Hydraulics B757
 

Good morning everybody.Having a look to the HYD system on the 75,I got sevral questions that I am sure somebody will know the answers to(at least to some of them)
I got 13 questions,I know it can be boring,but at least I hope to get some answers from the knowledge community and hopefully it ll be usefull for somebody else since discussions always make us think a little more.
Here they are.Thank you in advance:

1-It says that when Left engine and electrical Hydraulic punps fail,the Status page shows the PTU output.
---Does the Status page also shows the PTU output if only the L engine Hyd pump fail?

2-QRH says that if Lest System fails,PTU may be unavailable because of a Leak.
---What about then about the reserve fluid in the reservoir for the PTU?Can we also loose that?

3-Any idea of the reason why "USE DIFFERENTIAL BRAKING" is mentioned on the L hyd syst failure but not on the R Hyd syst failure?I thought they recommended differential braking because you loose that side s Reverser.But that s the same for both sides..

4-Why,in case of 2 simultaneous Hyd systems failure they put a Maximum Crosswind?is that because only one of the 3 actuators per control surface is available?

5-If Autobrake becomes INOP with a R Hyd system failure,why Autobrake does NOT become INOP when R and Center systems fail?

6-Theory says that Normal and Reserve Brakes (powered by R system)have total Antiskid capacity,but that Alternate Brakes(L system) have only laterally paired capability regarding antiskid.Then shouldn t all the QRH having a R syst failure(R,R and C or R and L) mention the reduced antiskid capability on the landing performances?

7-electrically speaking,where do the elec Hyd Pumps(the Left,Right and the 2 center ones) take the Energy from?what Bus?is that SBY?

8-any idea of the output(PSI) of the engine and the elec Hyd pumps?In normal operation they re all operating so I don t know if when the Engine starts running the eng pump overrides the electrical(more output) or if both work together

9-Pulling the fire switch not only cuts the HYD fluid to the pump but also the cooling fluid that still goes though when we select the pump off.So the question is:is Maintenance actio required after we pull the fire switch?what happens then if we push the handle back in?

10-Why we loose all Autopilots with (R,C) Hyd failure?why L A/P won t work?I thought every system powers its Servo

11-what is the real difference between reserve Brakes and accumulator Pressure?

12-The Diagram shows all the 3 systems linked to spoilers.How s that?L system to left spoilers panels,R to Right, and Center what?

13-Where are the Hyd fluid reservoirs located on the Airplane?How many Gallons or liters of fluid?

I told you it could be boring.
But thanks

Wow heavy ones, but interesting. I'll try to answer some of them from a mechanics point of view:
7/ The ACMP pumps gets power through respective control units ELCU. The ELCU gets 28V power from the switch you push in and this activates a relay that sends power to the pumps. Left pump gets power from AC bus R, right from AC bus L.
ACMP C2 gets power from AC bus R (through the ACMP C1 ELCU on/off function, that's why C2 cannot run on most conditions when C1 is running...) and ACMP C1 from AC bus L.

8/ EDP 3000PSI at 37 gal/min. ACMP 3000PSI at 6.5 gal/min. Both should work together. It's 3000 plus or minus something (125 I believe), and they only deliver the flow needed of course. Rest goes to case drain.

9/ If you pull the fire swith in flight, as long as the engine keeps windmilling the hyd pump will continue turning, without fluid or case drain fluid cooling going through it. So the pump may be damaged after 5 minutes. Pump filters must be checked for metal debris.

