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zorsan
23rd Apr 2010, 14:17
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

Piper19
24th Apr 2010, 03:17
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

Prober
25th Apr 2010, 23:18
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!

zorsan
26th Apr 2010, 20:39
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.

zorsan
26th Apr 2010, 21:32
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

zorsan
26th Apr 2010, 21:49
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..

Prober
26th Apr 2010, 22:44
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?

Swedish Steve
27th Apr 2010, 09:57
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?
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.

zerograv
27th Apr 2010, 14:55
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???

zorsan
27th Apr 2010, 15:55
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.

zorsan
27th Apr 2010, 16:03
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

zorsan
27th Apr 2010, 16:09
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.

zorsan
27th Apr 2010, 16:11
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.

Prober
27th Apr 2010, 22:37
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:

HAWK21M
28th Apr 2010, 08:05
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.

zorsan
28th Apr 2010, 15:57
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

zorsan
28th Apr 2010, 16:04
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.

Prober
28th Apr 2010, 16:40
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.

zorsan
28th Apr 2010, 19:25
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?

Prober
29th Apr 2010, 22:28
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.

Piper19
29th Apr 2010, 23:03
I don't know if pilots are told to be aware of this, but mechanics have to be very aware of the amount of fuel in the main tanks. If you put on hydraulics with the fuel/hydraulic heat exchangers not covered in fuel, it gets overheated! Overheated metal and fuel vapors don't go together. Do not operate hydraulics if main fuel tank has less than 1,8 tons of fuel, or at least keep an eye on the hydraulic fluid temp. (altough I hope you pilots never see 1,8 when flying). Mechanics can have this situation e.g. when we drain fuel to work inside the tanks.

BUT...to complete the story of the heat exchanger... There are 3 heat exchangers, one for each hydr system. Left and center systems in left wing, right system in right wing. The end result of your deicing fluid story remains the same:)

some more info about the acmp's...to start up an acmp it needs a lot of power. Try pushing in all 4 acmp switches at the same time, you'll find they all start at different times. All pump circuits have time delays of different values, built in for trigger happy pilots who want to save time by pressing all switches at the same time, without knowing they could overstress the elec system.
C2 acmp does not work when only 1 ac bus is powered, for the reason of needing a lot of power. There are situations where both C pumps can run, e.g. on takeoff I believe. Both will be running because in case of 1 engine failure, the remaining engine has not power enough to start up a center pump that would not be running. With engine failure on takeoff you need to get that gear and flaps up as soon as possible (I guess, correct me when wrong), and you need all center pumps running for this to happen quick.

accu pressure: with right hydr pressure on, the accu stores this pressure. With all hydraulics lost, you take the pressure that is sealed in this reservoir. If you don't press reserve brakes that is what happens. Next time when you're on ground with engines shut, check your brake pressure gauge. Shut off all hydraulic, now press the brake pedals several times. You'll see the brake pressure gauge go down. You're tapping off pressure from the reservoir. And suddenly, you'll hear "clunk" and feel no resistance in the pedals. Your pressure gauge will show about 1200PSI. This is residual pneumatic pressure, not hydraulic pressure, you've lost all your braking by now. This happens after about 7 brake applications. Now push the reserve brakes switch or the right acmp, and see the indicator go up to 3000 again. Make sure the wheel chocks are in place however, you don't want to have a rolling brakeless 757. The parking brake will also use this accu. There is also a lot of misunderstanding among pushback drivers. When they see the red light at the nose wheel they think parking brakes are set. But when the aircraft has been parked for 12 hours, all accu press is lost and parking brakes are NOT set, even if the handle is up and light on.

regarding antiskid; in normal and alternate you have each wheel in the system, in reserve you have 2 wheel pairs, on accumulator there is no antiskid.

That's more than you pilots should know, if you want to know more, get yourself an overall and some grease:ok:

edit: another one for you drivers; why do we need to put the EDP switches in on all the time, even when engines not running on ground?

zorsan
30th Apr 2010, 13:54
Thank s sir

Prober
30th Apr 2010, 19:03
I tell my conversion students that I seemed to know very little about the 757 on my first flight on that aircraft, especially after the old British ARB way of learning about every wire etc on one’s a/c. I also tell them, even after xx thousand 75/76 hours, I am still learning, and Thank You, Piper 19 for your most worthwhile post. It raises far too many very interesting situations to be discussed here – unless by general request – things which one only learns by (usually bitter) experience. Ref your parting shot – I had always assumed the EDP switches were left ON because there was no point in turning them off – turning off the engines had done that for you. Go on, tell me! And Zorsan, keep up your interest – there is an enormous amount that you are NOT told.

Swedish Steve
30th Apr 2010, 21:28
The EDPs on most aircraft (well all I know) incl 757 are powered off by a solenoid. So when you select them off the solenoid is powered. It will burn out if left off too long.

