757 Hyd Fluid Transfer

[underread east]

Complany manuals ask us to pressurise hydraulic systems from R to L and to depressurise from L to R to 'prevent fluid transfer between systems'.

My understanding was that these systems are isolated (apart from PTU powering L - and even then with no fluid transfer). Do any of you know of the possibility of this and, if fluids transfer is possible - does/can this result in major fluid loss from one to another (presume L to R given the direction of power up).

Looking forward to being enlightened!

Cheers U-E

[Mr.Brown]

Off the top of my head:
On the braking system there is a valve called the"Alternate brake selector valve" which when there is no R/H sys pressure it allows L/H sys pressure to provide brake (Alternate) pressure.
We (Engineers) can, by use of the park brake, take fliud out of either L/H or R/H sys and return it to the other by manipulating this valves position using Hyd differential pressure.:ok:

[RYR-738-JOCKEY]

Perhaps something to do with the Landing Gear Transfer Unit. On 738, normal LDG gear is on sys A, so if A is depressurized B will take over powering LDG gear, NWS...

[767-300ER]

The sequence in which you pressurize or remove pressure from the hydraulic systems can cause fluid to move between the center and right systems. This can cause a high fluid level in one system and a low fluid level in the other system. If you set the parking brake in one of these conditions:

- the right and center hydraulic systems pressurized
- only the right hydraulic system pressurized
- no hydraulic system pressurized

and then release the brake with only the center system pressurized, thefluid can move to the center system. If you set the parking brake withonly the center system pressurized and then release the brake in one ofthese conditions:
- the right and center hydraulic systems pressurized
- only the right hydraulic system pressurized
- no hydraulic system pressurized

the fluid can move to the right system. To keep the movement of thefluid between the systems to a minimum, pressurize the right systembefore you pressurize the center and left systems. Also, remove thepressure from the center and left systems before you remove the pressurefrom the right system.

[teamilk&sugar]

Mr Brown is correct. The fluid transfer takes place through the brake shuttle valve and can only happen (apparently) from left to right sides, hence the requirement to turn on your right ACMP first.

The PTU has nothing to do with it as this is merely a "power transfer" device, ie: a hydraulically powered motor pump using right system PRESSURE to enable the fluid on the left system to be used if required.

Rgds

[gas path]

The 777 is similar, as is the 747 (classic and 400) which will also transfer hyd. fluid across the brake shuttle valves.
Must be a Boeing thing :8

[Beaver man]

Not knowing the a/c I would tend to agree with Mr Brown. It certainly happens on the 146 and also the Q400. But at least on the 146 you can transfer it back again, which is quite handy!

[Piper19]

Mr Brown has it right, I do it often during the night when I don't have time to fill the right reservoir. You have a normal and a alternate antiskid/brake valve in the wheel well. The alternate uses left hydraulics, the normal and reserve brakes use right hydraulics. Left and right are a separated system, no fluid transfer, BUT, the two antiskid/brake valves connect to a brake metering valve, that chooses which brake valve to operate (normal or alternate). So between the brake metering valve and the brake itself is only 1 connection, which can contain left or right hydraulics according to which brake valve working. It is this fluid in the brake that you can exchange between the 2 systems. Since center hydraulics are not used in braking, you canot exchange fluid from the center system.

And for the fluid loss, the max drop I've seen during fluid transfer is 0.9, so not a big deal if you service the reservoir correctly first.

[Swedish Steve]

Its getting complicated.
On a Boeing there are one set of brake cylinders.
On an Airbus there are two sets.
So on a Boeing either left or right (B757) or centre or right (B767) hyds can pressurise the brakes, but there is only one return path.
On the B757 the return is to the right system. So if you turn on the L sys first, it will pressurise the brakes. When you turn off the hyds the fluid will slowly bleed back into the R system. So to avoid hyd transfer on a B757 always pressurise the R sys before the L sys.
Our crews don't, so we have to transfer the fluid back every week by setting the parking brake with the R sys and letting it off with the L sys.
Same can happen on B767 B744 and B777 (but different systems).
On an Airbus the main and alternate brakes have their own set of brake cylinders so it doesn't happen.
Also on an Airbus, when you set the parking brake on a nightstop it is still set in the morning. While on a B757 the accumulator is usually empty.

[Piper19]

Yes, on the 757 the brake accu works for a few hours, only good for a quick turnaround. Our Airbusses have 2 piston systems on the brakes, but on the other hand we can't even put on the parking brake. The brakes tend to be a lot hotter than on the 757 after landing, and by putting parking brakes on we get a lot of sticky brakes. (pistons unable to release when putting brake pedals off)