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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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    $\begingroup$ Electrical options are mentioned in the hydraulics vs. pneumatics question. $\endgroup$
    – fooot
    Jan 4, 2016 at 21:23
  • $\begingroup$ Check out this white paper on the topic $\endgroup$
    – Dave
    Jan 4, 2016 at 21:30
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    $\begingroup$ 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. $\endgroup$
    – Ron Beyer
    Jan 4, 2016 at 22:23
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    $\begingroup$ Electricity also can lead to fires (e.g. if the electric lines are severed -that can happen, too- and sparks contact fuel) $\endgroup$
    – orique
    Jan 5, 2016 at 9:53
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    $\begingroup$ What fluids are used in aviation hydraulics? $\endgroup$
    – Firee
    Jan 8, 2016 at 13:03

6 Answers 6

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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.

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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    $\begingroup$ 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). $\endgroup$
    – Steve
    Jan 5, 2016 at 3:49
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    $\begingroup$ Doesn't the 787 use an electrical aileron control package though (with hydraulic backup)? $\endgroup$
    – shortstheory
    Jan 5, 2016 at 4:45
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    $\begingroup$ @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. $\endgroup$
    – Jan Hudec
    Jan 5, 2016 at 7:38
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    $\begingroup$ @JanHudec Cool, I actually had the misconception that the control surfaces were operated by motors all throughout $\endgroup$
    – shortstheory
    Jan 5, 2016 at 12:52
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    $\begingroup$ The A380 and A350 use Electrohydrostatic actuators (EHA) as a part of all primary flight control surfaces. This is a local hydraulic pump and actuator. This eliminates weight from piping and the problem where a single leak can disable the whole system. These planes only have 2 hydraulic systems as a result. The 787 has a conventional triple hydraulic system, but adds direct electromechanical drive on several control surfaces. $\endgroup$
    – user71659
    May 15, 2019 at 17:13
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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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Weight-to-power ratio of the actuators, but things are changing rapidly.

The main advantage of hydraulic vs. electrical actuation of flight controls is the easy way of distributing power of hydraulics. One large pump can be mounted onto a motor or engine, and generate power for multiple small, light hydraulic actuators.

Hydraulics do have a disadvantage that a damage to the hydraulic pipes can cause loss of control.

True - however, this is more of a problem associated with the central/distributed architecture. In existing aeroplanes, electrical is also generated by generators connected to the engine, then distributed via 400Hz power lines.

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?

  • Electric actuators are still heavier than hydraulic ones: they require magnetic fields and metal in order to convert electrical into mechanical power, while the hydraulic actuator only requires a surface are for the oil to push onto. But the power-to-weight ratios of electrical actuators are rapidly increasing.
  • The safety concerns: there are fewer for electrical actuation, since it is fail passive. A hydraulic actuator can fail hard-over if the servo valve gets stuck, driving the actuator to one of its extremes.

own photo Note that simulator motion used to be done with hydraulic actuators. The hydraulic actuators would have an audible hiss, and maintenance requirements for preventing oil leaks. The hydraulic fluid in these systems was petroleum based, that for aircraft is synthetic which is not flammable but very corrosive requiring specific anti-measures in aircraft.

Every engineer and technician is know who has worked with both hydraulic and electrical actuation has been jubilant once the transition hydraulic => electric occurred, way easier to handle, tune and maintain.

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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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    $\begingroup$ 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. $\endgroup$
    – Jan Hudec
    Jan 4, 2016 at 22:03
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    $\begingroup$ The point about higher force for size is also questionable, since the hydraulic lines and fluid add a lot of weight. $\endgroup$
    – Jan Hudec
    Jan 4, 2016 at 22:04
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Electromechanical (EM) actuators can indeed replace conventional hydraulic actuators. There is also a intermediate, the electrohydraulic actutator (EHA) which consists of a local electric pump and piston actuator. Control is accomplished by servoing the motor rather than a hydraulic servovalve. EHAs used as part of all primary surfaces on the Airbus A380 and A350.

The Boeing 787 uses EM actuators for stabilizer trim and midboard spoilers.

In an Airbus paper on the electrohydraulic architecture of the A380, they point out that their main concerns with electromechanical actuators are with the mechanical transmission between the motor and the surface.

In particular, they note that EM actuators require gear drives which are difficult to assess for the risk of jamming and failure. They also are concerned that when the gear train wears out, it will have slack, which may lead to limit cycle oscillations. Finally, they say that with EHAs, they can be made to operate as a damper for standby and failure mode operations, which makes it easier to integrate with existing hydraulic systems.

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Electro-Mechanical Actuators are capable of performing as primary flight control actuation. BLDC power density and modern controller power electronics may be well suited for the role, but hydraulic actuation has a significant head start in heritage. I'm aware of an experiment a couple decades back that replaced hydraulic actuators on half of an F-18 to evaluate just this question:

https://www.nasa.gov/centers/dryden/pdf/88699main_H-2425.pdf

As aircraft move toward "More Electric" architectures, there may be a greater drive toward EMAs throughout to minimize the required plumbing, maintenance, and system weight of hydraulics or EHAs. That said, airplane builders are typically pretty conservative...

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  • $\begingroup$ Not sure how this answers the stated question. $\endgroup$
    – Ralph J ♦
    May 16, 2019 at 2:11
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    $\begingroup$ Sean, I find your answer interesting and covering an aspect that was missing in other answers. Problem is that in its present form, it hardly answers the question. It's more a comment. To make it fit as a self-standing answer, you could summarize why EMA were not used in the past, and elaborate a bit on the heritage. Then you could continue with the trend to and future importance of EMA. Just my 5 cents, of course. ;) $\endgroup$
    – bogl
    May 16, 2019 at 8:08

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