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I recently read about flap load relief systems in some aircraft that attempt to prevent flap overspeeds by automatically retracting flaps under certain conditions.

  • Are these systems common on all large aircraft or are they specific to just a few?
  • How do they work (i.e. under what conditions will they retract flaps)?
  • Do they prevent the possibility of overspeeding flaps altogether, or do they just make it less likely to happen?
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  • $\begingroup$ To the best of my remembrance there was no flap load relief system on 747-100s and -200s. Also, I can't find any reference to such in the documentation I still have. $\endgroup$ – Terry Mar 3 '15 at 17:28
  • $\begingroup$ Nor is there such a system on the ERJ-170/190 (to add a currently produced transport category aircraft to this discussion). $\endgroup$ – Porcupine911 Mar 3 '15 at 19:16
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To add on to this old question, here's the load relief schedule for a 777-200ER.

enter image description here

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I'm not sure how common flap load relief systems are - I know the Airbus A330 has them and I believe all the Airbus A3xx series do. I've also found at least one post that seems to imply it's common on most "large commercial" aircraft, which I assume encompasses most transport category aircraft.

On the Airbus the system is based on airspeed - When the maximum flap extension speed (VFE) is exceeded by a certain threshold the ship's computer automatically retracts the flaps.
My understanding is the retraction is staged, so if you've got full flaps selected and exceed VFE for that flap setting the system takes out one notch -- presumably it would continue doing so if you're still exceeding the operating limitations of the flap system until it reaches a point where it's convinced you're not going to damage the airframe.

When the overspeed condition is corrected (the speed has reduced to below Vfe by a set threshold) the flaps are returned to the selected position.

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