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I was reading about the bank angle protection mode in an A320. I have a few doubts about it.

I know that if the bank angle exceeds 33 degrees then automatic pitch trim is not available and hence of the side stick is released the aircraft would come back to a bank angle of 33 degrees and stabilise. But I don't understand what happens if the bank angle is BELOW 33 degrees. I read that the automatic pitch trim is available. What does that mean?

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In the context of the A320 as you are describing, pitch trim does not mean trimming with the horizontal stabilizer. Instead, it means that up to 33 deg bank angle, no aft stick pressure is needed to maintain horizontal flight, or more precisely, the load factor (for example, at 33 deg bank, the load factor is approximately 1.15g).

Up to 33 deg bank, the Normal Mode automatically commands the necessary load factor with the longitudinal side stick neutral. Since Normal Mode also ensures a neutral spiral stability up to 33 deg bank outside of High Speed Protection, you can maintain a coordinated level turn with side stick completely neutral.

Beyond 33 deg, longitudinal stick neutral no longer commands the necessary load factor for horizontal flight. Therefore, you would need to have aft pressure. Furthermore, there is positive spiral stability, which means non-zero lateral pressure is also required to maintain the bank angle.

For more information, you could consult: source 1 and source 2.

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  • $\begingroup$ No need to worry about pitch when maneuvering. Just the thing to degrade pilot flying skills even more. $\endgroup$ – John K Dec 24 '19 at 3:47
  • $\begingroup$ Ok, so if the pilot makes a 25 degree bank to the right and then suddenly releases the sidestick (to neutral), would the aircraft continue to fly at 25 degree bank or would it return back to 0 degree (level flight)? $\endgroup$ – Johnson Dec 24 '19 at 8:14
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    $\begingroup$ @Johnson If the aircraft is not overspeeding, then it would maintain 25 degrees bank with stick neutral (i.e. spiral neutral). Note that most, non FBW aircraft, would do this inherently, because spiral mode has a super long time constant. $\endgroup$ – JZYL Dec 24 '19 at 12:03
  • $\begingroup$ @JZYL Ok, so how would the pilot bring the aircraft back to level, should he pull the sidestick to the left till it's level? $\endgroup$ – Johnson Dec 24 '19 at 12:21
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    $\begingroup$ @Johnson Yes, lateral stick nominally commands roll rate (just like in any aircraft). $\endgroup$ – JZYL Dec 24 '19 at 13:14

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