A320 ELAC Fault

[Last Ditch]

Hi guys,
I'm an old foggie but new on the A320s.
The other day we had an ELAC2 failure in flight that was easily resolved by s/w off option (reset didn't work), however felt uneasy working on only one ELAC.
Post flight ground study revealed-
1. ELAC2 fault on ground is a no-go, while ELAC1 fault is a MEL. Any idea why? Could it be due to elevator actuators that these computers command? (ELAC2 -G & Y driven, ELAC1 -only B driven)
2. How come abnormal procedures don't cover ELAC1+2 fault?

Thanking you in advance. Inputs with references would be most appreciated.

[Torqueman]

You have pretty much answered your own question.

G + Y have independent power sources and a failure in one will still give you control of the other flight surface. If you MEL the ELAC controlling the independent sources (G+Y) and you happen to lose the blue system you lose the flight surfaces. It is probably more important for aileron control. The elevators have SEC's as back up as well.

Have a look and the hierarchy and failures of each system and which computer takes over the control surface next.

[Checkboard]

Originally Posted by FCOM 1.27.10.2000.3

ELECTRICAL CONTROL

- In normal operations, ELAC2 controls the elevators and the horizontal stabilizer, and the green and yellow hydraulic jacks drive the left and right elevator surfaces respectively.

The THS is driven by N° 1 of three electric motors.


- If a failure occurs in ELAC2, or in the associated hydraulic systems, or with the hydraulic jacks, the system shifts pitch control to ELAC1. ELAC1 then controls the elevators via the blue hydraulic jacks and controls the THS via the N° 2 electric motor.


- If neither ELAC1 nor ELAC2 is available, the system shifts pitch control either to SEC1 or to SEC2, (depending on the status of the associated circuits), and to THS motor N° 2 or N° 3.


MECHANICAL CONTROL

Mechanical control of the THS is available from the pitch trim wheel at any time, if either the green or yellow hydraulic system is functioning.

Mechanical control from the pitch trim wheel has priority over electrical control.

ACTUATION

Elevators

- Two electrically-controlled hydraulic servojacks drive each elevator.

Each servojack has three control modes :

• Active : The jack position is electrically-controlled.
• Damping : The jack follows surface movement.
• Centering : The jack is hydraulically retained in the neutral position.

- In normal operation :

• One jack is in active mode.
  
• The other jack is in damping mode.
  
• Some maneuvers cause the second jack to become active.

- If the active servojack fails, the damped one becomes active, and the failed jack is automatically switched to the damping mode.


- If neither jack is being controlled electrically, both are automatically switched to centering mode.

ELAC 2 fault is a no go, as relying on ELAC 1 gives you a "double jeopardy" in that either failure of ELAC 1 or Blue hydraulic will result in loss of ELAC elevator control.

[Max Angle]

2. How come abnormal procedures don't cover ELAC1+2 fault?

If you check out the ELAC 1 (2) fault procedure you will see there is a section for both ELACS failed, you end up in Alternate law with only spoilers for roll control.

[Last Ditch]

Thanks Everybody. Inputs were great.
For the first time the page 'Flt Ctl Architecture' in QRH is making some sense.
No substitute for burning the midnight oil I guess.

[dovish]

in FCOM 1 1.27.10 p8 it is well explained what servos are actuated by either ELAC. ELAC 1 G+Y servo and ELAC 2 only B servo.

How about when we have a double Elac failure? The secs come in but there is no information regarding what servos they will actuate. Looking at 1.27.10 p8 diagram it seems they will control the elevator by moving the THS and I assume the elevator go to damping mode. However in FCOM 1 1.27.10 p6 it looks like in case of failure the secs can control all servos.

Anyone have an idea how is the pitch controlled when no ELACS available??

thanks.

[Last Ditch]

Thanks. The diagram in FCOM 1 1.27.10 p6 shows SECs taking over all ELEC duties during ELEC1+2 fail. SEC- Spoilers and ELEVATOR computer better stand up to its name!! Couldn't find anything on pitch/ elevator deflection restrictions with ELEC1+2 failure.

[Last Ditch]

For all new Bus drivers like me, you may also like to have a look at summary pages in Abnormal Procedures for Flt Ctls.
FCOM3 3.2.27 Pg 20 to 22
Just found it... Great Stuff!!

[Alexlehay]

ELAC1 + G fail

Hey guys,

Very interesting, any idea on what would be the roll configuration if ELAC1 + G hydraulic system fail?
Obviously the L AIL will be inop but the R AIL will still be operative through ELAC2 (B system).

