bio15
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B737 Yoke Moving In Windy Ramp Conditions?

Tue Nov 20, 2007 4:14 am

I came across this picture:


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Photo © Danny Fritsche - Airplanespotters



On the remarks section the photographer says that it was windy, and thus the yoke moved blurrying out in the picture. I don't know much about the 737 or other aircraft with a similar architecture. But my initial thought is that if the hydraulic system is depressurized this shouldn't happen. Maybe even with pressurized hydraulics it shouldn't happen either? I don't really know what type of linkage exists between the yoke and the ailerons or elevator. Could anyone please explain to me a bit how the control surfaces are actuated trhough the hydraulic system, to understand how the event in the picture can happen? Thanks in advance


Please excuse me if my phrasing is confusing

Alfredo
 
ArmitageShanks
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RE: B737 Yoke Moving In Windy Ramp Conditions?

Tue Nov 20, 2007 5:24 am

What about the manual reversion system? That might have something to do with it...
 
YWG
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RE: B737 Yoke Moving In Windy Ramp Conditions?

Tue Nov 20, 2007 5:29 am



Quoting Bio15 (Thread starter):
But my initial thought is that if the hydraulic system is depressurized this shouldn't happen

The system doesn't just go to zero pressure once the system is turned off, but gradually loose this pressure. If the wind on the ground were doing this (this is not the case) then there would be a hell of a fight at cruise level with the massive winds aloft.
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tdscanuck
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RE: B737 Yoke Moving In Windy Ramp Conditions?

Tue Nov 20, 2007 5:31 am



Quoting Bio15 (Thread starter):
On the remarks section the photographer says that it was windy, and thus the yoke moved blurrying out in the picture. I don't know much about the 737 or other aircraft with a similar architecture. But my initial thought is that if the hydraulic system is depressurized this shouldn't happen. Maybe even with pressurized hydraulics it shouldn't happen either? I don't really know what type of linkage exists between the yoke and the ailerons or elevator. Could anyone please explain to me a bit how the control surfaces are actuated trhough the hydraulic system, to understand how the event in the picture can happen?

The 737 has manual reversion, hence this is entirely possible.

The control cable goes from the yoke to a control horn on the actuator. The actuator is rigidly attached to the control surface and the actuator rod is attached to the aircraft structure. If the flight control moves, it moves the actuator, which moves the cable, which moves the yoke. With the hydraulics depressurized, this should be more pronounced because you don't need to push the fluid around as much.

Tom.
 
Tristarsteve
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RE: B737 Yoke Moving In Windy Ramp Conditions?

Tue Nov 20, 2007 5:57 am

You can fly a B737 with no hydraulics. It is checked on test flights.
With ni hyd pressure all three control runs are connected with steel cables between the cockpit and the control surfaces.
If it is windy, the first thing you do on entering the cockpit is put on a hyd pump to stop the controil column from hitting you in the chest!
 
AAR90
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RE: B737 Yoke Moving In Windy Ramp Conditions?

Tue Nov 20, 2007 6:39 am



Quoting Tdscanuck (Reply 3):
The 737 has manual reversion, hence this is entirely possible.

Not only possible, but quite normal. Does not require very much wind to get the yoke moving when all hydraulics are turned off.
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HAWK21M
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RE: B737 Yoke Moving In Windy Ramp Conditions?

Tue Nov 20, 2007 7:57 am

Very Normal in windy conditions & Hydraulics off.
Remember Manual reversion exists on the B737 Ailerons.
regds
MEL
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OldAeroGuy
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RE: B737 Yoke Moving In Windy Ramp Conditions?

Tue Nov 20, 2007 4:20 pm



Quoting HAWK21M (Reply 6):
Remember Manual reversion exists on the B737 Ailerons.

And the elevators.
Airplane design is easy, the difficulty is getting them to fly - Barnes Wallis
 
wilco737
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RE: B737 Yoke Moving In Windy Ramp Conditions?

Tue Nov 20, 2007 5:21 pm



Quoting HAWK21M (Reply 6):



Quoting OldAeroGuy (Reply 7):

But not very comfortable... Tried that in real life (don't worry, was a maintenance flight and we had to do that) and I had to do it as well in the simulator. You need quite a lot of power to fly the airplane... But it is possible... But I am always happy when we have hydraulic Big grin
Oh, on regarding to the question: yes, the wind can move the elevator or ailerons and then the yoke moves...

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OldAeroGuy
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RE: B737 Yoke Moving In Windy Ramp Conditions?

