A320 Elevators

[B737NG_Pilot]

From DSC-27-10-20 Elevator Actuation

Damping : The jack follows surface movement.
Does this mean that elevator deflection is reduced? How is different from the active mode.

If neither jack is being controlled hydraulically, both are automatically switched to damping mode.
With loss of HYD how are these jacks being controlled?

If neither jack is being controlled electrically, both are automatically switched to the centering mode.
How are these jacks positioned if there is loss of electrical power?

Some maneuvers cause the second jack to become active.
Which Manoeuvres are these.

[Uplinker]

I have searched, but FCOM apparently does not tell us now.

The general principle is that each actuator cylinder has a feed of pressurised fluid to each side of its piston. These feeds are controlled by valves. The valves have four positions: open, closed, return to reservoir, and closed circuit. To move the piston, one valve is opened to the pressure feed, the other valve allows fluid on the other side of the piston to return to the reservoir, so the piston is pushed away from the pressure side, thus moving the surface it is connected to.

1. Damping prevents vibration, fluttering and 'bouncing'. On a car, dampers prevent the suspension springs from doing the same things as the car travels over bumps in the road. (car dampers are often called "shock absorbers").
On the aircraft, in damping mode both hydraulic valves go to the closed circuit position, so fluid can move between the valves and thus allow the piston to move, but against the resistance of the fluid flowing through the pipework circuit between the two valves. This resistance provides the damping force while allowing piston movement. Damping still allows full travel of the piston, and does not need system pressure from the Hyd pump.

2. If both valves are closed, fluid cannot flow so the piston will be held in one position. The valves probably fail to the closed position if electrical power is lost. We are not told how the system sets the piston to the centre position before both valves close, but there must be some sort of interlock.

3. Again, we are not told, but any commanded rate of control surface movement beyond what a single jack can provide will automatically employ force from the other jack. There is electronic feedback of the surface position to all the associated FBW computers. I can imagine that the secondary FBW computer monitors the primary and assists if necessary - in normal operation the primary jack provides its full force and the secondary jack might assist with, say, 10% of its force for very rapid movements. If the first computer or jack has failed totally, the secondary computer will need to energise its jack 100% to achieve the required movement and thus will take over completely, providing the back-up function.


As I say, this is a general principle of how hydraulic control surface jacks work.

[tubby linton]

https://cimg9.ibsrv.net/gimg/pprune.org-vbulletin/2000x1220/ce29a3ee_998a_43e7_8219_ac8b88d0a24b_71dff053ea854c4031166a4 0267df0797ca94f41.jpeg

[tubby linton]

The four fixed body servo actuators are equal and interchangeable. The servo actuators can operate in three modes. - active mode - damping mode - centering mode In active mode the jacks are electrically controlled. In damping mode the jacks will follow the surface movement, and in centering mode the jack is hydrauli-cally maintained in neutral position. In the event of high load-factor demand that would cause one servo actuator to stall, the second actuator in damping mode automatically becomes active, both actuators will thus be active. Active mode means solenoid valves ( 1 ) de-energized and pressure line clo-sure valve ( 2 ) open, return line closing valve ( 3 ) also open, connecting the actuator to pressure and return. The High pressure flow will thus activate the mode selector valve ( 5 ).The mode selector LVDT ( 11 ) will provide mode feed back to the ELAC and SEC's. Steering input from the side stick or the FMGC is routed via the EFCS com-puter in command to the servo valve ( 4 ), controlling the actuator piston. Servo loop feed back to the EFCS is provided by the position RVDT ( 10 ). The se-cond RVDT is back-up, used in case of failure. The servo valve transducer ( 12 ) is used for monitoring. Damping mode means solenoid valves ( 1 ) are energized, powered from the standing-by ELAC and SEC. The mode selector valve is displace due to the spring, this causes the intercon-nection of the two actuator chambers through the damping orifice ( 6 ). The mode selector LVDT identifies the mode change to the EFCS. A certain amount of reserve fluid is maintained in the actuator by the reserve reservoir ( 9 ) and the check valve ( 7 ). - to hold the volume of fluid if there is a leakage or if the hydraulic fluid temperature changes. When the Elevator surface is moved by the active actuator, the actuator in damping mode will follow the movement and provide a certain resistance to the movement.
Centering mode becomes active in case of loss of power to both controlling computers and hydraulic pressure are still present. The centering device ( 13 ) mechanically keeps the actuator in the center posi-tion, preventing movement of the surface. The deflection of the remaining surface is in this case limited in order to pre-vent excessive asymmetrical load on the tailplane and the rear fuselage.Numbers in text refer to the diagram in the previous post.

[B737NG_Pilot]

Tubby thank you so much for your effort. Your explanation made it quite clear. With regards to the Elevators... What are these manuevers that causes the jack to enter the dampening mode.

Also with regards to the Ailerons, the FCOM mentions the following statement "One of these servojacks per aileron operates at a time." but does no such mention for the Elevators. FCOM DSC-27-10-20.

So in case of the ailerons, how does the second the servo jack follow the aileron movement when it is in the damping mode, as only one operates at a given time.

[B737NG_Pilot]

Uplinker.... Thanks for the very detailed explanation. Yes the FCOM's don't mention much.

