Boeing Hydraulics
 

Hi everyone,

On 767/747-400 (and perhaps other Boeings I'm not aware of) when you pressurize the hyd system before start-up you should turn the right hyd system (No 4 demand pump on the 747) first by the Boeing normal procedure. The short explanation by Boeing is: "to prevent fluid transfer".
But fluid transfer between totally separate systems??? To my knowledge the system is designed to prevent transfer and loss of fluid in case of a failure (ruptured duct). And transfer from the left systems to the no 4 only?? And no transfer the other way round?
Studying the system diagrams in the AOM will not help.
Any explanation is welcomed,
b

Transfer occurs through the wheel brakes. Normal brakes are supplied from the RH system on nearly all boeings. If you pressurise the centre system first, the alternate brake system wheel supply fluid to the wheel brakes. When you apply RH system pressure an amount of fluid will transfer. The fix is a reverse of this scenario, and involves a lot of pushing of pedals and switching on and off of pumps.

Balaton,

On the 747-200, The #4 ADP (air driven pump), or #4 ACP (electrical pump) is used to pressurize the parking brake before start, and to provide braking during tow in the event of a tug failure.

The #1 ADP or ACP is used during pushback and tow to provide power to the body gear steering.

I don't know about the 767, but in the Classic 747 we can't transfer fluid. A veyr small amount can be transferred through the brake system, but it's inconsequential, and not the reason that the #4 hydraulic system is powered up prior to engine start.

balaton, the 767 and 737 have the same issue.Fluid transfer.
On those it does not take long to out it back where it came from if you dont get mixed up with your pump switching, brake application and release!

Many thanks for your quick response.
So, I understand, hydraulic fluid transfer - at some extent - is possible through the brake system if you don't activate sys 4 first. Still not clear technicaly how it could occur and why it doesn't occur if no 4 is activated first.
b

SNS3Guppy

You most certainly can, using the same procedure to transfer fluid from the No2 to the No4 system.

balaton

The reason for the transfer is the reserve brake shuttle valves. Also the restricters in the alternate brake source selector valves.
Selecting No. 4 (B747) or Right system (other Boeings) keeps the shuttle valves in place.

744
 

On the 747-400 course we were taught that to avoid transferring fluid from system #1 to #4 through the brake lines we indeed had to pressurize #4 first.

While the hydraulic systems are "separated" (by the antiskid shuttle valve) they do have a bit of common tubing, that of the brake lines downstream of the antiskid shuttle valve (which is why if there is a hydraulic leak in the brake lines, it is actually possible to lose systems 4, 1 and 2 in sequence).

The amount of fluid in the brake lines is small but the airplanes are pressurized various times each day...

MD

Same with the b757

Many thanks indeed.
b

Only a very small amount of fluid "transfers," and to do anything meaningful, one would need to switch systems repeatedly. Starting another hydraulic source prior to #4 doesn't create a continuous flow of fluid. System 3 has no effect, and system 1 will only work if the normal brake selector valve is selected to secondary system 1. It doesn't automatically pressurize the brakes unless selected.

What am I missing here?

There is a procedure in the maintenance manual to transfer fluid between no1 and 4 hyd systems , yes it involves a bit of switching, park brake applications etc etc but it is there and the fluid transfer does happen.

It seems to happen more on the 400 than the classic but that's probably because EICAS is easier to see and read the guages.:eek: Or that brake source selection is automatic.

quite often you will find no1 hyd at 110% + and no4 sys at 80% a few applications of the park brake and no1 & 4 ADP and it's back to 100/100.

Seems like it might be a lot easier to go down in the left body gear well and simply top off the resorvoir.

Nope easier to sit in the dry in the flt deck throwing a few switches and operating the brakes a few times.

We used to handle someone who did that and you end up with overfull systems.

Hmmm, looks like Guppy is misinformed, once again.:rolleyes:

Don't know if I should butt in because I work 90pc Airbus now with separate pistons for each brake system and no transfer.
But a few years ago I did nightstop checks on B757's and it was definitely worth transferring the fluid with the brakes.

Anyway no one has mentioned the brake accumulator. This is normally filled with the R hyd pressure, and returns into the R hyd tank. But after a nightstop, if the crews turn on the hyds from L to R then the Alt hyd system (L or C depending on aircraft) will fill up the accumulator. On the next nightstop it will empty into the R tank, and so quite a lot of fluid transfers. All Boeings seem to empty the accumulator pressure overnight. Perhaps they should check out the Airbus park brake system which lasts for days!

Many weeks with the L1011...clearly a superior design.:}

Yes, i'm sure your L1011's use that feature a lot these days :D

Thanks for contributing to the thread, as usual.

We don't use that method for servicing resorvoirs. We fill from the gear well.

Would that be before or after an engine falls off into Lake Michigan...or, the airplane runs off the end of the runway at BRU, due to a pilot screwup?

You're absolutely convinced you know what you're talking about (however irrelevant it is to the thread), and you really don't. Moreover, you inappropriately inject it into one thread after another. You seem to have a three-track mind: personal attacks, the L1011, and CKS. Seldom, if ever, do those things contribute to the thread. Have you any input on the Boeing hydraulic systems?