Tue Nov 20, 2007 8:45 pm



Quoting WILCO737 (Reply 8):
You need quite a lot of power to fly the airplane

I presume you mean muscle power rather than engine.

Of course manual revision is not a frequently encountered mode since it requires a two hydraulic system failure to reach this state,
Airplane design is easy, the difficulty is getting them to fly - Barnes Wallis
 
wilco737
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RE: B737 Yoke Moving In Windy Ramp Conditions?

Tue Nov 20, 2007 8:57 pm



Quoting OldAeroGuy (Reply 9):
I presume you mean muscle power rather than engine.

Yeah, I meant muscle power Big grin hehe

Quoting OldAeroGuy (Reply 9):
Of course manual revision is not a frequently encountered mode since it requires a two hydraulic system failure to reach this state,

I wouldnt even call it a mode! I would call it an emergency if that happens... its so difficult to handle ...

WILCO737 (MD11F)
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EMBQA
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RE: B737 Yoke Moving In Windy Ramp Conditions?

Tue Nov 20, 2007 9:36 pm

Quoting Bio15 (Thread starter):
Could anyone please explain to me a bit how the control surfaces are actuated trhough the hydraulic system, to understand how the event in the picture can happen?

Boeing still uses on many of its models control cables from the yoke to the respective flight control hydraulic power units. Boeing likes this idea as they say it gives the crew a 'feel' for the airplane. Airbus has gone to the all Fly By Wire, where only a wire bundle connects the yoke to the respective flight control hydraulic unit. There is a little more to the system, but this is the basics. Anyway........ if the wind blows hard enough the flight control will move slightly moving the input actuator from the control unit. This control unit is directly connected to the control cables which run up to the yoke in the cockpit. The movement is very slight and can only really be seen in this long exposure photo.

[Edited 2007-11-20 13:39:02]
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AAR90
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RE: B737 Yoke Moving In Windy Ramp Conditions?

Tue Nov 20, 2007 10:33 pm



Quoting EMBQA (Reply 11):
...the flight control will move slightly... ...The movement is very slight and can only really be seen in this long exposure photo.

It MIGHT be slight, but in the 737 it is usually FULL THROW. The reason one of the first things a pilot does upon entering the cockpit is to turn on one electric hydraulic pump to prevent injury from wind-blown control column movement! Even a "slight" wind can cause RAPID and FULL control column movement.
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JAGflyer
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RE: B737 Yoke Moving In Windy Ramp Conditions?

Tue Nov 20, 2007 10:53 pm



Quoting AAR90 (Reply 12):

Hans Moleman presents pilot getting hit by control column!

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OldAeroGuy
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RE: B737 Yoke Moving In Windy Ramp Conditions?

Wed Nov 21, 2007 4:34 am



Quoting EMBQA (Reply 11):
Boeing still uses on many of its models control cables from the yoke to the respective flight control hydraulic power units.

You're not describing the manual reversion system used on the 737 (or DC9/MD80 et. al. for that matter). For manual reversion, there is a direct mechanical link between the control column/yoke and the elevators/ailerons. If the control surfaces move, the control column/yoke will move in direct response and vice versa. This can take place with no hydraulic power involved. On the 707, the elevator was not hydraulically powered. It was flown manually full time with the help of servo tabs.

Quoting WILCO737 (Reply 10):
I wouldnt even call it a mode! I would call it an emergency if that happens... its so difficult to handle ...

On the other hand, I've seen a 120 lb female pilot fly the 737 in manual reversion without undue difficulty.
Airplane design is easy, the difficulty is getting them to fly - Barnes Wallis
 
wilco737
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RE: B737 Yoke Moving In Windy Ramp Conditions?

Wed Nov 21, 2007 5:43 am



Quoting OldAeroGuy (Reply 14):
On the other hand, I've seen a 120 lb female pilot fly the 737 in manual reversion without undue difficulty.

yeah, it is difficult to handle, but there are tricks to make it easier Big grin But I still prefer the flying with hydraulic Big grin

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OldAeroGuy
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RE: B737 Yoke Moving In Windy Ramp Conditions?

Wed Nov 21, 2007 6:39 am



Quoting WILCO737 (Reply 15):
But I still prefer the flying with hydraulic

As does everyone who flies the 737.
Airplane design is easy, the difficulty is getting them to fly - Barnes Wallis
 
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HAWK21M
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RE: B737 Yoke Moving In Windy Ramp Conditions?

Wed Nov 21, 2007 7:39 pm



Quoting OldAeroGuy (Reply 16):
As does everyone who flies the 737

Mx would prefer the cable as there would be no Skydrol  Smile

regds
MEL
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bio15
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RE: B737 Yoke Moving In Windy Ramp Conditions?

