I know aircraft commonly have rotary actuators to extend and retract the flaps. I am not sure how many but I think I read two per flap on a 747. My question is what is the result if one actuator fails? I don't know if more then one needs to fail in order for a flap not to extend or retract. I am mostly wondering if could cause an aircraft turn-back because somebody told me it could. However that does not seem right to me. I thought the flaps are extended before take-off so that worst case scenario prior to flight is a minor flight delay to replace it.

I suppose if it failed just prior to landing that would be the other scenario and am not sure what the worst impact on a pilots ability to land would be (assuming one failed unit would prevent a flap extension?).

Any experienced pilot probably knows more then I do and any insights are welcome.


1 Answer 1


The 747 has two motors per flap group. The primary means of moving them is through a hydraulic motor, while the second motor is a backup electric motor which is only used if the primary doesn't work correctly.

There are also two groups of flaps on each wing, and they are mechanically linked to the same flap group on the other wing so that they always extend at the same time.

You are correct about a failure on the ground. The pilots would take the airplane back for maintenance before they departed.

In the unlikely event that both the primary and secondary motors failed, or if the flaps were to somehow become jammed and not extend once airborne, the flaps would not be able to do their job. Since they wouldn't make the wing bigger or change it's shape to create more lift, the aircraft would be forced to land at a higher airspeed, which would cause it to use more runway. This isn't a big deal if there is sufficient runway at their destination, but if not they would have to divert to a suitable airport.

  • $\begingroup$ +1 thanks, is helpful. I am thinking your are right, there are probably hydraulic and electric sources of power, that power the many rotary actuators (I think I counted 32 for each wing 744). These probably use the electric or hydraulic power to provide high levels of torque, turning gears, that in turn, pull the chains that move each flap. When I am back at work I may have to confirm the details. In any case I think you helped me see the impact. I suppose some kind of flaps disagree message would result if one flap did not move properly. $\endgroup$
    – esé
    Commented Apr 26, 2014 at 15:59
  • $\begingroup$ Yes, they have three FCU's (Flap Control Units) that command and monitor each system and provide warnings if they disagree, as well as automatically switch to the secondary system. By the way, the title of your question asks about leading edge actuators but the body of the questions specifically mentions flaps, so my answer addresses trailing edge flaps. The leading edge slats are slightly different, but have similar consequences. $\endgroup$
    – Lnafziger
    Commented Apr 26, 2014 at 16:13
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    $\begingroup$ there is an incident with a A320 experiencing a flap lockout, they successfully landed overspeed on the longest runway avherald.com/h?article=47380aba&opt=0 $\endgroup$ Commented Apr 28, 2014 at 12:43

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