11/accu pressure is just what it is. Limited braking due to residual pressure left in the accumulator. This is what you get when right and left hyd systems fail. If you now push the reserve brakes switch, 2 isolation valves will isolate the right ACMP and dedicate that pump to give you brake pressure. One of the isolation valves will direct hydraulic fluid from the right reservoir BOTTOM offtake to the R ACMP. (normal offtake is somewhat higher in the reservoir so not all fluid can get out in case of fluid leak somewhere in the system). So in case of right hydr system loss due to leak, you still get fluid directed to the pump when reserve brakes pushed in.
If you push reserve brakes switch, you'll see that the right ACMP switch pressure light will go out even when the pump switched off. When R ACMP pump manually switched on AND reserve brakes on, check status page on ground with engines off, you'll see that the hydraulic pressure on the right system doesn't show 3000 PSI even if it runs. That's because the isolation valves did their work, and the fluid is transferred to the reserve brakes function (pressure transmitter bypassed). So if you ever see no pressure on the status page when you put on the right ACMP, check your reserve brake switch first before calling that engineer.

12/spoilers 3,5,8,10 on left hydraulic syst; spoilers 1 and 12 on center; 2,6,7,11 on right sys. Ground spoilers 4 and 9 on right system. Numbering starts at left wingtip.

13/ left rsrvr in left wheel well, right in right wheel well, center in hydraulic bay aft of left wheel well. L and R rsrvr hold 6.6 gal each, center one holds 3.5 gal.

I leave the rest for others, I'm getting a headache now:8

757 HYD
 

Q1. The PTU operates when the L HYD has fluid but no power. The fluid might be the complete L System or just the Stack Pipe reserve. The PTU is driven by the R System but there must be fluid available for it to work (and a serviceable R System).

Q2. Yes, if the leak is below the Stack Pipe, there will be no fluid left and thus no PTU. That is why the QRH says ALT Gear/Flaps etc MAY be required. You might not know where the leak is and thus not know until you select Flap 1 whether or not anything is going to happen.

Q10. Have a look at QRH Unsched Stab Trim. What do you do (without lingering - hopefully)? Think what those two switches are and therein lies the clue. It helps to work these things out yourself because that way you get a much better understanding of the whole aircraft.

It is some years since I last had a complete L HYD failure and I am sure there have been many mods since, but my experience was a total loss of fluid from below the stack pipe. That caused a Status PTU msg (indicating that it was NOT going to work) and zero L HYD pressure. EICAS got to page 3!

Hydraulics B757
 

Wow.
All I can say is Wow.
Thank you so much for the answers,I ll read them and try to digest them.
It s amaizing that,whatever is the level of the question,in this place you always find somebody who knows the answer.
Thank you Prober and Piper19,all the questions were answered except 3 to 6,and i think maybe reading the answered I ll be able to find out about the rest.
Honestly,the Tech Log section has a lot of "substance".I had never tried before,I used to be nor in the jobsearch.But this is clearly an excellent place to learn and know about other s experiences.
as I said,I ll read the answers carefully and get back.
Good night and thanks again.

Hydraulics B757
 

Good morning.
After reading(and as I expected) some more questions come to me.

prober:

1-perfectly understood.Thank you
2-Same.Thank you.
10-Excellent,I ll have a look.Honestly,I never though about looking at the Stab trim QRH.Thank you.Will do.

and you had complete fluid loss no no L HYD and no PTU:Do alternate everything?busy day...

Piper 19:

7-Sorry,I m very bad on electrics.Then the Center ones take from L AC and R AC?but nothing on SB AC Bus?what I m trying to find out is what happens with the Hyd Pumps when if you start loosing electrical Source.The Hyd engine punps work when engines running,but what about C1 and C2 if you loose AC?
And very interesting:could you explain why C2 cannot work normally when C1 is running?is that the same reason why C2 is the coosen one for load shed on engine start?

8-Excellent.Thank you.
9-Impossible to be more precise.Thanks
11-Excellent.Thank you.But then when they say that you have Normal,Alternate,Reserve and Accumulator Pressure Brakes,what s the difference between Reserve and Accumulator?I mean:in normal operation,normal brakes for R.Ok.If R fail,we have alternate(from L) and everything available except that no Autobrake abd laterally paired Anti-skid.But then if Normal and reserve brakes fail,what do we have?reserve Brakes?that is just the accumulator fluid?so Reserve and accumulator is exactly the same?
12-Thank you
13.Thank you.I expected much more fluid.6.6 Gal in L or C,and each eng pump is moving 37 Gal/min?so all the fluid circulates almost 6 times per minute,every 10 seconds?

