PBL, the PTU transfers hydraulic pressure (via shaft power) between the G and Y systems. There's no fluid transfer.
I think you might find there is actually some path whereby fluid might pass between G and Y or vice versa. And certainly if there is low or no fluid in the G system, there has been a transfer of fluid from G to somewhere. Where?
I could probably carry on all day like this, but I am not interested in playing language games. I was hoping that somebody who knew of a specific scenario might care to share it here.
Leave the hydraulic systems on, don't touch stick and rudder, use eng thrust to try it in the sim.
Wouldn't work, mate.. the FBW system compensates for thrust changes and keeps you the right way up - even with one at TOGA and one at Idle. Failing all three hydraulic systems might at least give some spoiler or aileron upfloat to make it interesting.. TP
BTW I'm very dissapointed about your system knowledge.
If you look a little more closely, you will find that I have given you no information about my system knowledge at all.
.................... I am here because I had hoped that on this forum of experienced aviators there might be some people who could tell me things I don't know, and some who might like to learn things which I could tell them.
[Edit: material on the discomfort I feel with what is, to me, the often puerile level of interaction on this forum, that I am surprised to find coming from people who self-describe as technical professionals.]
Hetfield, despite your reticence, I would still be grateful if you would share with me your personal knowledge of how both primary A320 hydraulic systems fail together in line service.
Last edited by PBL; 1st Jan 2008 at 09:11.
Reason: A somewhat lengthy description of problems and plea for different kinds of interaction has timed out
I have not read the entire thread so apologies if I am covering what has already been said but...
Airbus have warned of a situation that could indeed lead to a 'Dual HYD Failure'. The circumstances were a leak in the Green system after start that was not picked up all the way to Take-Off. On T/O the leak rate increased and total GRN system loss ensued but the warnings are inhibited because of flight phase. As has been pointed out before, the Yellow system began to drive the PTU, but with no fluid to drive in the GRN system it spins at an almighty rate and very quickly overheats the Yellow system. The aircraft now climbs above 1500' and lo and behold a Green system failure and Yellow 'Failure'! Of course the Ylw is actually an overheat that could be reinstated once cooled, but from memory the crew didn't bother with that they just went straight back to land.
In similar circumstances once the Ylw fluid had cooled and the PTU selected off the yellow could have been re-instated.
Oh yes another thing...
An Airbus rep told me of instances of Green to Yellow transfer but it is not 'intentional'. It occurs in the brake shuttle valves (Green / Yellow system) perhaps when seals fail internally? Sorry can't remember the exact details now. Happy New Year to all.
An Airbus rep told me of instances of Green to Yellow transfer but it is not 'intentional'
Thank you, idg. I wonder whether he was talking about known instances, or merely about what he knows about the hazard analysis in the certification basis?
Let me give some indication for those who might be interested but do not know, how a hazard analyst might go about analysing such systems.
First, heshe might look at the crude system diagram in 1.29 and observe that there are at least two devices which connect between the G and Y hydraulic systems. (idg is reporting a third, which I don't believe is indicated in those diagrams, but I can't be bothered to go check.) Now, for each of those devices the certification basis for the AC will include documents, amongst them one assigning the level of criticality of the device, and another giving an argument in detail as to how the device satisfies the assigned level of criticality.
Now, I don't know the detailed design of those devices; I have never seen such engineering documents. And I doubt anybody else here who is willing to say something has either, because those documents are prima facie proprietary and the people seeing them form an restricted group bound by non-disclosure agreement, and nobody who takes their code of practice seriously (i.e. all engineers except for the occasional crackpot) is going to violate non-disclosure for the sake of some guy asking questions on an anonymous web forum, unless there is something seriously, seriously wrong (which I doubt).
So my question to violator was a "leading question". In other words I already know that (as a hazard analyst interprets these words) there are paths between G & Y through which fluid can theoretically travel. Any, say, HAZOP-like analysis will explicitly consider such a scenario and its consequences, but as I said I do not know what techniques are used in the certification basis.
