View Full Version : B737 Brakes Hyd return
Anyone can help figuring out if using ALTERNATE BRAKES (737 classic) - System B press NOT available - the oil will return to Hydraulic Tank A or B?
Muchas gracias
alexban
8th Mar 2004, 02:02
Hy
As I know,when B pressure low,a alternate source selector valve will open and system A will supply pressure to the brakes.Also a valve will isolate the brake pressure accumulator.
From brake schematic you'll see that fluid will go back to system A
It's normal,to avoid losing also fluid from A due to a leak in B sys.
But you are on to something.You can transfer fluid from system B to A
A (if i'm not wrong) by using brakes (on ground).Something like this -push brakes,than select hydraulics B off- then release brakes.The fluid will go to sys A. You can use this if you want to raise sys A quantity if a little below 88%.
You can transfer fluid from A to B by using the nr 1 reverser -normal open,then STBY retract.I'm not 100% sure,I have to find my notes,but i think I'm right
Alex
The 737 classic brake system is separate for both A and B systems, so under normal circumstances, if B (normal) system is used fluid returns to B resevoir and vice versa if A is used.
However there is a common part to the system which is the piping down stream of the shuttle valves and the brakes themselves. Therefore as has been said, if the brakes are applied at the moment the B system fails (or is selected off) then the fluid under pressure that was in the lines down stream of the shuttle valves will return to the A system when the brakes are released.
Poor operational procedures can result in fluid transfer (i.e. selecting the park brake with A system pressurised but not B system). The A and B resevoir levels can be equalised by the operation of brakes and the correct sequencing of hydraulic power switches, but this is not a good maintenance practice as any contamination in one system could then be transfered to the other. I personally would NEVER transfer fluids by using the T/R's someone could lose an arm like that!
Roger that.
Poor operational procedures can result in fluid transfer (i.e. selecting the park brake with A system pressurised but not B system).
One thing puzzles me: on the diagram I see the Parking Brake valve is on the B return line.
If Normal brakes and Alternate brakes are two separate systems, is it possible to set the parking brake using Alternate Brakes ( B pressure not available)? Is there a Parking brake valve on the A return line?
You are quite right that the park brake valve is in the sys B return line. It is there to prevent system pressure bleed off when the park brake is set (i.e. long term)
When setting the park brake several things happen.
The left and right pedals must be depressed and the park brake lever pulled.
The partially depressed brake pedals operate pushrods and cables which open both the normal and alternate brake metering valves.
The pulling of the lever operates a switch which activates the park brake valve. It also moves a pawl locking pin which will keep the brakes depressed even when foot pressure is released.
Therefore which ever system is pressurised will apply presure to the brakes as long as that pressure is available.
So it is possible to apply the park brake when only system A is pressurised. When A system pressure is lost the accumulator isolation valve will allow accumulator pressure to feed the brake system via the B system brake lines and bleed off will be prevented by the isolation valve (park brake valve) in the B system return lines.
Hope this helps.
Hi mono,
I still have two problems:
1) If both systems A and B, are pressurized when we depress the pedals, how the airplane "knows" system B is the master?
I seem to understand the key to this problem lies in the NORMAL/ALTERNATE SHUTTLE VALVES.
But when both systems are pressurized, why the valves open the B pressure and not the A one?
Maybe the whole braking system is set in order to provide, with the same brake pedals deflection, a higher pressure on the B side?
If so, I need a harder push when using ALTERNATE BRAKES to get the same deceleration.
2) On the ALTERNATE BRAKE SYSTEM SCHEMATIC there's no PARKING BRAKE VALVE.
But there must be one in order to set the Parking brake with only sys A available!
Thank you for your time and valuable help.
LEM
Captain Stable
11th Mar 2004, 01:42
1) A and B hydraulic systems are independent. There is no exchange of fluid at the brakes, so B system fluid is returned to the B system, and A system's fluid stays there as well. The two systems don't need to "know" which system is in use. They are independent.
2) The parking brake can be applied with either A or B hydraulic systems pressurised. If neither, then it can be applied using pressure from the brake accumulator. Then when the parking brake lever is applied, not only does it close the parking brake valce but it also locks the brake pedals in that position.
avioniker
11th Mar 2004, 05:23
The normal brakes use the B system hydraulics and all fluid is routed from and to that system.
If you select Alternate Brakes then you are using A system and all fluid is routed from and to that system.
Some of the previous answers confuse the Power Transfer System with fluid transfer. The PTU uses A system pressure to drive a hydraulic motor which drives a pump in B system to increase the flow in times of increased need in the B System. No fluid transfers from A to B in that system.
If you apply the brakes with only B system hydraulics on, turn off B and turn on A then release the brakes the fluid will flow to the A return. Same for the reversers and spoilers.
There is an electronic shuttle valve to keep the systems isolated in normal operations but you can transfer fluid if doing it abnormally.
Here's a website that'll explain it to your heart's content
http://www.b737.org.uk/hydraulics.htm
Lem,
Again you are correct that the answer lies in the shuttle valves. These valves are designed such that the priority system ( in this case B) has a larger surface area on its pressure face than the alternate system. (think of the valve as a stepped shaft inside a tube with inlets at each end and an outlet in the middle). In this way when both systems are at the same pressure then the side of the valve with the larger pressure face has greater overall pressure acting on it than the smaller alternate pressure face. The valve then slides over to allow B system fluid to operate the brakes. When B system pressure falls to below 1500 psi (the ratio of the faces surface area is approx 2:1) the overall pressure on the A system side now overcomes the B system side and the valve slides over to allow A system pressure to operate the brakes.