And again thank you so much.reading answers makes think and that s how we little by little become better pilots so thank you

Hydraulics B757
 

Hi again.
Prober,I went through Q10 and to be honest I ll need some help.

Ok,I went throgh the UNSCHED STAB TRIM QRH
First thing you do is both CUTOFF,then A/P disengage,then try module by module to try to remove some CUTOFFS.Ok.

Then I went to FCOM.Ok,Stab Trim modules are driven by C and R Hyd,exactly the two systems that have to fail to bring us to an "all Autopilot inops" situation

And I found this sentence on Non Normals on FCOM:

If a single autopilot is engaged, electric trimming causes the autopilot to
disengage. If multiple autopilots are engaged, the electric trim switches are

inhibited. Alternate trimming does not cause autopilot disengagement.

So I know the answer is close,but I still don t see why the L Hyd providing Hyd Press to the L A/P Servo,why L A/P cannot work.

So come on,give a clue..

R & C HYD u/s
 

OK then. L A/P is working as expected. If you are in the cruise, it might work without problem for a bit, but eventually and inevitably you will get an AMBER A/P wng. This means that the A/P is working but is finding it difficult to do its job. (Think Unsched Stab Trim again - it is trying to keep you at your flight level.) Then, when you want to either climb or descend you will almost certainly get an A/P disconnect. Why? Because the Stab Trim is not responding (R & C). Does that help?

The eng pumps will move 37gall/min at full flow. But these punps (and the elec pumps) are constant pressure, variable flow pumps. Inside the pump is a swashplate that varies the stroke of the pistons, controlled by system pressure. So when the sys pressure is 3000, the pump produces very little flow. This is the state of the pump most of the time.

Hi Zorsan!!!

3. Left System provides Nose Wheel Steering. Through the rudder pedals you can get to turn the Nose Wheels "up to seven degrees in either direction".

8. In Normal Operation the Demand Pumps work when one activates systems that do require an higher output (higher demand) from the Hydraulic System such as Flaps and Gear (at least that the understanding that I have).

10. Without C and R Hyd systems there won't be Stabilizer Trim (Pitch Trim). Without Stabilizer/Pitch trim NONE of the Autopilots will work/function.

Hope this helps.

Take care,
Zero


PS. On a side note, how did things went at AirBaltic???

Hydraulica B757
 

Hi prober.
Thank you.
That s really useful.Yes,that makes sense.Of course.Left A/P has no reason to fail except that sooner or later he won t be able to cope with it without stab trim.
As you said,linking more than one system gives a better understanding.Thank you.

Hydraulics B757
 

Hi Zerograv

3-Thank you.So the fact of recommending you differential braking is because of the lost of nose wheel steering,not of the reverse.Excellent.Thank you
8-.Thanks
10-Thank you.

Oh yes,of course that helps.
Have a good day

Hydraulics B757
 

Hi Swedish Steve.

Thanks so much.
Yes,that makes sense.Things like "cte Pressure,variable flow",I have to read them, but I don t necessary think about them just reading the QRH.

So thank you.Now it makes more sense to thing that only 6 Gallons per system are enough.

Hydraulics B757
 

Het Zerograv,awesome memory.
yes,I got the job.I m half Convertion course and I ll have my first flight something like June first.

And I have to say that I am really impressed with the Airline.Very professional.

Thank you for asking man.

757 HYD
 

Here is another one to help blind your mates with science.
When you do the walk round, why, if there is frost on the underside of the right wing, do you get wet deicing fluid dripping down the back of your neck from the left wing? Hint - where is the fuel temp taken from? (It IS to do with HYD!!):bored:

1-
PTU will operate if LEDP looses pressure.

