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Airbuses use joystick-style side sticks for controlling the aircraft. Among the features of this system is that there exists no mechanical connection between the joysticks and the control surfaces.

In case of an electrical failure, what backup system is in place to make sure the pilot still has control of the aircraft?

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  • $\begingroup$ I read somewhere there are mechanical links for elevator trim and vertical stabilizer allowing minimum control the time the ram air turbine is deployed. I don't know if those control still exist on more recent fly-by-wire aircraft, but if not it means the fly-by-wire system is more reliable than mechanical controls. $\endgroup$ – Manu H Oct 12 '16 at 14:57
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In case of a total electrical failure, the aircraft can still be controlled using rudder pedals and manual pitch trim.

The airbus EFCS (Electronic flight control systems) are configured as below:

A320 EFCS

Airbus EFCS; Image from smartcockpit.com

From Airbus Flight Control Laws:

MECHANICAL BACKUP

In case of a complete loss of electrical flight control signals, the aircraft can be temporarily controlled by mechanical mode.

  • Pitch control is achieved through the horizontal stabilizer by using the manual trim wheel.

  • Lateral control is accomplished using the rudder pedals.

  • Both controls require hydraulic power.

The A340 also seems to have a similar system. From the A340 Flight Deck and Systems Briefing for Pilots:

Mechanical back-up

• To sustain the aircraft during a temporary complete loss of electrical power.

• Longitudinal control of the aircraft through trim wheel. Elevators kept at zero deflection.

• Lateral control from pedals. Roll damping is provided by the Back up Yaw Damper Unit (BYDU).

A340 backup

A340 mechanical Backup system; image from A340 Flight Deck and Systems Briefing for Pilots

Note that this is given only as a temporary measure. Also, See How are fly-by-wire airliners controlled in case of complete electrical failure? and How does the Airbus flight computer's voting system work?

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  • $\begingroup$ Do you want to add anything about using differential thrust as well? There was an accident (United Flight 232) some years ago due to a hydraulic failure where the pilots used differential thrust to safely (if roughly) land in a field. The fact that anyone survived (over half of the people on board survived) is a testament to getting the most out of the bad situation, per the NTSB. $\endgroup$ – KorvinStarmast Oct 12 '16 at 15:50
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    $\begingroup$ I wonder if a full electrical failure is in any FBW training curriculum... Rudder pedals and trim wheel control sounds like "make the lawyers happy" but I'm not so sure if its successful real-world outcome. $\endgroup$ – Ron Beyer Oct 12 '16 at 16:11
  • $\begingroup$ Well in the event of a total electrical failure, you would - I think - lose engine control as most, if not all, modern airliners use FADEC engine controllers with no mechanical throttle control. That's why they have multiple electrical buses which can be cross fed and powered by an alternator on each engine, the APU, and finally the emergency RAT. A failure of all of those systems is less probable than damage to mechanical or hydraulic controls rendering them non functional. $\endgroup$ – Carlo Felicione Oct 12 '16 at 18:15
  • $\begingroup$ @CarloFelicione -- the FADECs have dedicated engine driven PMGs for backup power $\endgroup$ – UnrecognizedFallingObject Oct 13 '16 at 0:28
  • $\begingroup$ @RonBeyer -- it'd be quite annoying to fly, but the Airbus test pilots did it. $\endgroup$ – UnrecognizedFallingObject Oct 13 '16 at 0:29

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