Hydraulics B757

[Piper19]

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

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]

Thank s sir

[Prober]

757 HYD

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]

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]

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]

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]

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]

757 HYD

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]

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]

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]

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]

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]

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]

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]

B757 Hydraulics

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