AA777223 wrote:113312 wrote:The answer to this is it depends upon the specific aircraft model and which surfaces.
The 744 flaps, as referenced in the OP would be a great start.
fr8mech wrote:The flaps on any aircraft, as a rule, will not droop.
Take a B747, for instance. The flaps are not driven by an actuator in the sense that the primary flight controls are. The flaps are driven on jackscrews running in transmissions (all that stuff lives under the fairing or canoe) that are driven by a power drive units (1 inboard flaps, 1 outboard flaps) that live in the wheel well. Loss of hydraulic pressure or fluid, for that matter, will not cause the flaps to droop. This is typically of Boeings.
Aircraft that have flaps that are driven by actuators…the MD11 or DC8 come to mind…will have check valves and/or restrictors that prevent the flaps from drooping with loss of pressure. These we do find drooped sometimes, and that tends to be an indicator that the system has a bypassing problem somewhere. How quickly they droop, depends on the severity of the problem. We’ve had MD11’s where the flaps fully droop before the flight crew is off the aircraft.
fr8mech wrote:Aircraft that have flaps that are driven by actuators…the MD11 or DC8 come to mind…will have check valves and/or restrictors that prevent the flaps from drooping with loss of pressure.
Horstroad wrote:The MD-11 had the issue when you would pressurize the #2 hydraulic system without pressurizing the other two hydraulic systems, the flaps would extend/droop after depressurizing the hydraulic system. I've not looked into why this is, I just accepted it as is and that this is something to be aware of. The flap lock valves should prevent extension of the flaps when pressure is lost, but in this case they don't.
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