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The A320 FCOM chapter 27 flight controls, normal law, protections, shows a diagram how the sidestick input is used when protections are active. When high angle of attack protection is active the side stick controls angle of attack (alpha protection to alpha max), so far so good.

In several places throughout the FCOM and that diagram I talked about Airbus states that the alpha protection has the highest priority.

Does this mean when alpha protection is active that pitch protection is no longer available? It makes sense that the nose of the aircraft is allowed to drop below the normally protected minimum of 15 degrees but is the maximum pitch of 30 degrees (or 25 depending on config and airspeed I think) still protected? When alpha prot is active is it possible to pitch up to more than the maximum pitch limit? The diagram would suggest yes.

A320 FCOM 1.27.20 P3, flight controls, normal law

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    $\begingroup$ It may be they didn't bother detailing the behaviour because the aircraft won't have enough power to climb as steep and therefore will pitch down on the alpha protection alone anyway. $\endgroup$ – Jan Hudec Nov 9 '18 at 20:49
  • $\begingroup$ When you fly empty and fast (say 230kt+) and then pull up quickly you can pull 2.5 gs (load protection) and if you then manage to get the angle of attack high enough to trigger the alpha protection you could have the alpha protection active with enough excess energy to pitch to a high pitch angle. Pitch rate will be limited by alpha max then of course.... And at least in my flight simulator I can actually do that dynamically. I tested two commercially available flight sims for this, both got me to 30 degrees of pitch well before reaching actual low speed conditions where I run out of energy. $\endgroup$ – Jan Nov 9 '18 at 21:06
  • $\begingroup$ But yes, the alpha protection does also factor in the speed trend and pitch as far as this video suggests: youtube.com/watch?v=9fqy8uPzW90 (awesome video by the way, 9:12 for alpha protection) So the alpha protection might force the nose down enough $\endgroup$ – Jan Nov 9 '18 at 21:10
  • $\begingroup$ Hi Jan. I tweaked the title of your question to try to better summarize what you're asking. Feel free to Edit further or to roll back the edit if you feel my edit changed your intent in any way. $\endgroup$ – user May 28 '19 at 14:45
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In normal law « Alpha prot » has the highest priority, if AOA is less than alpha limit, teta (pitch angle) is limited between +25 and -15 at long run. At long run, since in emergency descent you are allowed for a short time to go beyond - 15° pitch down, to do that you need to pitch down the side stick very quickly after having engaged a steep roll.

If AOA protection is active, I don’t see how you can exceed 25°or 30° pitch up, since the FBW will pitch down the aircraft to follow the AOA protection.

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  • $\begingroup$ The fundamental question is: does the AOA protection control angle of attack or does it control pitch and speed (trend)? If it does control AOA and nothing else it it physically possible to fly at 30° nose up and be at an angle of attack where the alpha protection kicks in and with a good enough thrust to weight ratio and a high speed (barely enough to fly above alpha protection) the protection would just follow your alpha command and since there is excess energy it could keep following that AOA and pitch above 30°. Unless there is an extra control loop on pitch angle. $\endgroup$ – Jan May 29 '19 at 6:23
  • $\begingroup$ Alpha protection is the highest protection, but lower priority protections will be followed if not contradicting higher priority. So the pitch CANNOT BE MAINTAINED ABOVE 25° for more than few seconds $\endgroup$ – user40476 May 30 '19 at 10:05
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Yes, it is active. I have seen and done high AOA exercises in the simulator and the green dashes (PFD markings) remain on the pitch limits even after the high angle of attack protection activates. That tells me as a pilot, the aircraft still has pitch attitude protections active. Here is a snapshot I took from a windshear recovery in an A320 (level D simulator) from YouTube with alpha floor activated. You can clearly see in the yellow box that the green dashes are still on. This gives the pilot an indication of the pitch attitude protection.

enter image description here

It makes a lot of sense. The protection adds thrust when it enters alpha floor. This means, it should not require a lot of nose down pitch attitude to recover. High angle of attack does not prevent you from a fully developed stall. It recovers way before you enter a full stall. So, you do not need to pitch down by a lot. Even if you pull the stick fully back, the aircraft will maintain V alphamax and slowly climb as TOGA thrust is added by the activation of alpha floor. Thus, a higher pitch attitude should not be required in an escape maneuver like in windshear. As a matter of fact, if for any reason (maybe due to an upset) if the aircraft pitches up more than 50 degrees or pitches down more than 30 degrees, the abnormal attitude law activates, which forces the aircraft into alternate law to help the pilot recover from what the airplane considers something beyond its protection envelope.

The next hint is found in the FCOM. It states:

Furthermore, there is no emergency situation that requires flying at excessive attitudes. For these reasons, pitch attitude protection limits pitch attitude: ‐ 30 ° nose up in conf 0 to 3 (progressively reduced to 25 ° at low speed). ‐ 25 ° nose up in conf FULL (progressively reduced to 20 ° at low speed). ‐ 15 ° nose down (indicated by green symbols “=” on the PFD’s pitch scale).

It further says that:

Pitch attitude protection enhances high speed protection, high load factor protection, and high AOA protection.

The last statement says it all, does not it? Airbus believes pitch attitude protection helps to enhance the effectiveness of other protections, including high AOA protection. Again, it is sensible. You need limitations for the protections to ensure that they do not go berserk.

enter image description here

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  • $\begingroup$ Thank you for the long and detailed reply. What I am thinking about is a situation when you fly 300kt or so pull up with full aft stick so that you trigger the high AOA protection just as you pitch through about 25deg of pitch and then you keep maintaining full aft stick. It now commands alpha max and if you have enough speed remaining (much more than for 1g stall) it should enable you to pitch beyond 30deg nose up. The question is, if alpha prot is active, is the pitch protection active. The green bars are on but the diagrams show otherwise. $\endgroup$ – Jan Sep 7 at 6:54
  • $\begingroup$ As mentioned in other comments it could be that in alpha prot the aircraft is not actually flying AOA but instead is a modified attitude hold that uses the speed trend, pitch and bank to define a desired max pitch angle and tried to fly that instead. In a demo video by airbus the pilot demonstrating the maneuver says that they are interested in stable attitude and speed. This means you can't actually pitch up even though physically it would be possible with enough speed. $\endgroup$ – Jan Sep 7 at 6:57
  • $\begingroup$ A320 is fly by wire and controlled by the computers. Sometimes we tend to forget that. The computers will limit the pitch even if you have the energy to increase the pitch attitude. But yes it can increase pitch attitude in an upset situation (due to a disturbance). And if it increases by an amount the control laws degrade. $\endgroup$ – Anas Maaz Sep 7 at 9:54
  • $\begingroup$ @Jan Which diagram are you talking about. I am not entirely sure which one it is. Thanks. $\endgroup$ – Anas Maaz Sep 7 at 9:55
  • $\begingroup$ I just now added the diagram to my question. $\endgroup$ – Jan Sep 20 at 9:00

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