Another B757 fact I learn't by experience.
Engine/APU bleeds.
There is an interlock so that, if the engine bleeds are not off, the APU bleed will not come on. You will see this in the bleed switches. If after shutdown there is no OFF light in one engine bleed switch, then the APU bleed will not open. The only way to overcome this is to manually wrench the valve closed with a spanner.
BUT, if you select the engine start switch to start, this interlock is overridden, and the APU bleed opens. This pressure will then push the bleed closed. (or start the engine if that is what you want)

Joetom
30th Apr 2010, 22:24
Just to add to S Steve's post, think my memory is correct.

Ref. No back feeding of APU bleed air.

If right eng bleed shows not "off " and no APU bleed avail, X bleed to closed and APU bleed comes on ok, X valve to open and and duct px will bang the right engine bleed to show "off "

If left eng bleed is problem, just select any eng start, when you get duct px for a few seconds, switch off selected eng start and all should be ok.

Note of caution, selecting start switches should be done with care, a little switch can cause many problems for those who choose to do this quick fix to overcome the bleed logic on the 757 when eng bleed valves fail to show the "off " indication. Take Care !!!!

HAWK21M
1st May 2010, 08:48
The LH & C Heat Exchanger for the Hydraulics system is in the LH wing & the RH Heat Exchanger is in the RH Wing.The C Heat Exchanger is Inboard of the LH wing.

Piper19
1st May 2010, 12:11
Swedish Steve, correct! The EDP switches will power the solenoid when in OFF but not when in ON (kind of reverse phylosophy), because in case of total electrical failure you'll loose all EDP's (on top of all acmp's) if it weren't so designed.

@prober, thanks! About the first time I don't get bashed for my replies here.

Prober
1st May 2010, 19:23
Thank you Piper19 and that makes sense. It is marvelous what you have to glean. Ref your post#21, with reference to the Park Brake. It is also worth noting here that the Park Brake Discrete light, though indicating the Park Brake is Set To Park, it is NOT setting the Park Brake to Park which brings on the light. It is the position of the Park Brake Valve. It is possible to have the Park Brake light ON but the Park Brake OFF – I know. And then you would have no Antiskid (14.5 tons T/O penalty IIRC).
Prober

spacegrand
2nd May 2010, 22:19
Reference keeping the hydraulics cool. I was told some 25 years ago the figure required in each main tank was 1,608kgs, i.e. never less than 3,200 total wing fuel. Does the engineering manual still say that?

mono
2nd May 2010, 23:00
Ref the park brake prober. Not any more.

There's an SB (mandatory I think) which puts additional switches in the circuit to ensure the park brake is really set.

Back in the day this was the case and all you had to do was pull the park brake handle to illuminate the park brake light.

Now the pedals need to be latched into the PB cam to illuminate the light. The only way it won't be set is if the brakes are set without hydraulic or accumulator pressure (or it was set and the brake pressure has depleted over a period of time).

zorsan
6th May 2010, 21:37
Hi everybody.

-First of all,thank you to everybody for keeping posting interesting ideas.
One thing that makes me think is that I m veeeery far from what I should know.And that s only Hydraulics.But at least I m in track,trying to leasrn more.In fact,no type rating syllabus tells you even anything about the physical things:yes,they tell you you have 3 HYDS.But not a word about where are the reservoirs,hoses,pumps imputs...

I feel like some of the things you can only pick them up here...
Thanks Prober for keeping up,thanks Piper19 for that very specific info,and same to everybody else.

anyway,I have a question that I hope is a good one:

-1-Before Taxi,I saw a lot of people making a Recall and Cancel.After the recall only the Parking Brake message is shown on the EICAS.Ok.Then people just cancel it and start taxiing.
My questionis the following:shouldn t we wait,do not cancel that message, and wait to see that the message goes off when releasing Parking Brake?I mean,let s say we cancel the EICAS,and the message that particular day would have NOT disapperared when releasing the brakes:could that mean that we have NO ANTISKID?what about an RTO then?If the antiskid valves are the same,it apperas to me than we should never cancel that PB EICAS message,then release PB,see that the message then disappears,so we know we have anti-skid availabe.

2-Another one(less interesting):why do we actually pressurize first the R Hyd system?it says to prevent hyd fluid transfer.From which side to which side?why that doesn t happen if we pressurize the L first?
and why then R Eng,C1,then C2 then L Eng?Why not R eng,C2,C1,L eng,so just from R to L?

