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I am learning for an avionics exam and I cannot find any differences between the direct law and the mechanical backup.

I will describe the laws that I learned such that it is easier to see if I have some misconceptions.

  • Normal law: Standard mode of operation
  • Alternate law: (at least two failures detected):
    • Loss of pitch and bank protection
    • $\alpha$-protection and high-speed protection replaced by stabilities
    • Load factor limitation is still active
  • Direct law: (at least two failures second not self-detected)
    • All protections lost
    • Manual pitch trim
  • Mechanical back-up: (all electric supply for control computers lost)
    • All protections lost
    • Manual pitch trim

Is the only difference between the direct law and the mechanical back-up that we do not have operating computers for control?

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  • $\begingroup$ feed back loop. $\endgroup$
    – user3528438
    Oct 9, 2017 at 16:35
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    $\begingroup$ Yes. Mechanical backup on Airbus encompasses the mechanical links between trim/elevators and pedals/rudder. Laws are related to the joystick and flight control computers. See this answer which should provide all the elements to answer the question. $\endgroup$
    – mins
    Oct 9, 2017 at 20:25
  • $\begingroup$ The last line should be “Manual pitch trim only”, because only the trim wheels and rudder pedals have hydromechanical linkage. $\endgroup$
    – Jan Hudec
    Dec 28, 2018 at 23:17

1 Answer 1

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The key piece of knowledge to understand a FBW system is that you need to differentiate between signalling and actuation.

signalling is what tells the control surface to move and to what position. In old airplanes, this was by steel wires. In FBW airplanes it's by copper wires carrying an electrical signal.

actuation is what actually moves the controls surfaces to the commanded position, the muscles. Traditionally this is hydraulics, but you can also find electric actuators these days.

For the Airbus, Normal, Alternate & Direct all use electrical signalling (whether produced by computers or not). But what to do if there's no electrics? That's Mechanical backup, where you have steel-wire signalling of specific surfaces.

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  • $\begingroup$ Does Airbus have mech backup? $\endgroup$
    – Caterpillaraoz
    Oct 10, 2017 at 10:14
  • $\begingroup$ @Caterpillaraoz - see this question and others like it here. $\endgroup$
    – FreeMan
    Oct 10, 2017 at 20:51
  • $\begingroup$ @Caterpillaraoz, the A320 does as does A330 and A340, but I had the impression that A380 does not. $\endgroup$
    – Jan Hudec
    Dec 28, 2018 at 23:15
  • $\begingroup$ Well, the important bit is that the side-stick is only connected to electric sensors, so in the mechanical backup mode it does nothing and only the trim wheels and rudder pedals work. $\endgroup$
    – Jan Hudec
    Dec 28, 2018 at 23:16

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