why hydraulic system in A320

[IFLY_INDIGO]

In A320, all flight control surfaces are moved by hydraulic pressure. They could equally be moved by electric motor only. what is the advantage of using hydraulic over electric?

[vilas]

I FLY _INDIGO
B707 was the last aircrft which did not have powered ailerons and elevators. It used servo tabs to lesson the force required. After that all aircrafts have used hydraulic powered flight controls. The amount of power that is needed and reliability required may be the factor favouring hydraulics. B787 is using electrics in place of pneumatics created so many problems with batteries getting burnt. If flight controls were powered by electrics it could have been fatal. But in future with more advances in technology anything is possible.

[IFLY_INDIGO]

limited capacity of the electric generators could be the one reason which favors the hydraulic system. Also the daily wear and tear would be more in case of electric flight controls compared to the hydraulic ones.

[toffeez]

“All Electrical Flight” has been performed (hydraulic pumps depressurised) according to Airbus.

[Fluffer]

vilas,when did you fly the 707,are you Mad???

[Dan Winterland]

Some British aircraft (Victor and Vulcan and later the VC10) used electro-hydraulic Power Flying Control Units (PFCUs) where electric motors drove hydraulic pumps to power the controls in a self contained unit. And now the A380 has very similar Electric Hydraulic Actuators to move the ailerons in abnormal situations.

Pure electric actuators would require a huge amount of power and need very large and weighty cables.

[grounded27]

what is the advantage of using hydraulic over electric

Hydraulic pressure is standing by waiting for a input to release at least 3000psi to control via small electric valves. The weight and demand of the multitude of electric motors on an aircraft is not yet feasible to my knowledge on commercial aircraft.

[vilas]

Fluffer
I flew B707 in 1986.

[vilas]

toffeez
I am not an authority on A380 but what I know about 380 flight control is that it is fitted with Electric Back up hydraulic Actuator or EBHA and Electro Hydrostatic actuator or EHA. The term used for this is more electrical because hydraulics is still involved. Only 380 pilot can shed more light on this.

[rudderrudderrat]

Hi vilas,

The A380 has three types of servocontrols:
• Conventional servocontrols that include:
􀀗 An actuator
􀀗 A hydraulic block connected to one hydraulic power supply of the aircraft
􀀗 A servovalve that receives orders from the flight control computers and controls the translation direction of the actuator rod.
A conventional servocontrol cannot operate if there is no hydraulic supply.
• Electro-Hydrostatic Actuators (EHAs) that include:
􀀗 An actuator
􀀗 A hydraulic block
􀀗 An electro-hydraulic generation system that receives orders from the fight control computers. The rotation direction and the speed of the electro-hydraulic generation system determine the translation direction and speed of the actuator rod.
EHAs are fully isolated from the hydraulic power supplies of the aircraft.
An EHA can operate when there is no hydraulic supply, but needs an electrical supply.
• Electrical Backup Hydraulic Actuators (EBHAs) that are a combination of a conventional servo-control and an EHA.
In normal mode, they operate as conventional servocontrols.
If there is a hydraulic failure, they operate as EHAs.
http://www.blackholes.org.uk/PP/A380%20actuators.jpg

[vilas]

rudderrudderrat
Thanks a lot. As I understand EHA has its own independant hydraulics, but due to it's construction needs much less. EBHA uses the main hydraulic system fluid but generates pressure electrically. It has replaced Blue system. Can you shed some light on this?

[rudderrudderrat]

Hi vilas,

Have a look at http://www.blackholes.org.uk/PP/A380_Briefing_For_Pilots_Part%202

[Uplinker]

My 2p worth is that hydraulics probably allow faster response, finer control, and far more power to weight ratio than a purely electric actuator.

Imagine a surface that has been commanded to move in one direction then has to stop and instantly move in the other - think inboard ailerons on a B767 for example, which move rapidly up and down on final approach. With an hydraulic actuator a valve just has to move a few mills either way to effect the change of direction. If the surface was being moved by an electric motor driving a screw jack; to reverse direction it would first have to bring the motor and a gearbox to a stop, then start up again the other way. So for each change of movement there would be the inertia of the whole drivetrain to overcome, and the system would not be as responsive as the hydraulic one.

There are linear electrical actuators available, but I don't know if these would be powerful enough.

This I suspect is why even the EHA's on the A380 still use hydraulics as the final operating force.

[clark y]

One of our A320 simulators has no hydraulics. When you think about how much a simulator bounces around, I'm sure there are electric motors strong and quick enough to power flight controls. I suppose the problems are weight, reliability, wiring, generator loading and so on.

On a side note, my understanding is that airbus flight controls are not balanced. Saves weight not having to carry all that depleted uranium around in your flight controls but what happens when all the hydraulic fluid has drained. I know there is no manual reversion, but does flutter become a major issue? I know the flight controls have a damping mode but does this require fluid in the lines or is it something within the actuator itself?

[Chu Chu]

Hydraulic actuators are simpler, cheaper, and (I think) lighter than electric actuators with the same effective power. Putting an electric actuator at each control surface -- or two or three for redundancy -- would probably be cost- and weight- prohibitive.