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On an Airbus aircraft, the Flight Augmentation Computers (FACs) control the rudder and have windshear detection, and the FACs also are responsible for speed computations. I know the FACs continuosly compute stall speed and compute the maximum speed but do the FACs also compute every other speed on the speed scale; VLS, alpha protection, alpha floor, green dot speed, flap retraction, etc., so basically the FACs compute every speed on the speed scale?

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    $\begingroup$ Do you have an idea of where the numbers would come from if they were not all computed? Knowing that would help us to understand what you are asking. $\endgroup$ – Wayne Conrad Jul 4 '18 at 16:06
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    $\begingroup$ @WayneConrad They can come from the FMS. In the A320 architecture, the FAC computes characteristic speeds based solely on sensor inputs like AOA, trim, altitude, fuel level. The FMS computes the same with the addition of user-input data like ZFW/CG. The FMS speeds are more accurate but are subject to user error. Which is actually displayed on the PFD depends on the model and generation of aircraft. $\endgroup$ – user71659 Jul 5 '18 at 6:18
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Yes. Kind of--the air data computers do some of the computations and display those based on hard coded limits. For example, an ADC will compute current AOA and may have the AOA limits hard coded therein.

The air data computers also provide input to the FACs, which further limit the aircraft speed envelope based on altitude, speed, and configuration. The FACs are necessary for configuration information and flight control positions.

A detailed explanation of the what the FACs accomplish is here. There is a related question.

Other examples of ADCs and their respective displays appear here and here. The first shows mechanical systems which compute a normalized AOA based, and later in the same article, a Gulfstream display shows feathers for Max AOA on the ADI, as seen here. Guflstream attitude indicator with "Pitch Limit Indicator", a function of AOA.

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  • $\begingroup$ So the FACs get all their information from the ADC and the FACs compute all the data they get and display it? Could you give me another example of an ADC computation and display? $\endgroup$ – Itzyoboi Jul 4 '18 at 21:20
  • $\begingroup$ Answer edited to include other examples of AOA displays. $\endgroup$ – Mark Jones Jr. Jul 4 '18 at 22:07
  • $\begingroup$ I dont understand what you mean when you say the AOA has hard coded limits? Doesn't AOA limits depend on altitude, weather, speed, configuration, and so on? $\endgroup$ – Itzyoboi Jul 4 '18 at 23:13
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    $\begingroup$ No, Max AOA is the same regardless of speed, altitude, bank angle, etc. An airfoil will always stall at the same AOA. $\endgroup$ – Mark Jones Jr. Jul 5 '18 at 0:26

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