View Full Version : How does the mechanical backup on the 320 work? + some questions

15th Oct 2012, 23:29
I keep confusing myself, I apologize for the stupid quetions.

Firstly, if you lose both engines, your only source of hydraulics is the blue electric pump. The RAT is supplying the electric for that, is this correct? (Assuming APU is inop.)

What if the RAT for some reason fails, you've got no hydraulic supply.
Then what?

Now if you only lose your engine generators, you've got no electricity, but the hydraulics are working. In this case if you had the APU, it would supply the electric supply to be able to work the hydraulics correct?
If the APU was inop, but the RAT was working, you'd still have your blue electric pump.

How do you get into the mechanical backup?
No engine generators, no apu, and no RAT?
How will the hydraulics work in this case?

Sorry for so many questions.
I would appreciate some help, thanks all!

15th Oct 2012, 23:42
Just to get an idea for the level the answers should be at, would it be correct to assume that you have no prior experience or knowledge of the A320 or aircraft in general?

15th Oct 2012, 23:59
There is no mechanical backup for hydraulic loss. Only for electrical control failure and then only on some systems.

If you lose all hydraulics you are on engine control only. Good luck with that. :eek:

16th Oct 2012, 04:52
As long as the engines are running, you have hydraulic power from the engine driven pump. The EDP is mechanically self controlled. You need electric only to "switch" the EDP OFF.

16th Oct 2012, 08:49
There is a mechanical backup, but only for the rudder. On the A320.

16th Oct 2012, 09:10

Thanks and yes I get that. But all the hydraulics are electrically controlled, that's why I ask what happens without any of the electrics.
If the RAT was the only souce of electric output, would you be in mechanical backup?

16th Oct 2012, 09:53
The RAT provides pressure to the blue HYD system, which in turn can then drive the emergency generator. It does not provide elec power itself.
On a large fly-by-wire aircraft, you usually require hydraulics to move the flight controls and electronics to tell the hydraulic jack to move.
There is no mechanical back-up for hydraulics on the fly-by-wire arbusses.
There was a mechanical back-up for the electronics associated with the rudder and the THS (trimmable horizontal stabilizer), but newer A320s have an all electronic rudder.

So, if you have no hydraulics (3 of which you started out with), you can only fiddle with the engines to alter the way the aircraft goes.
And, if all of the 5 (A330/A340) of 7 (A320) flight control computers fail, you are left with the engines and, perhaps, the mechanical back-up.

16th Oct 2012, 15:46
@ z.khalid
What do you mean with "all the hydraulics are electrically controlled"?
As i said: the hydraulic sorce (EDP) control itself mechanically.
By switching the EDP to OFF you energize a depressurizing valve via a solenoid and the fluid output goes to zero. If you now lost the electric for this solenoid (DC BUS 1 for the ENG #1 and DC BUS 2 for ENG #2) the EDP start to supply the system again (Failsafe). Or in other words: losing your DC BUS does not mean that you lose your hydraulic source.

BTW: DC BUS 1+2 are not supplied by the emergency generator.

16th Oct 2012, 16:44
If both AC BUS 1 and AC BUS 2 are lost and the aircraft speed is above 100 kt, the Ram Air Turbine (RAT) extends automatically.

Is it possible to manually extend the RAT? Can imagine a scenario whereby electric problem causes pitot heat to fail leading airspeed to become unreliable thus preventing RAT from extending....

16th Oct 2012, 20:58
Have a think about what you just typed.. Anyway, you can manually deploy the RAT

16th Oct 2012, 21:16
Don't let Lyman see that.....


16th Oct 2012, 23:43
Perhaps the most brilliant phrase in design: KISS: Keep It Simple Stupid!

while the question was on the airbus, I would like to contrast with another jet airliner.

The DC9 series.

IF all hydraulic pumps quit the plane retains rudder, aileron and elevator, you can still drop the landing gear and have hydraulic accumulators to extend the thrust reversers. Wheel brakes can also be used. you don't get flaps/slats or roll or ground spoilers...but you can still fly the plane quite well

IF all electrical quit, and you are vfr. you can fly quite well. If you have the battery you can fly night IFR in icing for 45 minutes. You still have full flight controls too. You can even work the pressurization system. Though stab trim is gone.

While the airbus is an interesting airplane, I simply can't see that it is inherently a better plane. IF things go wrong, you are in serious trouble.

Field In Sight
16th Oct 2012, 23:51
In mechanical backup you need hydraulic pressure to MOVE the rudder and stab.

This hydraulic pressure is controlled via servos connected by real wires to the rudder pedals and trim wheel.

22nd Oct 2012, 03:14
Field in Sight; absolutely correct old bean.

Mechanical back-up of pitch and yaw on the A320/321/330 STILL requires a hydraulic source - green or yellow for the THS, (blue or yellow on the 330), and green, or yellow or blue for the rudder. Mechanical back-up does not mean that the rudder pedals or THS trim wheel can wind the control surfaces alone in the absence of all hydraulic power - it merely commands the hydraulic motors in the case of the THS or the servos in the case of the rudder, to move.

This is possibly the most commonly misunderstood part of the A320 family control architecture.


22nd Oct 2012, 06:36

Your post implies that a hydraulic failure on a DC9 is any different to normal operations.

The -9 is ALWAYS flown using control tabs only, no hydraulics necessary, that's the way it was designed and a fine design it was but it was always a 'power steering off' Aircraft, same as the B707 / DC8/ MD80 and others.

22nd Oct 2012, 12:13
Totally agree with Uplinker about the misconception there in fact the rudder pedals connect to cables/rods and run all the way down the fuselage and don't even connect to the hydraulic jacks till below the fin. Brings a new meaning to fly by wire.

22nd Oct 2012, 12:50
If I'm not completely wrong ALL A320 family aircraft have a mechanical connection between the rudder and the pedals.
A330 and A340 do feature this only in the olde version. The "enhanced" one has an electric rudder with no backup.

22nd Oct 2012, 12:58
A320. There is a mechanical linkage between the rudder pedals and the rudder PCUs but without hydraulic power you won't be able to operate the rudder!:ok: