Hydraulic accumulator questions

[172_driver]

Hi all,

This is an ATPL question at my school:
If the charge pressure of a wheel brake hydraulic accumulator is reduced, the number of emergency brake applications available will:

*Increase.
Remain the same.
Reduce.
None of the above

Correct answers is A), increase.

Now to my concern. I can understand that if you reduce the charge pressure you can fit more hydraulic fluid in the accumulator (when charged). But also, when you start using the accumulator the gas pressure will drop quicker than if you had a higher charge pressure.

What is your thoughts about that question?

Thanks in advance for your help!!

[dixi188]

You are correct.

The number of applications will increase if the charge pressure is reduced, however the applied pressure to the brakes will be less with each application and may not privide the required braking force.

Also if the pressure is reduced to zero you will have no emergency or parking brakes.

The accumulator is primarily there for parking and emergency use and if correctly charged wil give about 5 to 7 applications on most A/C that I have worked on.

When charging an accumulator it is important to ensure that all the fluid is released back to the tank, often by applying the brakes on and off during charging. This ensures that the correct volume if N2 as well as pressure is in the accumulator.

Hope this helps.

[172_driver]

Thank you, it did definitely help!

Out of interest, do you have a value for a typical charge value? I assume this is the charge with a completely empty accumulator, more or less.

[NSEU]

750psi is normal for a pre-charge on a 747-400, but I don't know if this is typical.

Speaking of accumulator pressures...

Photos: Boeing 747-412 Aircraft Pictures | Airliners.net

Is this normal in this phase of flight? (I've seen pics of other 747-400's with 3000psi in cruise)

[Horwood]

I am not familiar with the 747-400 but with the 757. The Flight Deck brake accumulator pressure indication is typicaly sensed at the Hydraulic output of the brake accumulator,the brake acc precharge pressure by a mechanical direct reading gauge sensing the N2 pressure.The low inflight pressure in the 747-400 picture is probably a combination of a lower than normal N2 precharge pressure and a long inflight cold soak reducing the N2 precharge volume . From experience with the A300 and 757 I can positively say that a low acc N2 precharge pressure will reduce the number of effective emergency or parking brake applications available. This was the usual line engineers warning signs of a low N2 precharge pressure. With the brake accumulator precharge correct,but a hydraulicaly empty acc the flight deck gauge will read zero but the acculator precharge gauge (usually in the gear bay or wing/body fairing) will read N2 precharge pressure.
The 757 precharge pressure was reasonably critical but the A300 less so due to the different accumulator design and increased volumetric capacity.

[Robin Jain]

Typical precharge value is 1000 psi in Boeing 737-NG..& it normally goes to 3000 psi after getting pressure from hydraulic system. Precharge is Nitrogen. After 5-6 brake appln or after 8 hrs of parking brake appln it again comes down to its precharge value of 1000 psi...

Hope this helped u..
Regards
Robin Jain

[172_driver]

Thanks RJ,

I do understand the principle of operation (I thought at least) and was therefore surprised to see the B747 picture above. Even though the pre-charge is not correct, shouldn't the main hydraulic system pressurise the accumulator to system pressure?

[Horwood]

172 driver
Sorry ,rechecked the 757 AMM ,both the direct reading gauge AND the flight deck indicator pressures are sensed at the nitrogen end of the brake accumulator.The previous theory still holds good about the cold soak and slightly low precharge pressure .The separator piston in the accumlator has bottomed out towards the nitrogen end of the accumulator with the nitrogen pressure maxed out at 2500psi. There will still be hyd system pressure on the other end of the accumulator (3000 psi).

[Robin Jain]

172 Driver

Why you said precharge is not correct. Its the value of precharge pressure in Boeing-NG plane & i dont know the value in other aircraft. And yes by saying it goes to 3000 psi after getting pr from hyd sys, my mean is that total accumulator pr available for braking is 3000 psi as soon as
brake system hyd is ON (Sys B in Boeing NG). The indicator in cockpit shows the total available pr at any time which is composed of precharge & hyd fluid.

Thanks
Robin

[172_driver]

The separator piston in the accumlator has bottomed out towards the nitrogen end of the accumulator with the nitrogen pressure maxed out at 2500psi. There will still be hyd system pressure on the other end of the accumulator (3000 psi).

Is there a mechanical stop for the separator piston where it is "maxed out"? Otherwise there should be a standing force (500 psi*area) trying to compress the nitrogen further, up to hyd sys pressure? But I am no expert

Thanks for the additional info about the 757.

[Horwood]

You are correct in stating there will be a 500psi pressure drop across the separator piston in this instant only, because the piston is being restrained.Normally the piston is fully floating so the hydraulic pressure is the same as the N2. This Stop would be part of the accumulator design. It could be a change in diameter of the acc bore,shape of the piston or the accumulator end cap design.
The method of pre charging accumulators requires referring to a pressure\ temperature graph and charging the N2 with the parking /emergency brake system hydraulically depressurised. The N2 Precharge then forces thr separator piston fully towards the hydraulic end creating a known volume into which a known pressure is added. On some aicraft types a small temperature change will make a large difference in the precharge pressure.Most aicraft I have maintained have a precharge of around 1300psi with a variation for temperature of around +/-100psi.
I have referred to the type of accumulator used by Boeing and others.Airbus tend to use a round accumulator with a synthetic rubber membrane separating N2 and hydraulic oil.

[172_driver]

That explain things, thanks for the informative answer.