I'm assuming that all commercial Airbus planes use a similar, if not identical, system. Can someone give me an overview of the parts of the Airbus fly-by-wire system?

I've also heard of ECAM. Is ECAM related to fly-by-wire?


1 Answer 1


ECAM and FBW are not directly related, but information and alerts regarding the flight control computers and actuators are displayed on the ECAM screens, like any other information not visible on the primary flight display or the navigation display.


The fly-by-wire concept is actually simple:

  • In traditional aircraft, the control surfaces like ailerons or elevators are linked to the control inputs used by the pilot, e.g. the column. This link can be direct (cable) and/or amplified (hydraulic or electric actuator commanded by the pilot).

    enter image description here

  • In "FBW" aircraft, the control inputs are instead sent to calculators which deliver actual orders to actuators. Each FWB system, in addition to introducing calculators into the pilot-control surface chains, also gets information from sensors to measure the aircraft response to orders (feedback).

    enter image description here

This design has two advantages:

  • There is an actuator (e.g. an hydraulic jack), so the pilot (or the autopilot) has to exert on the control inputs only a fraction of the actual force that needs to be exerted by the actuators on the control surfaces.

  • More important for the concept, the calculator can supervise the pilot commands, so that the aircraft is never put in unwanted or dangerous configuration, e.g. stalled. This system is similar to the full authority digital engine control (FADEC) used for the engines.

The main calculators for Airbus A320 family are:

  • ELAC: elevator and aileron computer
  • SEC: spoiler and elevator computer
  • FAC: flight augmentation computer
  • SFCC: secondary flight control computer

They are redundant and some functions dedicated to one computer can also be executed on other computers.

enter image description here
Source: Fly By Wire Systems Explained

Actuators are moved using a transfer function from pilot input. In simple words, some correspondence links the actuator deflection to the pilot input. These correspondences are usually named "flight control laws". Airbus uses 3 laws:

  • Normal: When everything needed by the calculators is available. Maximum protection is available. Sidestick inputs by the pilot are converted into vertical G-load demand and lateral roll rate demand.

  • Alternate: When some calculators required data are not available. Some protections may be lost and sidestick lateral inputs are converted into a direct equivalence in control surface deflection angle.

  • Direct: When no protection can be provided by the calculators, due to calculator failure, missing input, actuator or control surface failure. Vertical and lateral sidestick inputs are directly converted into a proportional surface control deflection. Basically the aircraft is a non-FBW aircraft.


In parallel instruments in general have evolved from gauges to electronic displays. The elimination of the third crew member (the flight engineer supervising engines, fuel and other parameters) has led to automatic supervision of aircraft parameters and filtering of displayed information.

By default, only a few parameters are shown to the crew, other are replaced by alerts when something is unusual or requires attention. The crew has also the possibility to display parameters/status on demand, e.g. the status of the cabin doors or the fuel tanks. The electronic centralised aircraft monitor (ECAM) is just such system which monitors and displays alerts and important parameters.

enter image description here
A319 ECAM after an actual electrical failure, source

The ECAM system includes two displays that can be configured by the crew:

enter image description here

Relation between FBW and ECAM

Airbus FBW reconfigures laws when one of the flight control computers has failed. Because knowing this reconfiguration is vital for the crew and it follows some failures, this leads to alerts, information and suggestions being displayed on the ECAM screen. However this use of ECAM is not particular to flight control computers.

enter image description here
ECAM indication of autopilot being off, and laws being changed, source

For more technical information about this topic, see question: Is a Control Law Degradation in Airbus Planes displayed on the ECAM?

  • $\begingroup$ Thank you, @mins. I appreciate the time you put in your answer. Are the FBW computers, such as the ELAC, embedded systems like microcontrollers? Or do they run specialized Operating Systems? $\endgroup$
    – SaiMachi
    Jan 20, 2018 at 5:00
  • $\begingroup$ @MathFlyer: I don't have an answer to that, but you may ask a new question or look at the comments under this answer. $\endgroup$
    – mins
    Jan 20, 2018 at 12:32
  • 1
    $\begingroup$ @MathFlyer, each computer is composed of two dissimilar boards. On A320, one uses i386 and one a m68k CPU; I believe they use arm and some newer x86 version new models. Each board runs a different real-time OS (I think one is QNX; can't remeber the other) and each runs independent implementation of the algorithm. One board feeds the output and the other verifies and shuts down the unit if the outputs disagree. Each unit exists in multiple copies that work as fail-overs. $\endgroup$
    – Jan Hudec
    Jan 20, 2018 at 20:57
  • $\begingroup$ @MathFlyer, note that there is no pressure to miniaturize the boards, so they are similar to normal computer components and made for durability, but in programming style they are microcontrollers—each running a single, relatively simple (couple thousand lines in C) control loop. $\endgroup$
    – Jan Hudec
    Jan 20, 2018 at 21:16

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