Aeronautics, Digital Signal Processing, Electronics, Instrumentation, IT

Fly-by-Wire: what is it?

This post’s subject is an introduction to Fly-by-Wire systems on modern airplanes. It’s shown how it works and the advantages in relation to old systems.

Before Fly-by-Wire

In the past, pilot controlled airplane’s actuators (rudder, flaps, ailerons, elevators, spoilers and brake) through pulleys, cranks, steel cables and hydraulic pipes.

airplane parts
The main parts and actuators’ location. Source: Pinterest.

These mechanical systems added too much weight to the plane, in addition to maintenance be longer and more complex.

Simplified diagram of a mechanical actuator control system on airplanes. Source: Aviões & Músicas.
first aircraft with digital fly-by-wire.
This F-8C Crusader was the first aircraft with digital Fly-by-Wire (DFBW), flew for the first time in 1972. Source: NASA.

How it works?

The pilot moves and turns on cockpit’s controls (sidestick, pedals, etc.), they have sensors that convert motions in electric signals, which are transmitted by wires, until arrive to a flight control computer or primary flight computer to make calculations and process input signals. This computer sends signals for actuators to execute movements according to pilot’s commands. Sensors and actuators send feedback signals to computer. It’s a closed-loop system.

fly-by-wire vs. mechanical system
Schematics of a mechanical control system (above) and a Fly-by-Wire system (below). A/P computer is the autopilot computer. Source: Avsim.

Computers can fail due to electromagnetic interferences or pulses. To increase security, it’s used more than one flight control computer. A computer makes calculations and compare results with others. Defective computers, that obtain different results from others, are reinitialized or shut down.

primary flight computer
A flight control computer from BAE systems. Source: MathWorks.

Each one has 3 lanes, which are central processing units (CPU), they are from different manufacturers, but run the same software. A lane makes data processing, other is on stand-by and another monitor both.

Advantages of Fly-by-Wire

  • Substitution of mechanical systems by electronics reduce airplane’s weight. Therefore, saves fuel.
  • In case of commercial airplanes, the implementation of Fly-by-Wire allows more space for passengers and luggage.
  • Actuators control is more precise.
  • More stability and security on flight.
  • Less work load for the pilot. 

Other technologies

Also exist the Fly-by-Light system, which replaces wires for optic fibers, to reduce interference and increase transmission speed. At the moment, is being developed Fly-by-Wireless, which uses radiofrequency transmitters and receivers, to make communication between computers and components.

Kawasaki P-1
The Japanese maritime patrol aircraft Kawasaki P-1 uses Fly-by-Light.

About Pedro Ney Stroski

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