WebMaximus

A question on hydraulics

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In the real aircraft will the controls such as the yoke be equally hard to move with the HYD PUMPS off as compared to loss of hydraulic pressure?

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The 737 has mechanical reversion on the ailerons and elevator. These control surfaces are hydraulically boosted, but not hydraulically actuated. If there is a total failure of hydraulic pressure, the pitch and roll axes can still be controlled "the old fashioned way", by the cables running from the yoke to the ailerons and elevator. It would require a significant amount of muscle power though - kind of like driving a large truck when the power steering fails - it can be done - but it's a lot of work.

The physical effort in the roll axis would be compounded by the loss of roll spoiler augmentation, as those are exclusively hydraulic - as is the rudder - there is no mechanical reversion there. A complete hydraulic loss in flight on a 737 would be a serious situation - but the aircraft could still be controlled.

On an aircraft which uses hydraulically actuated controls, (like the 777), the yoke would be immovable in roll and pitch with no hydraulic power. 

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Thanks Jim for the very thorough answer but I'm still not sure I fully understand what the controls and the yoke in particular feels like in the real aircraft when the HYD pumps are off.

Reason for my question is I got myself a new FF yoke with lots of settings via the software that comes with the yoke and if the yoke in the real aircraft it quite hard to move until the HYD pumps are ON it would be cool if this could be replicated in the sim.

Edited to add I've seen how the virtual yoke won't move to its full extents when the HYD pumps are OFF so I guess this might be a way to visually simulate what I'm talking about in the NGX?

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I've never worked on a 737, but have a lot of experience with Gulfstreams - which use a similar mechanically actuated / hydraulicly boosted system on the ailerons and elevators.

On a GIV, on the ground (with no air load on the control surfaces), it is actually very easy to move the yoke with no hydraulic power. As easy as doing so in a Cessna 172. (In flight would be another matter).

I would think a 737 would be similar with hydraulics off on the ground.

In an aircraft with hydraulic actuated control surfaces, the yoke is typically completely frozen and locked up with hydraulics off. It can't be moved at all.

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Don't you think the size and thus the weight of the control surfaces on a 737 would make it a bit hard to move them with HYD off vs a smaller aircraft?

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What you really need is a real 737 pilot to confirm, but I suspect that on the ground (i.e. stationary) the surfaces should be reasonably easy to move, as Jim says. It is the aerodynamic load on the surfaces at flying speed which will provide most of the resistance.

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Yep, would be awesome if there's a real NG driver in here reading this that could comment on my question.

Thanks though for your responses so far guys!

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7 hours ago, WebMaximus said:

Don't you think the size and thus the weight of the control surfaces on a 737 would make it a bit hard to move them with HYD off vs a smaller aircraft?

You don't have to lift them and carry them around, all you are doing is rotating them about a control surface axis plus you have a small amount of mechanical advantage or leverage.  The mechanical design includes providing the ability to manually control the surfaces,  I'm sure it is a lot easier than the smaller B-24s that had pilots with large biceps.

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What you say makes sense. I guess I just assumed it would require a greater force to move the controls without help from hydraulics in a big aircraft vs a small aircraft.

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With the hydraulics off at the gate, the yoke will flop forward or aft, based on the wind. It is heavy to move. The ailerons stay centered as far as I've ever seen. I'm not sure if the hydraulics off heaviness at the gate is reflective of the what yoke would do in flight.

I'm sure I've done manual reversion in the sim, but that was probably 10 years ago and I can't remember what it was like. Probably slow and heavy, but doable.

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Many thanks! Just the information I was looking for and I'll now try to replicate this in the sim using the great software that comes with the yoke.

If not possible I'll forward this information to Brunner and maybe they'll be able to include it in a future version of their software.

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18 hours ago, Spin737 said:

With the hydraulics off at the gate, the yoke will flop forward or aft, based on the wind. It is heavy to move. The ailerons stay centered as far as I've ever seen. I'm not sure if the hydraulics off heaviness at the gate is reflective of the what yoke would do in flight.

I'm sure I've done manual reversion in the sim, but that was probably 10 years ago and I can't remember what it was like. Probably slow and heavy, but doable.

 

Pretty much what Matt said, with the HYD pumps off on the ground the control column will be difficult to move but in flight, if you decided to turn all the HYD pumps off (not advised) it would have the same affect as Manual Reversion, not only do you have no Hydraulic assistance, as Skelsey said, you have the aerodynamic loading as well.

To expand on manual reversion..... As a few people mentioned it's ******* difficult to fly with no Hydraulics but can be managed safely. You still have full rudder authority and electric trim available. When you start the checklist and move the FLT CONTROL switches to SBY Rudder and turn on the Yaw Damper you activate a system called WTRIS (Wheel to Rudder Interconnect System) which uses small inputs rudder to assist with turns.

Then you have to Plan a F15 Landing, Lower the Flaps using the SBY system and plan a Manual Gear extension and not to mention all the other things that don't work (check out the QRH)

During the Type Rating it's demonstrated and I don't know of any case where a complete loss of hydraulic pressure has happened on a 737, bad day in the office if it happens!

Hope this helped!

 

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