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I am looking at the schematics of computerized control for Aibus aircraft and I do not understand the relation between the various control law and the autopilot.

Under "normal law" can the pilot control the aircraft while the autopilot is engaged, or will control input from the pilot disconnect the autopilot automatically?

What is the meaning of "alternate law" for the autopilot? Will the autopilot be disconnected when shifting to alternate law?

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  • $\begingroup$ Alternate law isn't part of the auto pilot, it's a reduction in the protections built into the fly-by-wire system, for when some sensors are faulty or for some other reason the computer isn't confident that it knows better than the pilot. en.wikipedia.org/wiki/Flight_control_modes#Alternate_law $\endgroup$
    – Robin Bennett
    Commented Sep 23, 2019 at 10:27
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    $\begingroup$ @RobinBennett looks like the start of an answer. We prefer answers in the answer box rather than comments. $\endgroup$
    – Jamiec ♦
    Commented Sep 23, 2019 at 10:29
  • $\begingroup$ @Jamiec, fair point, but I can't answer the core part of the question and the bit about AL just seems to confuse the question. $\endgroup$
    – Robin Bennett
    Commented Sep 23, 2019 at 10:36
  • $\begingroup$ Dear Ahmed, welcome to aviation.stackexchange.com. I have changed the wording of your question so that it is a bit clearer. I hope I understood your question correctly. If I interpreted your question in the wrong way, please feel free to make further changes or roll-back to the original text. $\endgroup$
    – DeltaLima ♦
    Commented Sep 23, 2019 at 10:42
  • $\begingroup$ My understanding is that computer interpret those input given command law. Those input are provided either by pilot or by autopilot. (Both steps are separated) $\endgroup$
    – Manu H
    Commented Sep 23, 2019 at 13:21

2 Answers 2

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Moving the flight controls above a certain threshold will disconnect the autopilot.

From the A320 FCOM (1.22.30 Auto Flight - Flight Guidance, emphasis mine):

AP DISENGAGEMENT

AP1 or 2 disengages when:

  • The pilot presses the takeover pushbutton on the sidestick.
  • The pilot presses the corresponding AP pushbutton on the FCU.
  • The pilot pushes on the sidestick harder than a certain threshold or moves on the rudder pedals above a threshold.
  • The pilot moves the pitch trim wheel beyond a certain threshold.
  • The other AP is engaged, except when localizer/glideslope modes are armed or engaged, or when the rollout or go-around mode is engaged.
  • Both thrust levers are set above the MCT detent and the aircraft is on the ground.
  • The aircraft reaches the MDA-50 feet (MDH-50 feet), or 400 feet AGL if not MDA/MDH, with APPR mode engaged and a non-ILS approach selected.
  • One of the engagement conditions is lost.

Furthermore, in normal law with all protections available, the AP will disconnect if:

  • High speed protection is active;
  • Angle-of-attack protection is active (α prot +1° is reached);
  • Pitch attitude exceeds 25° up, or 13° down, or bank angle exceeds 45°;
  • A rudder pedal deflection is more than 10° out of trim.

The different control laws act in the F/CTL COMPUTER in the following diagram:

A320 Control Logic (source: FCOM 1.27.10 Flight Controls - Description)

They are independent of the autopilot and the protections are available regardless of whether or not the autopilot is engaged.

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When autopilot is engaged the pilot can't override the commands. If the autopilot senses manual forces on the controls (sidestick, rudders, ths) then it disengages.

Alternate law has nothing to do with the autopilot rather the flight by wire system. In alternate law there are less protection (like alpha-floor and others).

I don't know if the autopilot disconnects under alternate law.

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    $\begingroup$ What can cause alt law that wouldn't make the autopilot bail out anyway? $\endgroup$
    – curiousguy
    Commented Nov 14, 2019 at 16:47

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