2-
Depends on the location of the leak,yes you could loose all fluid from L system as its from the lower end of the reservoir.

3-
Left system supplies fluid for Nose wheel steering.

4-
You have only one PCU pressurized as only one System is functional.

7-
Will check the AMM & get back.

8-
Flow rate of EDP much higher than EMDP.

9-
EDP vane in pump will go to neutral position & stop fluid pumping.Not sure if the checklist permits resetting in flt.

11-
Reserve brakes comes from RH system.Accumulator pressure is Pneumatically pressurising the fluid for 2-3 brake applications.

12-
Spoliers are Symetrically operated on both wings for obvious stability reasons,So each pair gets INOP together.If not would need deactivation of the paired opposite side.Spoilers #1 & 12 are powered by Ctr system.

13-
L & R Reservoirs are located in their respective MWW.Ctr Reservoir is in the Wing body fairing aft of Left MWW.

Hydraulics B757
 

Hi Hawk21M

Thank you.We re kind of building an Hydraulic Mater here.

-1,2,3,7,8,9,12,13:understood.Thanks.

-4 So the reason is because only one PCU unnit is operating?So it has to do with Spoilers Control Modules?assymetric spoilers deployment?that makes sense for a Max crosswind limitation.But still one piece missing to me:there are 6 SCM s,and just one SCM failure will cause assymetrical spoiler deployment.And spoilers are driven by the 3 Hyd Systems.So why there is no crosswind limitation with one Hyd system failure and there is with 2 Hyd failure?Plus I m looking at the diagram, and all the spoilers are symetrically Hydraulically driven:L HYD drives 3,5,8,10,R Hyd 1,12 and C hyd 4,9.So lets say you loose C and R Hyds,with the remaining L Hyd you should have spoilers 3 and 5 on L side and 8 and 10 on R side.may be I m mixing things.The question is why no crosswind limitation with ! system INOP

-11-Ok,then when Normal and Alt brakes fail and we push RSVR BRK,we get the reserve brakes,the C2 is isolated for the brakes and full brake authority is available.But then what is the accumulator?I though it was the level below the stack pipe,but that s the liquid used for reserve brakes right?I mean,when do you loose Reserve brakes to have only available the accumulator Pressure"aand pneumatically pressurized?you mean that when you get the RSV light,other than a low qty,that could indicate low Hyd Fluid pressurization Pressure,that can tell you that if you loose Normal,Alt and reserve,you could have problems with your accumulator Pressure for your 2-3 applications?

Thank you like always

Hydraulics B757
 

Hi prober.
What?well,we have to admit our limits,and on first approach I have to admit that I don t see the relationship.
But let me think a minute...

When you do the walk round, why, if there is frost on the underside of the right wing, do you get wet deicing fluid dripping down the back of your neck from the left wing? Hint - where is the fuel temp taken from?

Ok.fuel temp is taken from the Right Side.But I don t see the Fuel-Hyd system link.
But this Hyd inquiry is being extremely useful.
No,I don t see it.Frost on R side(where Fuel T is measured),deicing fluid from L side..I don t know...all that comes to me is the Fuel Heat exchanger working when the R side sensor tells that the R side is cold(frost)...No,no idea.But looking forward because it looks like a really good one.

L Wing Drips
 

There is a HYD heat exchanger in the L Wing Fuel Tank so, if the aircraft has been flying recently, the left wing will always be warmer than the right one. So you get melting ice etc dripping during the turn round. The right wing fuel tank does not have this and so the true fuel temp can be measured here.

Hydraulics B757
 

Wow,that s good one.That s why they measure Fuel temp on the right side.
Are we talking about the Fuel?Oil Heat exchanger?

Heat Exchanger
 

I am not an engineer and this sort of detail is not covered in the pilots' conversion course any more and we have to learn these things as we go along. Investigation of some vaguely related problem led me to ask our Quality Control engineer about this and he assured me that there was a Fuel/Hyd heat exchanger in the L Wing.