names are type teaching, not an EASA-issued law table.

Normal law is the dispatched electrical law when computers, sensors and hydraulics are serviceable. Pitch is typically a load-factor (C) demand*: stick displacement commands g, not a fixed elevator angle, and autotrim holds the demanded trajectory. Roll is typically a roll-rate demand. Protections are active: high angle of attack, high speed, excessive pitch attitude and excessive bank are limited so that full stick cannot easily stall or overspeed the aeroplane in the protected envelope.

Alternate law is entered after specified dual failures (for example dual air-data or dual inertial, or certain hydraulic and computer combinations — exact logic is type-specific). Some or all hard protections drop out. Load-factor demand may remain, but the stall and overspeed protections of normal law are typically lost or reduced to warnings. The crew is told that protections are degraded.

Direct law makes sidestick deflection proportional to control-surface deflection. Autotrim of the THS is lost, so the crew uses manual pitch trim. A characteristic memo (often taught as USE MAN PITCH TRIM) appears. Direct law is the electrical last resort before mechanical backup.

Mechanical backup on many Airbus types is a cable path to the THS trim wheel and to the rudder, allowing the aeroplane to be kept in a stable attitude if electrical signalling of the elevators is lost. It is a survival mode, not a dispatch law.

Boeing FBW teaching uses normal, secondary and direct modes rather than Airbus law names. Envelope protection is generally soft: the column can be overpowered through a protection. Module 5 requires recognition that laws and modes exist and that they degrade with failures, not memorising every interlock.

Independence, redundancy and inceptor philosophy

Independence means that one failure must not remove all signalling, all computation and all actuation together. Typical provisions include:

• Electrical independence: computers and ACEs are fed from different AC and DC buses so that a single generator or bus loss does not blank every computer.
• Hydraulic independence: surfaces that must keep working after one system loss are powered from more than one hydraulic system, or have backup electrical actuation.
• Computational redundancy: multiple computers, often with dissimilar hardware or dissimilar software, compute in parallel. Voting or monitoring isolates a failed lane.
• Sensor redundancy: air data and inertial sources are triplex or better; a single bad angle-of-attack vane must not command a false protection.

Sidestick versus yoke is type context, not a syllabus preference. Airbus uses a sidestick with no mechanical link between captain and first-officer sticks; dual-input is resolved by priority pushbuttons and by summing logic until priority is taken. There is no tactile column fight. Boeing FBW retains a control column (yoke) that is mechanically linked and often back-driven, so each pilot feels the other's input and the autopilot's commands. Neither inceptor proves that the computers are healthy: a sidestick that moves freely can still sit in front of a failed ELAC, and a moving Boeing column can still sit in front of a failed ACE.

Associated BITE and the hangar close-out

FBW computers run power-up and continuous BITE. Typical hangar evidence is a flight-control fault on ECAM or EICAS, a computer-reset procedure, a hydraulic-page disagreement, or a CMC/CFDS fault history naming an ELAC, SEC, PFC or ACE. Replacing a computer is a configuration-controlled LRU change: software part numbers must match the approved standard (Chapter 12). After replacement, a BITE-initiated test, actuator cycling as required by the Aircraft Maintenance Manual (AMM), and confirmation that the law or mode is again normal are the familiarisation-level close-out. Do not treat a law degradation as a simple display fault: the law is the computers' assessment of remaining redundancy.

[!WARNING] A flight-control computer swap is not complete when the box is seated. Confirm electrical connectors, hydraulic status, BITE pass, software identity and that the aeroplane has returned to the dispatched law or mode before signing the task.

Level 1 does not require a type-course memory of every law interlock. It does require you to know that computers command actuators electrically, that laws or modes degrade after failures, that power, computation and hydraulics are split on purpose, and that sidestick versus yoke is only type context around the same FBW idea.