Hydraulic system loading
 

A few times I've seen quips about avoiding using multiple hydraulic systems (like gear and flap movement) simultaneously, because of the extra wear and tear the increased load can put on the system. But this doesn't make sense to me, and I'd like to make sense of it, or confirm that it's bunk. With other types of systems I can see the issue. With electrical, we have increased current and heat flowing through the components. With mechanical systems, we get closer to the yield limits, increased chance of fatigue cracks, etc. With an engine, the implications of loading it more are obvious.

But with hydraulics, we have have fluid, pushed by a pump, staying at a regulated pressure, and running through actuators. With multiple systems using it simultaneously, of course it takes more flow to get the job done (and maybe the pressure would decrease), but there's no more pressure or stress on the lines or actuators. As for the pumps, if anything I see this easing their load as they're pumping against less resistance.

Maybe I'm looking at this simplistically, but what am I missing?

For certain abnormals, you'll notice the decreased system response time. I move gear, flaps, and primary flight controls simultaneously on the A320. Never had a problem. We have some pretty esoteric limitations in our books. If hydraulic loading posed a practical (as opposed to theoretical) problem, I'm sure it'd be in the book.

I think it has to do with the caudal of hydraulic liquid available in the system. You can have same pressure, but with lesser quantity some systems (heavy users), might not work completely fine.

I was a fan of switching in the yellow electric hydraulic pump when on the APU during an emergency landin with both engines flame out (only in the sim!), until one day the checker asked an obvious question:

- "Why you doing that Iggy?"
- "For the benefit of having the yellow hyd system working in a situation when control of the airplane and braking power is needed the most?"
- "Don't you think that, if doing that was somehow helpful, they would have written down it on the list?"
- "But, they are French, and we know better than them in this airline, right?"
- ...
- "Right?"
- "Look, if it is an obvious thing to do, but it is not written, there must be a reason for it. And in this case, the reason is that the caudal (the amount of liquid, I'm not sure if you know the word caudal, Iggy), that the electric pump can deliver is around four times less than the caudal delivered by the EDP. Understood?"
- "Yes Sir!"
- "Ok, now do me 50 push ups, you are fat".

So, not being in the book means has more to do with people like me being allowed in the cockpit more than being esoteric.

I remember someone in the forum posting the official reply from airbus for the matter, which replied that not only the liquid is less than required to operate heavy users on the yellow, but the operation would be questionable, and unpredictable when deploying high lift devices as the leak measurement valves would run out of pressure locking them into unpredictable intermediate positions

Watch the system pressure when operating multiple systems. Especially landing gear. There may be your answer.

Sometimes it’s a case of mechanical sympathy versus ‘approved operations’.
Best action vs ‘technically not a limitation’.
Although whatever you do to enhance the lifespan is negated by the knucklehead before you and the one after you.
If the name on the paycheck is the one that’s on the tail then you follow whatever their procedures are.

I'm not sure if any of the rest of us do, either - have you just made it up ? :O

:ugh::ugh::ugh::ugh: That's the Spanish for discharge, which I don't know if also can apply to the topic. I guess the checker was right telling me to do push ups and zip it :O:O

What would the correct term be in this context? Thanks!

Pump specifications
 

https://tinyurl.com/2p967x24

I would assume the ability to operate multiple systems simultaneously will be limited by the maximum output flow (discharge).

Fluid turn over. To your yellow pump use in double flame out you were not far wrong. I asked this question Airbus replied to me saying when in dual flame out case when eventually G+Y pressure drops low ECAM will ask for Yellow electric pump with PTU OFF because the fluid turn over is less for powering two systems. Then they replied to another by saying the flap may lower erratically and may not lower to desired position but for yellow brakes surely that will help.

Swash plate pump? higher demand higher angle of plate, more friction on slipper pads, case drain filter blockage, more flow generates more heat; excessive high system demand can lead to knock / vibration; jerky operation of hydraulic motors can exacerbate drive shaft wear and cause system lock out ( mainly flaps). Could cause priority valves, where fitted, to hammer leading to such items as extending gear extension time, longer unclean door configuration flight mode.

Thanks Vilas!!!

If you want exact figures then Yellow EDP delivers 140L/mt while electric pump delivers 32L/mt.

I think aeromech3 mentions good reasons for operating one hydraulic service at a time.

