Hydraulic Backup systems

[Officer Kite]

Hello all,

I ask as an ATPL ground school student so please pardon my ignorance.

Anyway, despite being able to smash out quite high scores on the question banks on hydraulic systems I do feel my knowledge on hydraulics could be better. I have one question I can't seem to find a direct answer to despite searching the web and various historic threads.

Basically, considering we have all these wonderful backup systems, 3 on Airbuses I believe at least, why is it such a big deal when we lose the main system? Why can't we just switch to the 2nd back up, if that goes, then switch to the final back up? It happens quite regularly, most recently with an Aer Lingus A333 out of Dublin, that when the main hydraulic system fails, it requires a return to base and there doesn't even seem to be enough pressure in the hydraulic system to raise the landing gear bay door, which I believe are even blown out by an accumulator (which I had in mind as being last resort for when :mad: really hits the fan and you've nothing) ... but what about the 2 back up systems? are these not as good?

Finally, where exactly does the RAT fit in all this? From my knowledge I understand it provides basic pressure to the primary flight controls and other key systems, but why is that the first go to when there are 2 fully capable back up systems to the main one?

Thank you all for your time!

[FlyingStone]

Having 3 identical hydraulic systems would increase the aircraft weight by a lot. Heavy duty hydraulic pumps, wide pipes to allow for appropriate flow of hydraulic fluid, etc. Also, if you would get a catastrophic failure on one of the (relatively unimportant) flight control surface, you could loose all hydraulic fluid and thus systems - highly undesirable.

The hydraulics of the aircraft are usually designed with redundancy in mind. So the loads are split between smaller hydraulic systems, so the failure of one or two hydraulic systems doesn't affect aircraft ability to fly.

On top of all, hydraulic failures are pretty rare, so the odd return to base or diversion is not really an issue.

[AtoBsafely]

Consider the following design points:

The more items powered by a hydraulic system, the more likely it is to fail. There are more pipes and couplings to break and lose system pressure.

So critical systems (flight controls, brakes) need to be powered by multiple systems, of which at lest one will do little else.

Each hydraulic system should have at least two sources of pressure. The engine driven pump and something else. The something else's could be an auxiliary pump (electrical or air driven), a power transfer unit (hydraulic pump driven by another hydraulic system) or the RAT. Obviously secondary pressure sources aren't any help if the oil has gone.

The loss of a single system will mean you've lost some services and other critical services will probably be downgraded (e.g. Less effective flight controls)

A RAT is there as the final source for critical services. It may also be able to supply critical electrical power.

[sonicbum]

Basically, considering we have all these wonderful backup systems, 3 on Airbuses I believe at least, why is it such a big deal when we lose the main system? Why can't we just switch to the 2nd back up, if that goes, then switch to the final back up? It happens quite regularly, most recently with an Aer Lingus A333 out of Dublin, that when the main hydraulic system fails, it requires a return to base and there doesn't even seem to be enough pressure in the hydraulic system to raise the landing gear bay door, which I believe are even blown out by an accumulator (which I had in mind as being last resort for when :mad: really hits the fan and you've nothing) ... but what about the 2 back up systems? are these not as good?


Hi,

As you correctly interpret, technically speaking losing 1 hydraulic out of 3 is not a big deal per se, but, like most of the time in aviation, it all depends on the context. There are various aspects to consider in that case, such as flight time and conditions (would you go for a 12 hours etops overwater flight with the minimum amount of en route alternates on 2 hydraulics ?) , systems affected by the hydraulic loss (maybe now you're performance limited and your landing distance at destination and/or alternate(s) is not ensured), multiple failures (maybe you have left with an MEL item and with another failure you find yourself in a degraded condition), maintenance facilities at destination (no maintenance and/or much more expensive to have the airplane tech there than coming back to land at home base, this is generally discussed with your company ops) and so on. During your training you will learn how to integrate the pure technical aspects of the airplane with all the other environmental factors and come up to a decision with your colleague(s).

[Metro man]

Being grounded after landing at an outstation with no maintenance available and having the aircraft unavailable for a couple of days while engineers are brought in comes into the equation.

When one system fails the remaining back up is under higher than normal load which may cause additional failures, like opening a can of worms.

Whilst the Airbus A320 has three systems, one of them, the blue system has much lower capability than the other two and is basically a last resort emergency back up which does a bit of work to help out the other two.

Flying on the RAT is a Mayday situation and the emergency services will be standing by as you land.

Due to Airbus fly by wire being so heavily dependent on electrics and hydraulics, a higher than normal level of redundancy is required and provided for.

[MurphyWasRight]

One other (often glossed over) consideration is that the redundant systems are usually considered to be independent as far as failures go but may not be.

Using orders of magnitude worse failure rates to keep the number scomprehensible a simple example is:
If each system has a 1 in 100 chance of failure (per hours/flight/whatever) in theory there is only a 1 in a million chance of losing all 3.

The 'gotcha' in this is that the failures causes are not truly independent for several reasons:

1: Already alluded to is higher load on remaining 2 systems.

2: Maintenance errors affecting all 3 systems. (Famouse case is missing O rings on oil filters on all 3 engines)

3: Manufacturing or (undiscovered) design defects affecting all systems.

Lastly is a design or maintenance error causing independent systems to not be as independent as they should be.

Others have mentioned very significant safety and operational considerations when deciding to press on or divert.


Anybody else ever cringe when some supposed expert describes an aviation event as a 1 in one million happening.
Considering total number of commercial flights per year I would hope for much better than that.

[Officer Kite]

Thank you all for the replies

So what I gather from everything is that we do not actually have 3 clones of the same system as this would be too heavy and a few other reasons as mentioned. This is news to me as I was of the impression that identical backup systems were in place.

Also, am I correct in now saying that all 3 systems are used continuously then in flight with the loads being split across the 3 of them and if one goes the other 2 try to pick up the mess and split it between them?

[Fursty Ferret]

Also, am I correct in now saying that all 3 systems are used continuously then in flight with the loads being split across the 3 of them and if one goes the other 2 try to pick up the mess and split it between them?

Spot on. But it's not so much a case of picking up the mess as a loss of redundancy.