[tubby linton]

https://cimg1.ibsrv.net/gimg/pprune.org-vbulletin/1239x1164/3252ff35_ec32_4811_90db_6866e6a9ea98_c7310c33883eee082821f82 323a8efd8fa7de41c.jpeg

[tubby linton]

The above shows how the servos for the elevators are signalled.I will do some research for the ailerons and will post later if I discover the answer.

[Jet II]

Tubby thank you so much for your effort. Your explanation made it quite clear. With regards to the Elevators... What are these manuevers that causes the jack to enter the dampening mode.



I think you are getting confused - large manoeuvrers cause the 2nd actuator to enter 'active' mode.


https://i.ibb.co/hBFcKCc/a320.png (https://ibb.co/0CyKfQK)

[tubby linton]

Aileron servo

Active Mode

Activation of the unit requires power to the solenoid valve ( 1 ). This will cause the pressure line closing valve ( 2 ) and the return line closure valve ( 3 ) to open and the mode selector valve ( 5 ) will change to active position. The LVDT ( 11 ) supplies a signal identifying this position. The two chambers of the actuator are now connected to the servo valve ( 4 ), controlled by the ELAC. The feedback transducer ( 10 ) gives the servo loop feedback.

Damping Mode
The solenoid valve is deenergized. The mode selector valve moves to the re-laxed position by the spring. The two chambers are now interconnected through the damping orifice ( 6 ). In case of electrical failure the solenoid will deenergize and the unit goes to damping mode. In case of Ioss of hydraulic pressure the pressure and return line closing valve will close, causing the unit to revert to damping mode. The servo valve, solenoid valve and the mode selector valve are all LRU's. After replacement of the servo valve, adjustment of the feed back transducer is required. A adjustment device is located on the actuator piston end ( adjusting nut ).

Aileron Servo Valve
The aileron command from the ELAC is received by the servo control valve. The valve consist of a jet nozzle springloaded to the neutral position. The steering signal from the ELAC wiil activate the magnetic coil and reposition the jet nozzle sending the hydraulic pressure to one of the sides on the control sleeve. The control sleeve will guide the hydraulic pressure to either side of the aileron actuator and will connect the other side to return. Steering feed back signal to the ELAC is transmitted by the feedback trans-ducer located inside the aileron actuator

[B737NG_Pilot]

Thank you tubby for all your effort. To check if I have got it right, In case of ailerons, only one servo jack controls each aileron. The other one remains on standby. Only in case of a failure, the other jack becomes active.

In case of elevators, both servo jacks control the elevator, one in active mode one in damping mode. This is true during normal manoeuvres.

During large pitching Manoeuvres both follow the elevator. Shouldn’t one be in dampening mode to prevent excessive movement.

[tubby linton]

In pitch ELAC 2 will be in command in normal configuration, operating the two inboard servojacks supplied by the green and the yellow hydraulic system. ELAC 1 will be in monitor mode and the two outboard servojacks ( blue hydrau-lic system ) will be in damping mode. In case of fault on ELAC 2 or the servojack, the backup will become active and will maintain full control of the surface. If one servojack looses its electrical sup-ply it will be switched to centering mode. In case of failure on both ELAC l and 2, control of the Elevators are automati-cally switched to SEC 1 or 2 depending of the status.
As stated above if you demand a high load factor through the side stick both actuators become active.

[tubby linton]

https://cimg4.ibsrv.net/gimg/pprune.org-vbulletin/2000x1250/ec2f16b9_94ba_4a29_bb08_d31190539e95_6e9235fdc4c806c838c280f d1abdc68578f13be1.jpeg
Pitch law

[tubby linton]

Ailerons
Each aileron can be actuated by two different servocontrols. In normal operation, one servocontrol per aileron is active ( controlled by ELAC 1 ), the other is in damping mode - on the left aileron, the blue servocontrol is active - on the right aileron, the green servocontrol is active The servocontrols, controlled by the ELAC 2 , green on the left side and blue on the right side are in damping mode.
The ailerons are normally controlled by the ELAC 1, the ELAC 2 is in standby and the associated servocontrols are in damping mode. In case of ELAC 1 failure, the control of the ailerons is automatically transferred to the ELAC 2 which becomes active through the left green and right blue servo controls.
In that case the servo controls dedicated to the ELAC 1 revert to the damping mode. In case of double ELAC failure, or blue and green hydraulic system low pres-sure, all ailerons are in the damping mode.
If ELAC 1 can only drive one aileron, the ELAC 2 drives the other aileron from the order computed by the ELAC 1.

[Uplinker]

@ #11

The ailerons and elevator flight control surfaces have two servo jacks each.

Normally, one jack is active - moving the surface, the other jack is passive - in damping mode.

Damping does not restrict the travel of the surface or the other jack; it just damps it - stops vibration or flutter.

Have another read through of the chapter starting with DSC 27-10-10, in particular the General Architecture diagram at DSC 27-10-20. The diagram shows which surface is controlled by which Hyd system and which FBW computer. The arrows next to each surface show the sequence in which the computers take over from each other if the primary computer or Hyd system for that surface fails.

Take your time and read the whole chapter through several times slowly, with a cup of tea or something. It looks complicated but the principle is quite simple :ok: