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.