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Direct Flight Law:

What does the following characteristics exactly mean regarding sidestick input at a very low airspeed (let us assume near stall speed).

'control sensitivity depends on airspeed'

Btw. is direct flight law only achievable with gear down and both FACs off?

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  • $\begingroup$ Remaining in direct law and with gear down at relatively low altitude could against official documentation any protections kick in automatically? Could the plane assume landing mode and enable alpha protection overwriting the pilots inputs? $\endgroup$ – Horizontal Vertikal Aug 23 '15 at 4:48
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    $\begingroup$ If you want to ask more questions, ask more questions, don't add them as comments. $\endgroup$ – Jan Hudec Aug 23 '15 at 11:35
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    $\begingroup$ You have posted four questions as one. Jan has answered the main question. I suggest you post the others separately and consider identifying the aircraft model of interest. $\endgroup$ – RedGrittyBrick Aug 24 '15 at 7:24
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In aircraft with mechanical controls, the position of the control column corresponds to position of the control surfaces and the force on the control column corresponds to the force on the control surfaces. At higher speeds, the same deflection generates larger force and correspondingly stronger response from the aircraft.

However in Airbus the forces are not carried and the side-stick is spring-loaded. In normal law the side-stick deflection commands specific wing loading (which mostly corresponds to pitch rate) in pitch and roll rate in roll. So you get the same rate or pitch/roll with the same deflection independent of speed.

But in direct law, the position of the side-stick corresponds to the position of the control surfaces. Because the same deflection causes larger aerodynamic forces, it leads to higher pitch/roll rate at higher speed. Significantly higher, because the control forces grow with square of speed.

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  • $\begingroup$ So does this mean that at the transition to Direct Law there will be somewhat of a step change in how the control surfaces respond to the stick? How does Airbus handle this transition? The boundary condition does not seem to match from the two sides of approach. e.g. A the same stick deflection would suddenly correspond to a grossly different control surface deflection? $\endgroup$ – curious_cat Aug 23 '15 at 16:45
  • $\begingroup$ @curious_cat when transitioning between laws, there is period where the signals are blended. In the beginning the original law provides 100% of the control signal, but over time the new law will gradually get a stronger weight until it is at 100%. The switch over period varies between laws. $\endgroup$ – DeltaLima Aug 23 '15 at 17:49
  • $\begingroup$ @DeltaLima Ahh! Very smart. Thanks. $\endgroup$ – curious_cat Aug 23 '15 at 18:41
  • $\begingroup$ @DeltaLima what duration are the periods of transition? $\endgroup$ – Pugz May 25 '16 at 1:38
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    $\begingroup$ @Pugz That would be a good question to ask on this site. That way the information given in the answers is much easier to find using the search functions. $\endgroup$ – DeltaLima May 25 '16 at 7:41
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As to the second part of your question, AFAIK the aircraft will only go into Direct Law or Alternate Law in the case of a malfunction, either in the physical control systems, sensors or computer systems. In the case of Direct Law all flight envelope protections are lost. Since the controls are not connected with cables in a FBW system you still need the FAC's since they are actually making your control inputs happen.

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Direct law:

When the data for the C* law is no longer valid, or upon landing gear extension when the alternate law is active, the direct law is activated in pitch. The side stick directly controls the elevator position. Therefore the side stick position is no longer generates a load factor demand as a function of the position of the side stick and inertial feedbacks. The THS is mechanically controlled and no flight envelope protections are included.

In lateral control, the side stick movements directly control the position of the ailerons and the spoiler 4 and 5.

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