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B757 Hydraulics tricks

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B757 Hydraulics tricks

Old 22nd Oct 2010, 15:12
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B757 Hydraulics tricks

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
zorsan is offline  
Old 24th Oct 2010, 08:37
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#3

I have never flown a 757.

But I believe it will be similar to the 747 in terms of some spoilers being unpowered with some hydr systems unpowered. Excessive pitch-up moment on touchdown if the Speedbrake is armed and you get only the partial spoiler deployment that results with lost hydraulic systems. So touchdown, get the nose down, then manually deploy the SB.
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Old 24th Oct 2010, 09:39
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B757 Hydraulic Tricks

Thank you.That makes sense.SO it would to avoid excessive pitch up on the flare.That means that spoilers would deploy too early if we arm them...ok,thank you
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Old 25th Oct 2010, 05:32
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#5 think about your comment.

If the RAT doesn't work below 130 KIAS why is there any concern about Vref at light weights? The rudder might be ineffective???

If you're flying on the RAT, and it's minimum effective speed is 130 KIAS, what do you think you're minimum Vref should be at 160,000 lbs? 170,000 lbs? 180,000 lbs?
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Old 26th Oct 2010, 00:46
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I defer to the 757 experts, but...

I assume the rudder and other flight controls on the 757 have triple redundancy where hydraulics are concerned. There may be limitations on the centre hydraulic system, but surely the Left and Right windmilling engines would provide hydraulic power to the rudder below 130kts?(enough to get you onto the ground safely).

The speed advisory might simply be to alert you to the fact that not all control surfaces might be working (e.g specific spoilers).
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Old 31st Oct 2010, 07:28
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B757 Hydraulics tricks

Hi misd-agin.
Thanks.
Ok,then Vref should be 130 for light weights regardless of Vref30 if we re on RAT?
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Old 31st Oct 2010, 08:37
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B757 Hydraulics tricks

I found answer to

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

Answer could be:

The main gear trucks remain un-tilted during alternate gear extension, which is the final signal for the automatic speedbrake extension. If the speedbrakes were armed, alternate extension used and the thrust levers subsequently closed at some point during the approach, the speedbrakes would extend fully

But now another question comes up:

Ok that if we loose L and R Hyd,we ll need Alt Gear extension so no Tilt so we should not arm speedbrake.
But why does QRH also mention "Do not arm speedbrakes" with the other combinations of 2 hydraulics failure?
I mean:If we loose C&R we don t need alternate Gear,and if we loose C&L and PTU works we don t need Alt Gear either.SO why do they still say "do not arm speedbrake" f Gear will be putted down with normal opertion so tilted strucks?
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Old 31st Oct 2010, 08:59
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B757 Hydraulics tricks

Looking closer at the QRH:

-It does NOT say "Do not arm speedbrakes" when R&C Hyd Failure.Logical,since normal gear extension.

But still 2 things that do not match:

-Hyd L&C Failure:
It says "Do not arm Speedbrakes" and it says "manually extend speedbrakes after landing".Ok,both messages go together.
But is it assuming that PTU will not work?
Later it talks about Alternate Gear and Flaps "if required"(if PTU not working).But then the " do not arm speedbrakes" should be onli "if required",so if PTU does not work.
What I mean is that the Normal Gear operation and the autspeedbrake funcion should be together,and the QHR puts "do not arm speedbrakes" in the all procedure and the Alt Gear only if required.
If PTU works,we should be able to Arm speedbrakes normally right?

-Hyd L&R Failure:
Totally agree with "Do not arm speedbrakes" in this case.
But then why the "Manually extend speedbrakes after landing" message is missing?

Thank you
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Old 31st Oct 2010, 09:18
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B757 Hydraulics tricks

By the way,does anybody knows how to transfer Hydraulic Fluid fromL to R Reservoir?
I hear that there is a way,using pumps and Parking Brake,to transfer from side to side.
But I don t see how
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Old 31st Oct 2010, 09:32
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B757 Hydraulics tricks

I found answer to:

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..

If nosewheel steering is inoperative, then differential braking will be required to keep the aircraft straight on the runway. The autobrake system can only provide straight-line braking and differential braking is not available during autobraking.

But,again:This "Do not use Autobrake" appears on L Hyd faluer and in L&C Hyd Failure.
My question is:in both cases,if PTU works,we would have Nosewheel Steering so we would not need Diff braking and we would be able to use Autobrake right?
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