Thu Nov 22, 2007 1:53 am

Hi everyone, I appreciate the responses.

First I would like to say that I am ignorant in the systems on such aircraft, but I understand some terminologies and have a good idea of the engineering behind systems and mechanisms.

----

Quoting Tdscanuck (Reply 3):
The 737 has manual reversion, hence this is entirely possible.

The control cable goes from the yoke to a control horn on the actuator. The actuator is rigidly attached to the control surface and the actuator rod is attached to the aircraft structure.



Quoting OldAeroGuy (Reply 14):
For manual reversion, there is a direct mechanical link between the control column/yoke and the elevators/ailerons. If the control surfaces move, the control column/yoke will move in direct response and vice versa. This can take place with no hydraulic power involved.

To be honest, I have no idea of what manual reversion is  Smile. From what I get of the responses, it's a way of having direct mechanical control over the control surfaces in case of a hydraulic failure, right? I get a vague and general idea, but an image is worth a thousand words, does anyone happen to have any diagrams? I find those very interesting and clear.

Alfredo
 
AAR90
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RE: B737 Yoke Moving In Windy Ramp Conditions?

Thu Nov 22, 2007 3:28 am



Quoting Bio15 (Reply 18):
To be honest, I have no idea of what manual reversion is

Try reading this
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HAWK21M
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RE: B737 Yoke Moving In Windy Ramp Conditions?

Thu Nov 22, 2007 3:22 pm



Quoting Bio15 (Reply 18):
From what I get of the responses, it's a way of having direct mechanical control over the control surfaces in case of a hydraulic failure, right

Thats true.
regds
MEL
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bio15
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RE: B737 Yoke Moving In Windy Ramp Conditions?

Thu Nov 22, 2007 7:48 pm



Quoting AAR90 (Reply 19):
Try reading this

Thanks! It also contains interesting information on control feel design by Boeing.

Quoting HAWK21M (Reply 20):

Thats true.
regds
MEL

Something similar in the article posted by AAR90:

"In the event that both independent hydraulic systems “A” and “B” become unavailable the ailerons, elevator controls automatically revert to a simple mechanical reversion backup system. The control cables move the ailerons and elevator flight control surfaces directly, requiring higher pilot input control forces to move them."


When in the mechanical reversion mode, the control cables move the control surfaces directly. Are these the same cables that drive the hydraulic PCUs? Or are they separate cables that "come into action" by some sort of mechanism under hydraulic failure?

I'm thinking that if there's only one cable per control surface, it should be some kind of bifid cable, one end attached to the control surface and the other attached to the PCU. So in case of PCU inability, only one cable end becomes unusable and the other cable end is still directly attached? Is my imagination going too wild?  Smile


Alfredo
 
tdscanuck
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RE: B737 Yoke Moving In Windy Ramp Conditions?

Thu Nov 22, 2007 10:58 pm



Quoting Bio15 (Reply 21):
When in the mechanical reversion mode, the control cables move the control surfaces directly. Are these the same cables that drive the hydraulic PCUs? Or are they separate cables that "come into action" by some sort of mechanism under hydraulic failure?

As far as I know, they're the same cable. The PCU is fixed to the control surface, not the aircraft. So, even if the PCU goes dead, the cable is still pulling directly on the PCU, which pulls on the control surface. Each control surface also has two cables going to it (A & B channel).

Tom.
 
PGNCS
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RE: B737 Yoke Moving In Windy Ramp Conditions?

Fri Nov 23, 2007 12:22 am



Quoting OldAeroGuy (Reply 14):
On the other hand, I've seen a 120 lb female pilot fly the 737 in manual reversion without undue difficulty.

She must have been extremely strong for her size to not experience "undue difficulty," because I am a fit 220 lb male and I think it is extremely taxing and difficult to fly anywhere near precisely. The DC-9/MD-80 system, though, is never a problem for anyone I have seen with or without hydraulics.
 
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HAWK21M
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RE: B737 Yoke Moving In Windy Ramp Conditions?

Fri Nov 23, 2007 6:47 pm



Quoting Bio15 (Reply 21):
When in the mechanical reversion mode, the control cables move the control surfaces directly. Are these the same cables that drive the hydraulic PCUs

They are the same cables that operate the PCUs,when Hydraulics are not functioning,an extra mvmt mechanically moves the Links to operate the Cables & the Contol surfaces.
regds
MEL
I may not win often, but I damn well never lose!!! ;)

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