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Duration of 747 Hydraulic Pressure Reduction?

Posted: Wed Oct 27, 2021 2:24 pm
by AA777223
Forgive me if this is a super rookie, non-technical question to ask:

I was looking at this sad picture of the last UA 744s being scrapped and started to wonder something.



I know have read somewhere in my decades on the forum that aircraft surfaces such as flaps or ailerons will begin to droop over time when parked due to a lack of hydraulic pressure in the various control systems. However, when you see aircraft parked at a gate or hardstand overnight (or sometimes for much longer), this doesn't happen. My question is, how long does it take for these systems to depressurize in a long term storage situation vs. a temporary parking situation? Or are there "locks" or some sort of valves that allow the pressure to be maintained in a static state?

I know it is not uncommon to see A330/340 aircraft with drooping ailerons, even when parked at the gate, but I assume the impact of a dropping outer aileron is deemed far les disruptive to ground ops than a 747s triple slotted flaps would cause in a fully deployed state. Perhaps a design difference between Boeing's cable/pulley system and Airbus's FBW?


I look forward to feedback from the experts. Many thanks!
AA777223

Re: Duration of 747 Hydraulic Pressure Reduction?

Posted: Wed Oct 27, 2021 3:07 pm
by 113312
The answer to this is it depends upon the specific aircraft model and which surfaces.

Re: Duration of 747 Hydraulic Pressure Reduction?

Posted: Wed Oct 27, 2021 5:34 pm
by AA777223
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.

Re: Duration of 747 Hydraulic Pressure Reduction?

Posted: Wed Oct 27, 2021 6:18 pm
by fr8mech
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.

Re: Duration of 747 Hydraulic Pressure Reduction?

Posted: Wed Oct 27, 2021 6:18 pm
by StereoTechque
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.


Flaps generally are position specific ie. have discrete outputs per the Flap lever Position in the Flight deck.
Primary control surfaces like the Aileron, Rudder & Elevators do not have a particular position and are Continuous per the Control wheel/Sidestick input. Hence the mechanism and actuators are quite differently designed for the specific roles.
Flap/Slats on the A320 for eg use Rotary Actuators and tracks for actuation.

Re: Duration of 747 Hydraulic Pressure Reduction?

Posted: Wed Oct 27, 2021 7:54 pm
by Lpbri
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.



Flaps on MD-80s drooped all the time, within minutes of hydraulic depressurization. If you’ve been around them, you can easily hear the creaking as they droop. At hydraulic power up, you wanted to be clear as they retracted. The flaps are driven directly by linear actuators. During overhaul, a check involved hooking up bypass hoses to the actuator return lines and counting the drips with the flaps retracted and hydraulic pressure on. Some was allowed. Rigging flaps and slats on DC-9/MD-80s was done with hydraulics on.

Re: Duration of 747 Hydraulic Pressure Reduction?

Posted: Wed Oct 27, 2021 8:07 pm
by Horstroad
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.

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.

The slats on the MD-11 were also always drooping after some time without hydraulic pressure.

Regarding the original question, it always depends on system design, as said before. Anything driven through a ball screw or gearbox will generally not droop. Anything driven by a linear hydraulic actuator will eventually droop, unless something is preventing it, like a check valve.

The elevators of the MD-11 are a good example. The inboard ones droop after hydraulic pressure is removed, because they are powered by hydraulic actuators. In addition to the actuators, the outboard elevators have hydraulic dampers. The purpose of the dampers is to prevent sudden elevator movement caused by flutter or high frequency buzz while in−flight or sudden high winds while the aircraft is on the ground. They also prevent the outboard ailerons from drooping when hydraulic pressure is removed.

Re: Duration of 747 Hydraulic Pressure Reduction?

Posted: Wed Oct 27, 2021 11:56 pm
by fr8mech
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.


Yup, big problem...well not that big...McBoeing says it's not really a safety of flight item and we've received a "No Technical Objection" (NTO) to operate the aircraft until it's convenient to figure it out. We've changed pumps (#2), restrictors, lock valves, control valves, etc. Eventually it stops doing it. We've had a couple of different aircraft do it.