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Airbus has normal law, alternate law and direct law. What was the need for specifically making each of these three laws? What benefit does each one offer over the others?

Also, why does Airbus degrade to alternate law upon a failure, and then degrade further, to direct law, on landing gear extension? What benefit is gained from this? For example, with a dual hydraulic failure, it goes to alternate law, and then goes to direct law on landing gear extension.

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2 Answers 2

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It is pretty simple: if everything works, no malfunctions, no false signals, then the aircraft is in normal law. You have all the stability and protections that Airbus intended the aicraft to be operated with. In real life, the aircraft will be in this state for 99.999% of the time. In my 12 years of flying, I have never encountered alternate or direct law in line flying. Only in simulator training.

If certain things go wrong (malfunctions, detected wrong signals), the flight control computers cannot function properly in normal law. There may be important signals missing to compute the proper reactions to stick inputs or environmental disturbances. For example, if you lose some of the speed signals, or lose flight control surfaces (hydraulic actuation or electric control), the aicraft reverts to a simpler flight law.

You have less protections in alternate law, and no protections at all in direct law. The aircraft reverts to this state, because it cannot guarantee that the protections will work the way they were designed, and they could even do harm (if all speed indications are lost or wrong, a low or high speed protection would make the aircraft unflyable).

Some Airbus aircraft revert to direct law with gear down (A320), and some do not (A330). With direct law, you have -- as the name suggests -- direct stick to surface control. Sometimes, the aircraft goes straight into direct law (loss of all ADRs for example).

There are even more flight control laws, that are not indicated to the pilots. You have flare law (when below 50 ft radio altitude for landing), ground law, abnormal attitude law, mechanical backup...

To sum it up: Airbus has implemented flight control laws other than normal law to cope with malfunctions and abnormal situations.

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  • $\begingroup$ In case of dual hydraulic malfunction, why could not the aircraft land in alternate law ? what would be the problem to be specific $\endgroup$ Mar 21, 2023 at 13:21
  • $\begingroup$ Dual Hydraulic is a heavy and complex abnormal. The A330 can actually land in alternate law with this abnormal. The A320 cannot, because the flight control computers are not as sophisticated. At the time when the A320 was developed and certified, there were probably not enough resources, processing power or other reasons to program the alternate law to handle this kind of scenario. $\endgroup$
    – Flo
    Mar 21, 2023 at 14:44
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    $\begingroup$ Also, you lose a wide range of systems and control surfaces. Which exactly are inoperative or remain operative varies with the type of abnormal. My guess is that the FCCs of the A320 would not be able to handle every possible combination of malfunctions to remain in alternate law for landing. So, as a general rule, whenever you are in alternate law, it will revert to direct law after gear down. Apparently, the A330's FCCs can handle most losses, which is why it rarely reverts to direct law in abnormals. $\endgroup$
    – Flo
    Mar 21, 2023 at 14:50
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    $\begingroup$ An example: the aircraft must be controllable in the flare. If the aircraft remained in an alternate law, which does not take into account a reduced pitch authority if you lose one elevator, for example, you would most probably not make a safe landing. In direct law, you have the full flight control surface deflection available and can compensate. Of course, it's difficult to control, but it's the maximum controllability you can get. I guess that the A330 has a more finely tuned alternate law that can better cope with a loss of various control surfaces. $\endgroup$
    – Flo
    Mar 21, 2023 at 14:56
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    $\begingroup$ @SachinChaudhary I don't have specifics on the why, but I can give you an idea on how they made the decision to go to direct law. From the development of normal law, they have an exhaustive list of all available controls and signals. They can systematically remove said control and signal to determine whether safe flight can continue. Out of this extensive list, they can determine which failures that alternate law cannot handle and will switch to direct law. Given that they have access to a working simulator, it is a matter of grunt work to calculate all scenarios. $\endgroup$
    – Nelson
    Mar 22, 2023 at 1:03
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Firstly I want to say it's not just Airbus FBW Aircraft that has 3 primary FBW laws/modes. Boeing also has 3 modes for their FBW 777 and 787; Normal mode, Secondary Mode, and finally Direct mode. The 777X will be the same. The same is true for the Gulfstream FBW business jets and various other business jet FBW aircraft. As for why 3 modes/laws for these aircraft, I don't think anyone really knows the true answer to that except the engineers who built these planes. The only 2 FBW aircraft I know of that has only 2 primary FBW modes is the A220 airliner (ex Bombardier C Series) and the Bombardier 7500 business jet. Why only those 2 with only 2 primary FBW modes, I don't know.

A FBW aircraft degrades to alternate law (or secondary mode in the case of the 777 and 787) cause the computers would not be able to calculate data accurately to provide flight envelope protections, enhanced flying qualities and stability etc to remain in normal law/mode. If certain things fail or the computers receives faulty data like if the pitot tubes are blocked, it can't continue in normal law/mode as the computers won't be receiving accurate data or lacking certain data so it downgrades to the 2nd lowest primary FBW mode/law (alternate law/mode/secondary mode) where many protections are lost and handling qualities in manual flight are degraded a little bit. As for why the FBW Airbus (only A320 family apparently) dumps you to direct law upon gear extension when in alternate law, according to some Airbus pilots online, alternate law lacks a flare mode where normal law has it. Flare mode memorizes the attitude you had at 50ft and freezes the autotrim when coming in to land and then shortly after, the computers will introduce a 2 degrees nose down pitch via elevator deflection within 8 seconds, where the pilots have to pull back the sidestick to pitch the nose up to flare afterwards which feels conventional as said by Airbus pilots.

Since alternate law lacks flare mode, you're dumped to direct law upon gear extension where the autotrim of the horizontal stabilizer stops to avoid maintaining pitch up inputs when the sidestick is released and elevator deflection is proportional with aft sidestick inputs. According to some Airbus pilots, this would allow the A320 to nose down a little on it's own and then you pull back the sidestick to flare. This also allows the pilot to land the plane like a conventional aircraft like flare mode in normal law. Some Airbus pilots online said it's would be awkward to land with autotrim still operating by the time you had to land so Airbus wanted to give a conventional feel so they introduced flare law and will dump you in direct law if you was in alternate law. That's what I gathered.

I heard the A330 and the rest of the widebody FBW Airbus aircraft would remain in alternate law even if the gear was deployed and I have to assume it's because the computers are more advanced than those on the A320. The widebodies in alternate law I read online would introduce their own slight nose down pitch when the plane is coming in to land, requiring the pilots to pull back to get the feel of landing conventionally.

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