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Old 19th Sep 2008, 14:13   #1 (permalink)
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Join Date: Jul 2006
Location: Norway
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737-800 Hydraulics


Our FCOM states that the electric hydraulic pumps should be switched on in sequence 1 - 2, and switched off in sequence 2 - 1, to minimise the transfer of hydraulic fluid between system A and B.
It isn't a problem operationally from what I've heard, but a procedure we utilise to not create an increased workload on the tech staff.
My guess is that there has to be a check valve somewhere in one of the hydraulic systems, but I haven't figured out which one.

Does anyone here know which system?

All inputs are welcome
KristianNorway is offline  
Old 19th Sep 2008, 16:28   #2 (permalink)
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never seen that--

i've been on 737's since the early 90's, 100,200,300,400,500 and now the 800 and i have never seen that in any books i have been given.

must be a company added change and someone in your company should know.
stator vane is offline  
Old 19th Sep 2008, 19:27   #3 (permalink)
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Yes.. I got the impression this was company - specific. One wouldn't find it in the FCTM.

I'll try our tech staff, but if anyone knows about the procedure please let me know.

I would appreciate it
KristianNorway is offline  
Old 19th Sep 2008, 19:35   #4 (permalink)
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Brake shuttle valve (between main/alternate brakes)?

Ok, I am a classic driver
Cough is offline  
Old 20th Sep 2008, 08:57   #5 (permalink)
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with the brakes,
if you set the park brake with system B, then secure sys B,
turn on sys A, and release the park brake, there will be a small transfer of hydraulic fluid, or vice versa!

The same effect will be had with the reversers, since they can be operated open with a 'normal' hydraulic system, depending on the side, and closed with the standby system.

But these are things one would do intentionally, not something done unintentionally!

In response to your Q, I don't know of any otherway to transfer fluid from one hyd sys to another, on the 737NG.

Think of your current procedure when being pushed/towed without the steering disconnect pin in place, don't you toggle the system A pumps off and on, without changing the on status of system B!

Flight Detent is offline  
Old 21st Sep 2008, 12:48   #6 (permalink)
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Thanks Cough and FD.

Cough was right. I spoke to one of our techs who said the initial thought was to limit fluid transfer via the brake shuttle valve.

On the other hand the applicability of the procedure is questionable since the unpressurized system pressure must drop below 1500 psi before the brake shuttle valve shifts hydraulic source.
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Old 21st Sep 2008, 15:15   #7 (permalink)
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Yer can always rely on Capt 'S' Hydraulics
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