Now, I have never seen any hazard analysis on any piece of complex equipment which was completely correct, without exception. It may be beyond the bounds of current human capability to devise one for such a thing as a commercial aircraft that was exceptionlessly correct. Most of them have faults. Some of them, occasionally, have glaring holes. And I know, for certain specific aircraft that interest us, that there are hazards whose severity (technical term) is catastrophic (also a technical term) that have not been mitigated, because we have reverse-engineered and identified them.
I thank idg also for explaining the PTU-overheat scenario in a little more detail. My next question if I were to follow that line would be of course what the consequences are of that abnormal state that the PTU gets into, but as I said, I doubt anybody here with either the detailed engineering design or the certification basis documentation sitting in front of himher would be inclined to answer.
Sorry for the length of this note. I'm just trying another writing technique to try to avoid this pointless, and to my mind rather silly, one-sentence repartee. I do hope I can get back to brevity; I'd rather just ask a simple question and hope for a sensible answer.
well, there are obviously people here who don't know the FCOM in detail either. But what has this issue to do with knowing or not knowing the FCOM? As people have pointed out, losing three hydraulic systems is not considered operationally.
An Airbus rep told me of instances of Green to Yellow transfer but it is not 'intentional'. It occurs in the brake shuttle valves (Green / Yellow system) perhaps when seals fail internally? Sorry can't remember the exact details now.
It is my understanding that this would only occur when the brakes are applied or pressure is sent to the shuttle valve from the Brake control unit. Normally not pressurised. Transfer would stop when brake pressure released.
Also, I'm not sure if the brake hydraulic fuses are before or after the shuttle valves. It's been a while. If they are before they will stop the flow when it gets beyond a normal rate.
In regards to the green system loss. If the warning is inhibited due to flight phase screening to 1500'. The plane does not fly around long below 1500' agl. (maybe a pilot type might tell us how long from takeoff to 1500') I'm tipping it's not long.
Your yellow system might get to an overheat but it still takes time. Depending of course on the amount of work it does in the PTU and other controls. It also depends on the rate of loss of fluid from the green system. It's generally going to keep tyring to make pressure until the reservoir is empty. A small leak will take some time to dissipate the fluid. The Engine pump will continue to make pressure though. The PTU will not come into play.
The Airbus warning was from an actual event when the 'dual failure' occurred. I'll try and get some more info but it actually happened, and happened quickly enough to be present when the a/c climbed above 1500'. Probably about 1min 30 secs.
I'll also try and get a tech diagram of the shuttle valve.
...An Airbus rep told me of instances of Green to Yellow transfer but it is not 'intentional'. It occurs in the brake shuttle valves (Green / Yellow system) perhaps when seals fail internally? Sorry can't remember the exact details now.
A so called "brake shuttle valves" between green and yellow system does not exist in the A320 braking system.
Last G+Y loss scenario which I know hapens at BA PRG-LHR (A320-100)flight one or two months ago.. Succesfull landing at PRG with full emergecy, one tyre blown-out due temperature. Airbus solution is inhibiton PTU operation below 1500´, probably on new planes only...
Last edited by františek dobrota; 3rd Jan 2008 at 18:41.
I know hat a "Dual Shuttle Valves" exists, but you have mentioned a "brake shuttle valves" whitch is connected to green and yellow hydraulic. For Images simply use : ImageShack® - Online Photo and Video Hosting The "Parking Brake Operated Valve" is not so complex: Parking/Ultimate Emergency Braking - Schematic:
Last edited by IFixPlanes; 29th Dec 2012 at 15:10.
Reason: Links repaired...
Hi All Forgive my ignorance, But unlike the 737 classic series which can allow fluid transfer depending on which system you use to apply the parkbrake and which system is running when you release the park brake. I was under the impression that the A320 use of yellow and green hydraulic systems to the brakes were completly seperate systems with no possibility of fluid transfer, right down to two seperate supply lines to the brakes, and operating seperate pistons. If someone can explain it to me about the A320, i would aprecciate it.
Sorry if I haven't made my query clear, its my first post