View Full Version : A320 Green Hydraulic System Superiority
Hi all,
In case of dual hydraulic failure, when G+B LO PR or G+Y LO PR happens, aircraft is in alternate law but when B+Y LO PR happens, aircraft is in normal law. What is the logic that makes green system far superior over other two systems?
Maybe the reason is that green system has more connections with flight control surfaces compared to other two systems or something related to flight computers?
FlightDetent
4th Apr 2020, 19:31
QRH OPS 07A: Hydraulic Controls Architecture
If you only had
BLUE
- no THS and only one elevator
YELLOW
- no roll control, except two spoilers per each wing (no 2 + 4), no Slats
GREEN
- apart from one elevator side lost
- all the rest OK
The green system has been designed to cover practically all users around the structure. Unlike the other two.
Free memory trick for you: Sort the HYD systems alphabetically
1) B-G-Y
2) imagine the wing cross-section, slats front, flaps at the back
3) the first two B-G control the slats, the last two G-Y control the flaps.
Courtesy of an exquisite gentleman M. B. tasked with my assimilation to the collective 15 years ago right this time in Toulouse. I never needed to remember any more about the hydraulic, save for Wheel doors being on Green and NWS G or Y.
Do not lose too much sleep over OPS.07A.
Thanks for answer FlightDetent.
I was aware of hydraulic system and flight controls architecture but i wasn’t sure about the superority of green system in terms of dual hydraulic failure. Thanks again for detailed explanation.
Speaking of hydraulic system i have one more question.
Say we have dual hydraulic failure and what a bad luck aircraft gets into mechanical backup. If we have only green or yellow system after dual hydraulic failure, we can fly the aircraft by using rudder and THS but if we have only blue system after dual hydraulic failure, we can’t fly the aircraft in mechanical backup because blue system can’t control THS.
Why doesn’t blue system have connection to THS? Getting into mechanical backup with blue hydraulic system maybe has very very low probability but not impossible. Why did Airbus decide such a flight controls architecture? Is there an explanation?
FlightDetent
5th Apr 2020, 11:29
Cannot answer that as I only have observations, not explanations. Why did they design the way they did, that's well beyond my knowledge AND designed by people far smarter than me.
Think redundancy.
- If "B" is the only one remaining, it remains connected to both elevators! So pitch control is possible with good authority. No need for THS.
- If "Y" or "G" would be the last one you have, either of them will give you THS and one side of the elevator.
Think reliability.
- The more complex and connected system you create, the more chances and places where it can go wrong. That is why, I assume, you do not want to have a line of HYD connected all around the craft as the probability of losing it is increased then. The G actually is, but G and Y are most definitely not. Do not forget that compared to dual failure with a very capable single source remaining, it would be miles better not to have a dual failure at all. By connecting two systems at one delivery user, you are already creating a possibility of a dual loss at that point.
Connecting three lines to one place makes no sense. Redundant and Independent are two legs on which Reliable stands. Keep them apart for better balance. The layout seen on OPS.07A is done by dilithium-belt system engineers.
I do not have the stamina for discussion on Mechanical Backup, apologies. BTW what is your interest, enthusiast or professional? Which field?
Think redundancy.
- If "B" is the only one remaining, it remains connected to both elevators! So pitch control is possible with good authority. No need for THS.
- If "Y" or "G" would be the last one you have, either of them will give you THS and one side of the elevator.
These conditions aren’t valid in mechanical backup. In mechanical backup you have only rudder and THS. With blue system you can’t control THS in mechanical backup situation. You have to fly aircraft with only rudder. I am just wondering why Airbus designed A320 that way. Rudder is supplied by three hydraulic systems but not THS. Why?
Maybe they think having mechanical backup with blue system has a very very low probability so they designed that way or they thought THS supplied by three systems could create another problems like complexity but there is a probability having mechanical backup with only blue system. Never say never :)
As a person getting A320 type rating, i want to know what engineering idea is behind that design.
The chances to end up in mechanical backup are so remote that EASA does not even require any flight training in mechanical backup during the type rating or recurrent training. And i guess that is the reason behind the design. To move the rudder you need in any case hydraulic power, hence the three system connection. For the elevator you have, as so eloquently described above, usually either one side and the THS powered, or both sides on the blue system.
FlightDetent
5th Apr 2020, 22:01
About Mech. Backup all I know is
- it was demonstrated during training, and controllable to do a 360 level turn with careful inputs
- the book says it is a mode for unforeseen combination of faults, that might theoretically prevail for a short duration while pilots reset and re-engage some F/CTL computers
- the instructor told me, once factory test pilots managed to land it
Discussing Mech. Backup ON TOP of a dual hydraulic failure is triple useless. And there are plenty of "standard" abnormal situations and protocols worth studying.
Suggestion: For the training go and print a physical copy of FCTM and QRH. To be used in combination with electronic FCOM. Write your lesson notes directly into the appropriate chapters of FCTM and QRH.
The FCTM contains what the instructors will tell you and what you will not find in the FCOM later or elsewhere, like the cement between bricks. If marked as per above, this will be your most valued resource for the next 2-4 years. Once you master what the FCTM contains, you're on a TRI level knowledge-wise.
Wish you well and all good luck!
Suggestion: For the training go and print a physical copy of FCTM and QRH. To be used in combination with electronic FCOM. Write your lesson notes directly into the appropriate chapters of FCTM and QRH.
The FCTM contains what the instructors will tell you and what you will not find in the FCOM later or elsewhere, like the cement between bricks. If marked as per above, this will be your most valued resource for the next 2-4 years. Once you master what the FCTM contains, you're on a TRI level knowledge-wise.!
FlightDetent, thank you for suggestions and i will take them into account 👍