As the title says, why only the outboard ("low speed") aileron drops when there is no hydraulic pressure on the 748?

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  • $\begingroup$ Please could you be a bit more precise in your question, what phase? Are you considering the aileron DROOP function that is the flaperon Function of ailerons? What do you mean no hydraulic? Just for this aileron or more than that. However many servo controls become floating in absence of pressure. On ground it drops by its weight. In flight not to twist the wing, it will float. $\endgroup$
    – user40476
    Jun 14, 2019 at 14:05
  • $\begingroup$ @user40476 On the ground, without hydraulic pressure, only the outboard aileron drops due to its weight. I don't talk about the droop function. I think the answer lies in the fact that the inboard aileron is actually a flaperon $\endgroup$
    – DeepSpace
    Jun 14, 2019 at 14:43
  • $\begingroup$ Interestingly, it's the opposite on the 777 :D And it's easier to explain on the 777, with the slow-speed ones having a lock-out mechanism (although I haven't checked if that is really the reason). $\endgroup$
    – user14897
    Jun 14, 2019 at 15:21
  • $\begingroup$ At high speed flaps up a 28 volts signal locks out the outer ailerons only, may be what happens on ground, hydraulic off is just a consequence of the servo control design. $\endgroup$
    – user40476
    Jun 14, 2019 at 18:11
  • $\begingroup$ If the servo control has an idle circuit they will droop unpressurized. If no idle circuit they are hydraulically locked when there is no movement of the servo control out of null and they will stay rigid while unpressurized. $\endgroup$
    – John K
    Jun 14, 2019 at 18:44

1 Answer 1


The Boeing 747-8 (unlike the 777) is not a full fly-by-wire aircraft, but some fly-by-wire controls have been added to the original 747 design. The inboard ailerons are mechanically connected to the flight controls, while the outboard ailerons are not. This allows them to droop without hydraulic power.

From the Boeing 747-8 FCOM (9.20.6 Flight Controls - System Description):

Roll Control

Inboard ailerons are mechanically controlled and hydraulically powered. Outboard ailerons and all spoilers are electronically controlled and hydraulically powered.

Aileron and Spoiler Roll Control

Rotating either control wheel positions ailerons and spoilers. Above approximately 258 knots/.53 Mach, the outboard ailerons are commanded to the neutral position to prevent overcontrolling. The outboard ailerons become operational at lower airspeeds. Inboard aileron deflection is limited when flaps are extended beyond 10.

To improve takeoff performance and reduce noise, the outboard aileron is deflected down for flap detent positions 10 and 20. The downward deflection is removed if the angle of attack is above stick shaker. The inboard aileron is deflected down when the flaps are selected to 10 or greater.

All spoiler panels operate with the ailerons. Spoiler panels are deflected up or down based on Speedbrake lever and control wheel inputs.

747-8 Ailerons

The following image from modernairliners.com shows all new features of the 747-8 wing including the Aileron droop.

747-8 New Wing Elements


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