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In an answer, a user linked to this video of the gust alleviation system (GAS) on an A380. This seems like a complex system with many moving parts—all the ailerons move completely independently. What happens if the GAS fail, if for instance two ailerons get stuck in an extreme position?

Or maybe even worse: imagine a sensor failure that produces two opposite extremes on each wing, such that e.g. the left wing have all its ailerons in the up position, and the right wing ailerons in the down position.

  • Could this be fatal?
  • Are there procedures for this in some checklist?

This is a follow-up for my other question, What is a gust alleviation system?.

This is not question Why do some aircraft have multiple ailerons per wing? as far as I can possibly see (though it was marked as such).

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    $\begingroup$ I read somewhere the GAS works with the ailerons and the external spoilers to adjust the lift distribution all over the wing span. This is only a way to use split aileron, and if faulty, the non-faulty part may be used as traditional ailerons $\endgroup$ – Manu H Nov 16 '16 at 17:55
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    $\begingroup$ Following your edit, you should enhance what part of your questions are not tackled in this answer. $\endgroup$ – Manu H Nov 17 '16 at 14:08
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You have a few questions here so I will address them independently. But first off in a more general sense you should understand that everything on an airplane is generally designed with a failure mode this helps to ensure that if something fails it fails in a way to create the least adversity to flight. However there are event scenarios where multiple units fail in such a way that an adverse condition can be encountered. Generally (if you read accident reports) these can be avoided if an emergency landing is executed earlier on.

What happens if the GAS fail, if for instance two ailerons get stuck in an extreme position? Or maybe even worse: imagine a sensor failure that produces two opposite extremes on each wing, such that e.g. the left wing have all its ailerons in the up position, and the right wing ailerons in the down position.

In this case the plane still, to an extent controllable. I don't know for the Airbus, but generally speaking all systems like this have a manual override and can be disconnected. So this is implying that the system is somehow stuck on and not responding. It should be noted that in the Airbus this system does not independently actuate the flight control surfaces and most likely is bused as input to the fly by wire system. On that note a sensor failure generally does not cause extreme inputs but instead causes no input. Most things like this are at least triple redundant (so that you can figure out which sensor is bad) and if all are providing inputs that disagree the system usually disconnects and becomes inop. So to create the case you ask about you would need to have 3 independent sensors provide the same erroneous input which is highly unlikely.

Could this be fatal?

Its hard to say, but generally speaking no, the direct failure of a single system would not be fatal.

Are there procedures for this in some checklist?

More than likely but I don't have access to the official checklists so I cant say for sure.

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You seem to overestimate the complexity and moving parts. The GAS is a feedback system, it has sensors that provide information to the flight computers. The flight computers can then use this to adjust the ailerons as a part of its normal flying. GAS is not a completely separate, standalone system with its own control surfaces.

Gust Alleviation System (GAS) Failure

Redundancy and good programming is key here.

By having multiple sensors, it reduces the effect of failure of one sensor and makes it easier to detect a failed sensor. If you have 3 sensors saying you have an upwards gust and the 4th saying you have a side gust your software should be capable of detecting the outlier and act accordingly.

In addition to this, the amount of adjustment the system can make is also likely to be limited to a certain value and therefore it should be incapable of asking for full aileron lock.

All of this combined makes the chance of this system being capable of causing a fatal crash highly unlikely. At best it could be a contributing factor.

System checks

As for checking the system, it is likely to be self checking. The only components directly linked to the system are the sensors and computers. If either fail the aircrafts maintenances logging system should detect it and log it or alert the pilot.

Aileron failure

This is a completely separate issue to the GAS failure.

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    $\begingroup$ Related: Why are critical flight computers redundant? $\endgroup$ – a CVn Nov 16 '16 at 13:06
  • $\begingroup$ “This is a completely separate issue to the GAS failure.” I was referring to the aileron that is a part of GAS. $\endgroup$ – Jonas G. Drange Nov 16 '16 at 13:46
  • $\begingroup$ @JonasG.Drange that would be all of them. There aren't separate ailerons for the GAS, it's the same ailerons. i.e. If to the flight controller system says it wants 5 degree deflection on port aileron and 6 degrees on starboard to maintain flight, then GAS says -1 degree for gust alleviation, the ailerons would move to 4 and 5 degree deflection accordingly. The GAS is a feedback system, not a standalone system. $\endgroup$ – Notts90 Nov 16 '16 at 13:52
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Flying without the GAS switched on will increase the fatigue load on the wing such that it will accumulate more stress over time. This is completely uncritical for the duration of a single flight and will only add up when the aircraft is permanently operated without the GAS working.

Basically, the GAS gives the wing a slightly longer service life. If it fails, the safe life time of the wing's structure will be shortened. Failure means that the GAS will not add its additional deflections to the control commands from the pilots or the autopilot. A failed GAS means that the control surfaces behave like those on GAS-less aircraft, i.e. no more "valse des ailerons", but no hard-over.

Now what happens when the aileron control fails is a much more severe failure case and consequently made much more unlikely by full redundancy. Every actuator, every pushrod, every control computer and every control surface is doubled such that failure of any single component will not be critical. Only a multiple failure could affect safety, but is proven to be highly unlikely.

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