# 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?

• feed back loop. – user3528438 Oct 9 '17 at 16:35
• 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. – mins Oct 9 '17 at 20:25
• The last line should be “Manual pitch trim only”, because only the trim wheels and rudder pedals have hydromechanical linkage. – Jan Hudec Dec 28 '18 at 23:17