[VintageRed]

FCOM PROC-ABN-27 P 50/54 tells you all the possible configurations with various faults for the elevator and stab control.

[Alexlehay]

Thank you VintageRed,

I would like to find the same table for the ailerons/spoilers but I don't think there's one.

As I was saying above in case of ELAC1 + G fault, only the right aileron will be operative and in that case I'm wondering if the roll axis is only controlled by the right aileron OR there is a kind of lock (damping mode) and the roll is controlled by the spoilers through the SECs respectively on each wing.

[Rocket3837]

Hi all
E LAC 2 is classified as no go item because in case of Emer Elect Configuration there will be pitch control until RAT is fully deployed (8 seconds).

Also it worth mentioning that ELAC1 & ELAC2 are equivalent computers, which means that in case of ELAC2 fault in flight, it can be swapped with ELAC1 after landing to make aircraft dispatchable.
Thanks

[Rocket3837]

Correction for my post
there will be NO pitch control until RAT is deployed

[Cough]

Trim wheel will still work...

[Rocket3837]

Cough
The above is Airbus official answer to my company similar question (about 20 years ago).
I guess they meant ELEV control which is a primary control unlike THS which is a secondary control.

Thanks

[VintageRed]

Actually you have a 30 second latching of ELAC 2 to Battery 2 (standby mode).

****CAVEAT - Im not entirely sure what functions you get in this 'standby mode'****

That's discussed in the AMM, and is only briefly mentioned in the FCOM. After 30 seconds the latching is removed, and as a result if for some reason the RAT GEN doesnt come on line then you have nothing, but during 8 seconds of deployment you still have ELAC 2, albeit for only 30 seconds.

Thats what i can see so far.

I still havent found out what happens to the Ailerons if you lose ELAC 1 and the Green system. The pages from the AMM which i have dont mention that configuration anywhere...im still researching it though.

[Uplinker]

In the flight controls architecture diagram in the QRH, it shows that by losing ELAC 1 and green hydraulics, you will lose the left aileron and both outermost roll spoilers, but all other roll controls will still be operational - i.e. three roll spoilers per wing and the right aileron

Additionally from the FCOM, DSC-27-10-20 Architecture/Roll control: if either an ELAC or a hydraulic service fails, the relevant aileron servojack will automatically go into damping mode, where it follows the control surface but damps its movement.

[pilotho]

I don't know if anyone is still reading this thread but from the way I understand it, in roll the 2 ELACs does not control different hydraulic systems like it does in pitch.

If you lose ELAC 1 and G then you'll still have both ailerons as ELAC 2 will take over with Blue system making the aileron dance.

The ailerons will only go into damping mode if you lose both ELACs or both G + B HYD. ref dsc 27-10-20

[Uplinker]

It was explained to me by a very experienced training captain and fleet manager that there is a subtlely of the Flight Controls Architecture diagram in the QRH that is not readily appreciated, (or taught):

Each controlling ELAC or SEC only operates one hydraulic jack in each flight control surface - the one shown above the ELAC/SEC number. So for example, the left aileron is normally controlled by ELAC 1 operating the Blue hydraulic jack. If ELAC 1 fails, ELAC 2 takes over operation of the left aileron, but it does so by operating the Green hydraulic jack - not the Blue - i.e. each ELAC or SEC is hard wired to only one specific jack in each location.

To explain another example; the right elevator is normally controlled by ELAC 2 operating the Yellow hydraulic jack. If ELAC 2, or Yellow hydraulics fail, then the right elevator will be controlled by ELAC 1 operating the Blue hydraulic jack.

With this knowledge, the diagram becomes even more useful and informative when working out the results of computer or hydraulic failure scenarios.

PS, I think there is a mistake in my electronic FCOM in the schematic at the bottom of DSC 27-10-20 showing the SERVOLOOP PRIORITIES table for the elevators; in that they have not drawn the control line from the right elevator Yellow jack all the way to ELAC 1, so it looks as if ELAC 2 might control the Blue and Yellow right elevator jacks, but I don't think it actually does.

[sierra_mike]

If you lose ELAC 1 and G then you'll still have both ailerons as ELAC 2 will take over with Blue system making the aileron dance.

no, left aileron is lost in that case. airbus even uses this example in the "use of fuel penalty factors" chapter in the QRH

Example:
• Dispatch with the ELAC 1 inoperative under MEL
• HYD G SYS LO PR ECAM caution in flight
• These two failures lead to the loss of the left aileron
• INOP SYS will display “L AIL”