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asadkamal

Flaperon droop on ramp

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In the sim, soon after you depressurize the hydraulic systems the flaperons droop. Travelling almost every week out of Toronto airport, I see a lot of the Air Canada 777s on the ramp, where I see that the rudder is deflected but the flaperons and ailerons aren't drooped. Is there a reason for that ?

 

Thanks in advance!!

 

 

Regards,

Asad Kamal

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In the sim, soon after you depressurize the hydraulic systems the flaperons droop. Travelling almost every week out of Toronto airport, I see a lot of the Air Canada 777s on the ramp, where I see that the rudder is deflected but the flaperons and ailerons aren't drooped. Is there a reason for that ?

 

Thanks in advance!!

 

 

Regards,

Asad Kamal

The only way that would happen is for one of the Hyd systems to be pressurized. I don't believe there is any way to lock these surfaces up, and also it is not standard practice to have live Hyd systems at the gate unless they are testing something. 

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Also I think I'm right in saying that it takes a while for pressure to bleed away after use so there may not be immediate droop

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The only way that would happen is for one of the Hyd systems to be pressurized. I don't believe there is any way to lock these surfaces up, and also it is not standard practice to have live Hyd systems at the gate unless they are testing something. 

 

I think he means that the flaperons droop when they should not, from his experience looking at 777's on the ramp.

 

The flaperon "logic" of the 777 is complex. I don't know if they should droop or not once pressure is removed and/or under which conditions they do so. Maybe they are locked in place to prevent unwanted movement which could be potentially dangerous.

 

Here's some pictures, we don't know how the hydraulic panel was set when these pics where taken though, I think it's safe to assume that the panels where in the standard OFF state (everything OFF except for L and R engine primary pumps). It's hard to find a clear picture of the trailing edge with the aircraft parked on the ground...

 

http://cdn-www.airliners.net/aviation-photos/middle/0/5/1/2661150.jpg

 

http://cdn-www.airliners.net/aviation-photos/photos/0/5/8/2472850.jpg

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Powered flight controls are moved by servo valves which port pressure to one side of the actuator or the other. If there is no path to return the hydraulic fluid in the actuator can't move and the surface will not droop, even if there is no hydraulic pressure.

 

This photo shows a 777 with a drooping flaperon and non drooping elevator.

 

https://www.planespotters.net/photo/428266/vh-vph-virgin-australia-boeing-777-3zger

 

It may well vary from airframe to airframe.

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Powered flight controls are moved by servo valves which port pressure to one side of the actuator or the other. If there is no path to return the hydraulic fluid in the actuator can't move and the surface will not droop, even if there is no hydraulic pressure.

 

Thanks Kevin.

 

May I ask, what about elevators and rudder? Why do those "droop"?

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Gravity and wind, respectively.

 

Yes, that's what I thought, but what happens if the hydraulic circuit doesn't allow for actuator displacement as is the case for the Flaperons like Kevin said? They should be "locked" in place. Obviously they are not, so what am I missing?

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so what am I missing?

 

True or false: each hydraulic pump serves each and every flight control surface.

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True or false: each hydraulic pump serves each and every flight control surface.

 

False. But I still don't get it (sorry). Maybe a hydraulic diagram with the lines and all will help :)

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Yes, that's what I thought, but what happens if the hydraulic circuit doesn't allow for actuator displacement as is the case for the Flaperons like Kevin said? They should be "locked" in place. Obviously they are not, so what am I missing?

I didn't mean just the flaperons, I meant all powered flight control surfaces. Whether they droop or not may depend on how much leakage there is in the system, if the servo valve was centered when pressure was removed, and so on. It may be elevators are more likely to droop, I don't know. Only someone with a very detailed knowledge of the aircraft would know for sure.

 

The column is angled forwards slightly when neutral. That would mean a slight bias from the mass of the column towards elevator nose down as pressure is removed, which could open up a return path past the servo. That might be why the elevators droop. All speculation on my part as I don't know the detailed design of the system. If there are bypass valves in the system then they would positively open a return path and allow the surfaces to droop.

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Thanks Kevin,

 

Yes, the systems are complicated. To really understand what's going on one would need the actual hydraulic system diagrams (the maintenance/engineering diagrams, not the watered down diagrams for the pilots), and I don't think those are easily obtainable by the general public.

