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I'm sure it's possible to use servo motors to get precise control of the ailerons, elevators or the rudder. But still, hydraulics are widely used instead.

Hydraulics do have a disadvantage that a damage to the hydraulic pipes can cause loss of control. Even with redundancy, there are cases where all three hydraulic lines get damaged at the same time and there's complete loss of control (can't remember the example). I think in some smaller aircrafts servo motors are used. But why not in the larger ones. Is it because of the torque required?

What are the issues with using electric motors? If used what are the safety concerns? In terms of safety, how different are they from hydraulics?

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Check out this white paper on the topic – Dave Jan 4 at 21:30
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Electric motors are usually very heavy, requiring quite a bit of copper and magnetic material to operate. On top of that, servo motors typically require a lot of power, especially when being operated quickly like ailerons, rudders, and elevators need to be. That being said, there are servo-hydraulics (servo valves, hydraulic actuators) in aircraft. – Ron Beyer Jan 4 at 22:23
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Electricity also can lead to fires (e.g. if the electric lines are severed -that can happen, too- and sparks contact fuel) – orique Jan 5 at 9:53
    
What fluids are used in aviation hydraulics? – Firee Jan 8 at 13:03
up vote 13 down vote accepted

I'm sure it's possible to use servo motors to get precise control of the ailerons, elevators or the rudder.

No, it isn't. Primary flight control actuators require both rather high forces and quick response time. Electric motors can provide either, but both at the same time is a problem. Hydraulics is still better for that combination.

Note that other actuators that don't require the fast response time (like flaps, gear or horizontal stabilizer foreplane (elevator trim)) are electric in some aircraft.

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Is it possible that fewer hydraulic motors are required because they can be shared throughout the plane? I would think each surface would need it's own servo-motor (or linkages). – Steve Jan 5 at 3:49
    
Doesn't the 787 use an electrical aileron control package though (with hydraulic backup)? – shortstheory Jan 5 at 4:45
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@shortstheory, I suspect you are confusing fly-by-wire with the actual actuation. The system is fly-by-wire, meaning that control column is sensed electrically and computer processes the commands to the control surfaces, but the actuation can still be hydraulic. The computer uses solenoids, that have the precision and fast response, to control hydraulic valves, which does not need much power, and uses hydraulics to provide the forces. I have not studied 787 systems, but I am certain that's how it works in FBW Airbus types. – Jan Hudec Jan 5 at 7:38
    
@JanHudec Cool, I actually had the misconception that the control surfaces were operated by motors all throughout – shortstheory Jan 5 at 12:52

As mentioned already, hydraulic systems are fast and powerful but you need to also consider how extremely efficient and simple they are. Reducing complexity reduces maintenance costs which is a large factor. Flaps, trim devices, and landing gear doors are the only good places to use motors.

I believe the specific example you're looking for was the Sioux City DC 10 accident in the 80's (https://en.wikipedia.org/wiki/United_Airlines_Flight_232). The DC10 had an achilles heel in that all of the hydraulic lines ran parallel to each other in one section of the tail. When the tail engine suffered a catastrophic failure a fan blade struck through all 3 (?) hydraulic lines which led to a complete hydraulic system loss. Airplanes since then have been designed with the hydraulic lines running in separate places and further apart (i.e. different sides of the rudder) to prevent total flight control system failure.

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Planes use hydraulics because of the immense pressures on the control surfaces during flight. Hydraulic systems can deal with higher loads than motors of similar size. An electric motor can get "stuck" when it encounters a load that is greater than it can move, possibly causing a crash if it happened in an airplane.

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A hydraulic actuator will get "stuck" just the same when it encounters a load that is greater than it can move. That is a matter of using suitably powerful actuators rather than choice of their type. – Jan Hudec Jan 4 at 22:03
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The point about higher force for size is also questionable, since the hydraulic lines and fluid add a lot of weight. – Jan Hudec Jan 4 at 22:04

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