# How is a fly-by-wire system realized in big planes?

For example if whole system is build on CAN protocol that give maximum rate of 9000 messages per second with IMHO is quite low number for big plane.

Can you give me any example how this problem was solved? Do they divide system into subsystem or use protocols like Ethernet IP? Or maybe that number of messages is enough because they're sending only critical information?

• 9000 messages per second with IMHO is quite low number for big plane - why? What reference do you have? What number do you expect? – Simon Feb 8 '16 at 11:33
• The problem with CAN for aviation purposes is that one bad device can disable the whole bus, because it relies on the common clock for arbitration. That's good enough for ground vehicles that can simply stop, but not for aircraft. – Jan Hudec Jun 5 '19 at 19:15

In fact, ARINC 429 is used in the "federated" avionics, this is the "old" system used in many aircraft like A320, B737, ...

• Federated avionics require a lot of wires which generates a lot of weight. In ARINC we have approximately one wire per device, so the speed (100Kbps or 12,5Kbps depending on the selected mode) is sufficient.

ARINC 429 Architecture :

• In "new" aircraft like A350 or B787 the industry use the Integrated Modular Avionics in order to reduce weight and reduce the number of S/N to stock.

• In this kind on avionic all computers are merged in a couple of "cabinet" composed of multiple core processor units, shared ROM, shared RAM, shared graphic cards.
• All these cards are linked, in the cabinet, with the Backplane BUS (ARINC 659), which operates at 60Mbps as a commercial aviation bus
• All cabinet are linked ,each other and with actuators, using AFDX (ARINC 664) who look like Ethernet protocol and work at the speed of 100Mbit/s.

There is a lot of different IMA architecture with the same principle, but some differences. This is one on them, in this picture you can replace PCI Ex. by ARINC 659 :

Regarding CAN bus, Airbus use in in this A380 (http://www.aviationtoday.com/av/issue/feature/can-bus-in-aviation_31468.html)

Sources :

http://www.interfacebus.com/Design_Connector_Avionics.html

http://www.modern-avionics.com/

https://en.wikipedia.org/wiki/Avionics_Full-Duplex_Switched_Ethernet

Fly By Wire is not necessarily based on digital computation, as an example the CONCORDE had FBW with analog computation.

With modern aircrafts the iteration rate is increased by increasing the number of buses, thus in ARINC 429 systems every computer receives several buses and may use 2 or 3 output buses.

Most FBW as conventional control are hydraulically powered, the difference is in the control, instead of being fully mechanical, the stick is connected to electrical transducers that send an input to a computing system. The servo control instead of having a mechanical control, it has an electrical control, the main power remaining hydraulic.