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Thanks a lot for all the replies. Don't think it's a 'time' thing, was on an AC 77W yesterday from Toronto to Vancouver and when I boarded the airplane had been on the ground for atleast an hour and half if not more on the turnaround. The flaperons nor the ailerons were drooped. I have observed the same behavior on Saudi 777s at Jeddah last year.

 

Not complaining about the PMDG behavior at all, I actually quite like the droop :), just trying to understanding why the real world behavior seems to be different.

 

Will try and catch a picture today on my return flight, unfortunately I am not on a 777 today but might be able to catch one on the ramp at Vancouver.

 

Regards,

Asad Kamal

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Don't think it's a 'time' thing

 

Hydraulic pressure takes time to fall, if it is allowed to fall.

 

 

 


just trying to understanding why the real world behavior seems to be different.

 

Likely because their config is different from the config you have in the sim.

 

Some operators do things differently. Some operators will turn off system A, but leave system B on, for 737s, as an example.

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It all has to do with the lunar cycle just as it affects wave generation.

When the moon is full and its exerting the most gravitational force on the earth, the surfaces are more prone to droop when not pressurized.  

I know this is true because I read it on the internet. :P  

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It all has to do with the lunar cycle just as it affects wave generation.

When the moon is full and its exerting the most gravitational force on the earth, the surfaces are more prone to droop when not pressurized.  

I know this is true because I read it on the internet. :P  

 

haha, good one.

 

I have a friend of mine who's an IT technician. Whenever a computer does something that nobody understands, the first thing he says is: Okay people, let's try walking out of the room, and getting back in, maybe that will do it!

 

Surprisingly enough, sometimes systems work like that! haha

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haha, good one.

 

I have a friend of mine who's an IT technician. Whenever a computer does something that nobody understands, the first thing he says is: Okay people, let's try walking out of the room, and getting back in, maybe that will do it!

 

Surprisingly enough, sometimes systems work like that! haha

 

When I was an avionics tech in the Navy, we'd sometimes blame radio communication issues on sun spot activity....sometimes it was actually true.  :wink:

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Unlike the other flight controls the only modes the flaperon PCUs have are normal, and bypass. When there’s no hydraulic pressure the PCUs are in bypass mode which allows the flaperon to move freely. They will slowly droop under their own weight when parked. If the PCUs go into bypass mode in flight aerodynamic loads will pull them up. A good example of this is the flaperon during takeoff which is in bypass mode until 100 knots.

 

The other flight controls have also have a blocking/damped mode which behave differently depending on the control surface. The elevator drooping at shutdown it depends on its position when hydraulic pressure is removed. In the case of a failure of both PCUs on an elevator, the PCUs will go to bypass mode until airloads move the elevator within two degrees of being flush. At that point the PCUs will go into blocking mode which hydraulically locks the elevator in place. When you see elevators drooped it’s because the the elevator was more than two degrees trailing edge down when hydraulic pressure was removed. If you remove hydraulic pressure with the elevator up it should not go past the neutral position.

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Unlike the other flight controls the only modes the flaperon PCUs have are normal, and bypass. When there’s no hydraulic pressure the PCUs are in bypass mode which allows the flaperon to move freely. They will slowly droop under their own weight when parked. If the PCUs go into bypass mode in flight aerodynamic loads will pull them up. A good example of this is the flaperon during takeoff which is in bypass mode until 100 knots.

 

The other flight controls have also have a blocking/damped mode which behave differently depending on the control surface. The elevator drooping at shutdown it depends on its position when hydraulic pressure is removed. In the case of a failure of both PCUs on an elevator, the PCUs will go to bypass mode until airloads move the elevator within two degrees of being flush. At that point the PCUs will go into blocking mode which hydraulically locks the elevator in place. When you see elevators drooped it’s because the the elevator was more than two degrees trailing edge down when hydraulic pressure was removed. If you remove hydraulic pressure with the elevator up it should not go past the neutral position.

Thanks for the detailed explanation Brian. So why are flaperons often seen not drooping when apparently without hydraulics, as the OP asked?

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I don’t have a definitive answer other than there’s nothing system wise that would cause them to droop on one shutdown and not on the next. It most likely comes down to environmental conditions vs.mechanical resistance.

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I have pictures from last week at Vancouver. Couldn't get an AC shot but fortunately there was a Air New Zealand 777 on the ramp. These pictures were taken at least 2 after shutdown if not more. Whats the best way to attach a picture to a reply? , my apologies in advance for my ignorance. 

 

Regards, 

Asad Kamal

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