Does that help??
I understand the the shuttle valves. These valves are designed such that the priority system ( in this case B) has a larger surface area on its pressure face than the alternate system functioning, and that's a step forward, but forgive me, I'll insist as this matter is not 100% clear to me.
Correct me if I'm wrong:
When B pressure is available, the ALTERNATE SOURCE SELECTOR VALVE (not the normal/alternate shuttle valves ) is closed and the alternate brakes system is not pressurized.
If B pressure falls below 1500psi, the ALTERNATE SOURCE SELECTOR VALVE opens and allows A pressure to the alternate brakes.
Is that valve electronic, as per avioniker's post, or is it working on the larger surface on one side principle as per mono's post?
If both normal brakes and alternate brakes were always pressurized, the priority function could be accomplished by the NORMAL/ALTERNATE SHUTTLE VALVES (the ones close to the wheels on the diagram), but I think that's not the case, because this priority function is accomplished by the ALTERNATE SOURCE SELECTOR VALVE.
I still have a problem with the parking brake!
The parking brake is set by blocking the return of fluid in the return line (and yes, by blocking the pedals with a pin).
When we use system A pressure only (ALTERNATE BRAKES), we must trap oil pressure in the A return line, right?
But I don't see the blocking valve on the ALTERNATE BRAKE SYSTEM SCHEMATIC.
I seem to understand you are telling me there's only one PARKING BRAKE VALVE (a blocking valve), and it's located on the B return.
So my question is: how can we block the A return?
There must be a blocking valve on the A return as well!!
"There must be a blocking valve on the A return as well!!"
I share your confusion, LEM, but, as far as I can see, the blocking valve, if you look at the brake system schematic, is only required when there is no continuous pressure on the brake pedals or the hydraulic systems have been turned off.
When the brake pedals are depressed as part of normal park brake application, they are held down by a mechanical latch. Therefore, you have continuous pressure on the brake pedals. If you then have hydraulic pressure, the blocking valve is not required....it's just like normal pedal braking. If you remove pressure from the pedals or remove hydraulic power, then you have no braking.
Let me expand/paraphrase....
When the brake pedals are pushed and held by the park brake mechanical latch and you have pressure from system A (B off), then you have braking (the latch simulates your feet on the pedals). However, if you switch off System A, you no longer have any brake pressure and therefore no braking. This is where the brake accumulator comes into play (on system B). Assuming there is pressure in the accumulator, then the shuttle valves will position themselves to allow pressure from the accumulator to reach the brakes (pressure removal from System A will allow the shuttle valves to move to System B/the accumulator circuit). The normal park brake valve on System B can now come into play (if I interpret the diagrams correctly).
The only problem I can see is if there is a loss of fluid or pressure in the accumulator braking circuit.
I stand to be corrected.
Rgds.
NSEU
The Park Brake Valve is located in the return (system B) line of the NORMAL Anti-skid valves.
It`s purpose is to block return fluid from leaking through the normal anti-skid valves back to return (to system B reservoir).
By blocking the return line of the normal anti-skid valves the park brake valve ensures that the accumulator when fully charged(3000psi) will hold the aircraft for eight hours with-out the use of chocks.Parking for longer periods requires the outboard wheels of the MLG to be chocked.
The park brake valve will have some affect if the brakes are parked using system A.System A pressure over time will bleed down through the Alternate anti-skid valves as there is no park brake shut off valve preventing the leakage of pressure to return to system A reservoir.
Pressure leakage will not occur through the Normal or Alternate Brake Metering Valve Assy`s.
When the A system pressure within the brakes drops below the accumulator charge pressure, the accumulator isolation valve opens allowing accumulator pressure to hold the brakes in the parked position.As the accumulator uses and is in the NORMAL (system B) hydraulic system the park brake valve stops the accumulator pressure from returning throught he normal anti-skid valves just as it would if the brakes had been parked normally.
Regards DDG
NSEU and DDG,
Thanks a lot Gentlemen for your outstanding posts!
At last I got it clear.
The NORMAL PARKING BRAKE SYSTEM is redundant , as we rely on the pedals locking pins and the parking brake blocking valve.
The ALTERNATE PARKING BRAKE SYSTEM is not redundant itself, as it relies only upon the pedals locking pins, but the whole system IS redundant, as the accumulator pressure will come into play after the A pressure has bled off, even with the airplane sitting dead on the apron.
Great!
As I'm always eager to learn more, let me ask a few little more questions:
1) I now see clearly on the diagram the ACCUMULATOR ISOLATION VALVE has a larger surface area on the A pressure side, so (as explained by mono), after a few hours the accumulator pressure comes again into play when A pressure has bled below.... what exactly?
Around 1500psi, is that it?
2) At the risk of asking a stupid question:
Parking for longer periods requires the outboard wheels of the MLG to be chocked.
Why the outboard ones?
Thank you again for your valuable help, really appreciated.
:D :ok: :ok: LEM
LEM
1)To be honest 1500psi sounds correct but i can`t find an exact figure in my notes,all it says is when the A system no longer supplies pressure.
2) Outboard wheels are recommended to be chocked as these will provide the most resistance to the yaw effect of cross winds on a shut-down aircraft and is the Boeing proceedure,however many companies will use their own policy.
Regards DDG