Other reasons might be :

Hydraulic pressure might drop so the services might not reach the intended positions correctly. This might produce a warning or it might not, but it could cause a lock-out - for example of flap or slat WTBs - leading to problems you would prefer not to have.

If, say the gear and flaps were operated together and the aircraft suddenly banked over un-commanded; which service caused the problem, and therefore which will remove the problem ?

Best to operate one service at a time if you can and confirm the correct operation, travel and lock of each position before operating the next. (Apart from normal ailerons, elevators and rudder, obviously, which are designed to be operated together).

Then how come Boeing say “Gear down, Flap xx” simultaneously on a bunch of their fleet?

But the book says so.

WHEN FLAPS ARE AT 2...........L/G DOWN ORDER​​​​L/G lever .....................................​​​​​SELECT DOWN
AUTO BRK....................................CONFIRM
If the runway conditions have changed from the approach briefing, consider another braking mode. GROUND SPOILERS ARMEXTERIOR LIGHTSNOSE sw T.ORWY TURN OFF sw ON
WHEN LANDING GEAR IS DOWN
FLAPS 3 ...........................................ORDER
FLAPS 3 ............................................SELECT
ECAM WHEEL SD page ...........CHECK WHEEL SD page
​​​​​​appears below 15 500 ft when landing gear is extended. Check for three green indications on the landing gear indicator panel. At least one green triangle on each landing gear strut on the WHEEL SD page is sufficient to indicate that the landing gear is downlocked. Rely also on the “LDG GEAR DN” green LDG MEMO message to confirm that the landing gear is downlocked. If residual pressure is indicated on the triple indicator:RESIDUAL BRAKING PROC APPLYDue to the accomplishment of the alternate braking functional test after the landing gear is downlocked, brief brake pressure indications may be observed on BRAKES PRESS. FLAPS FULL .............................ORDER
FLAPS FULL ..............................SELECT

I don’t read that as “wait for three greens to move the flaps”.

I still believe that if a problem existed, it would be specifically prohibited.

At both companies I’ve flown the plane, we call for gear and F3 simultaneously, and more often than not, Flap full before we have 3 greens.

Haven’t heard of anyone having a problem so far.

I think if you read the Airbus SOPs like that then you would not be able to lower the gear for drag if you were hot and high. Because what you’re saying vilas, is that you can only select F3 once the gear is fully down and you can only select gear down once F2 is fully set. So how can you put the gear down if you haven’t got F2 set?

If you say you can select gear down without F2 being fully set then the argument that that’s what the book says is invalid.

It’s semantics to a point but I don’t read the SOPs like that. What do you think?

You are misinterpreting the sentence. It's only giving the sequence when Flap2 is in transit. By itself gear can be lowered any time as in even EMER DES.
Check Airman
Nothing has happened philosophy is not used to deviate from the book. There many things one can do differently and normally nothing happens and yet something can happen.The book specifically tells you to take Flap3 when gear down. If it had no meaning then there was no need to mention WHEN GEAR DOWN select Flap3. In Airbus we lower flaps through SFCC and gear through LGCIU. So we make a selection we confirm flap number or call gear down by three Reds for gear to confirm the computer is working but when it says when Flap2 or when gear down then the surface has to be checked at the correct position or three greens for gear. Gear is a high demand system so a pause is indicated. Besides when gear is selected down you need to check AB selection, if required then change it , spoilers need to be armed turn off taxi lights to be put in. If there's a short cut something will be missed.



​​​​​

And I think you are reading the placement of a sentence in a sequence of SOP as a “must be done this way” approach.

Another example would be the flight control check - in the FCTM Airbus state up/down, left/right, rudder - left/right. Would you be wrong to check the flight controls right/left or down/up? Or does Airbus have to write the sequence a certain way and the sequence is not important - it’s just written down that way and nothing prohibits you from checking down/up, right/left. Everyone does it the way it’s written but would it make any difference whatsoever to do it another way? What if Airbus decided to write it the other way? Would the old way be “wrong” and the new way “right”?

If it was a problem and had associated threats then Airbus could put in a Limitation, or a Note, or in the FCTM Preventing Identified Risks section but they don’t.

Perfectly happy to to be happy to disagree with you.

Has anyone ever experienced an issue selecting gear and flaps at the same time? Any first hand evidence?

Ps. The gear takes 15-21 seconds to extend - AMM 32-31-00-720-002A. I’m pretty sure most pilots can check the AB, arm the spoilers, turn on two lights inside 15 seconds. And if necessary select F3.