Good night to everybody

Swedish Steve
7th May 2010, 09:56
why do we actually pressurize first the R Hyd system?it says to prevent hyd fluid transfer.From which side to which side?
Its to do with the parking brake accumulator.
When a B757 is parked, the brake accumulator will slowly lose pressure during time. After a nightstop, the pressure will be down at 1000psi. As this happens, hyd fluid is returned to the R hyd tank. If you then turn on the L hyd pump first, fluid from the L hyd tank will fill up the hyd brake accumulator. At the next nightstop, this will drain back into the R hyd tank. No fluid is lost, it just transfers from L to R.
Us engineers fix this by transferring it back, by turning the pumps on and off and putting the park brakes on and off in the right order, the fluid will go back.
So if the crew always start by turning on the R hyd pump, this won't happen.
This happens on most Boeings, as they have one set of brakes, but two sources of fluid.
On Airbusses there are two sets of brake cylinders, so fluid transfer through the brakes doesn't occur. Airbusses also have a different system of setting the park brake so that pressure is not lost overnight.

why then R Eng,C1,then C2 then L Eng?Why not R eng,C2,C1,L eng,so just from R to L?
On the ground, only one of the C pumps will run. If both are on, C1 will run.
So if you select C2 first, then C1, C2 will start to run, but stop to let C1 run. Starting these electric pumps takes an awful lot of amps and starting two quickly could overload the electrical system.
For this reason, it is a good policy to not turn on more than one electric pump when you are on ground power. Wait until the APU is supplying before you select a second pump.

Prober
10th May 2010, 22:31
Three comments for what it is worth. Yes, Zorsan, you will never stop learning! Second, RECALL and CANCEL. I always instructed students NOT to cancel (if only PARK BRAKE is showing), for exactly the reason you quote. The PARK BRAKE discrete is not very noticeable, especially on a sunny day. My Boeing manual (admittedly a very old one), says that the PARK BRAKE light indicates the position of the PARK BRAKE VALVE (nothing to do with latching on the pedals or whatever). If the PARK BRAKE VALVE gets stuck, then when the PARK BRAKE is released (and the aircraft taxies away), the light will not go out. Maybe this has now changed – I am afraid I don’t know, I’ve now hung up my flying helmet. Third, on my conversion course (in the days even before the aircraft had VNAV functioning), we were taught by Boeing instructors and they told us to switch on the HYD pumps as you describe. The 3 reasons were as follows: individually because of the electric overload, xfer of fluid as described and Loadshed. R HYD, then C2 –wait for it to show – then C1 – wait for it to show – then L HYD. This will loadshed C2, showing you that C2 works, C1 works and Loadshed works. I apologise if I am not correct, I have now passed my “sell by date”.

zorsan
18th May 2010, 10:09
Thanks again prober and Swedish.
Extremely clear.
All this makes me think that TR courses shouls last half year if they wanted us to know a little bit what s behind.But new phylosophy is just to operate without knowing why.
But awesome to have your answers,it gives sense to every single task and that is really good.
Thank you.

I will take some time to digest everything and in the mean time I ll bother with other subjects.
Thanks.really.

zorsan
19th Oct 2010, 20:12
Good morning everybody.
Back to study,and once again on Hydraulics,some questions raised and I kow there is no question that cannot be answered here(by experience) so I hope to don t bother too much.
Here they are:

1-Boeing QRH says"Autobrake INOP" if R Hyd failure.Ok.Nut wy it sys:"DO NOT USE AUTOBRAKE" when L Hyd failure?what do they mean?Because if you only loose L Hyd,you still have R Hyd so you have Normal and Reserve Brakes so we should have full Autobrake capability..

2-If we loose R Hyd,we ll only have Alt Brakes(with only laterally paired antiskid capability).Why then there is nothing on QRH saying that anti-sid action will be reduced?

3-Why QRH says "Do not arm Speedbrakes· everytim we loose 2 Hydraulics?

4-What is the real reason to have a "DO NOT AUTOLAND" with only 1 Hyd falure?because we have 3 HYDS required for Cat 3B,but only 2 HYDS required for Cat 2,and from cat 2 we can land either manual or Autoland.So does it means that with one HYD failure we are limited to Cat 2(300m) and that we ll have to lad manually?And why?what is the change from disconnecting or not the A/P just before landing?

5-RAT gives enough power above 130 kts.Ok.What about below?Being ligh Vref will be below 120 so does it means that rudder will be almost useless on the last portion of the approach?

6-When we are nt using Flaps or Gear(high demand),are the Demand Elec umps C1 and C2 working at all?or they re just waiting on a low Pressure?

7-And my favorite:CANCELLING the PARKING BRAKE EICAS message before taxi.Let s say we lost Antiskid and let s say we cancel the message without waiting to see the message disappearing when we release Parking Brake.And lets say we don t put the PB anymore during Taxi.We could then hide the Anti-skid problem?So it s not a good idea at all to ancel it.Actually Boeing says "Recall/cancel" before start but only"Recall" before taxi.Is that the reason.
And,if that s the case,if PARKING BRAKE EICAS(being PB released) means Anti-skid failure because the valves are the same,what does ANTISKID EICAS message means?What is he difference?

Thanks so much,I hope somebody will find it interesting.Good flights