What are the differences between direct law and mechanical back-up? 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?
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 &amp; 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.